Amperage for 5HP table saw?
_firs...@lr_dot_los-gatos_dot_ca.us
single phase circuit?
My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
at 19.8 or 20 A. Given that a circuit is supposed to be loaded 80%
(look it up in the NEC sometime), this means a 25 A breaker would be
borderline sufficient, and that a 30A breaker should be generous.
With a slow breaker (not a fuse), there should be no problems with the
startup surge of the motor tripping the breaker.
The reason I'm asking is that I've recently read a few posts claiming
that a 5HP tool needs a 40A or even 50A breaker. There are a few
reasons not to do that. First, it requires 8 gauge wire (for 40A) or
6 gauge wire (for 50A), which is considerably harder to work with.
Second, it requires a different connector, which means less
flexibility in moving tools around. For this reason, I had been
planning to make all the 240 V outlets be twist-look 30A outlets in my
soon-to-be-built shop. Third, it is unsafe to use too large a
breaker: If something goes wrong, there is more current around to fry
things or start a fire.
While we are it it. I have converted on of our waterpumps (a 1.5HP
pump) from single-phase to three-phase motor, with a VFD (or inverter)
driving it. This gives me a really nice slow start, with no current
surges (which is vital when running on a generator). I'm planning to
convert my drill press similarly, one of these days. This would give
me the ability to run at variable speed (within a range of maybe a
factor of two), without having to change belts, and it would again
remove the startup surge and the mechanical stress associated with it.
This brings up the following question:
Is there a point using a 3-phase tablesaw motor on an inverter? There
is no point running a tablesaw at reduced speed, is there? So the
only point would be slow start (which is not worth the extra couple
hundred $$$ for the inverter). Also, in principle, one can buy 3 HP
inverters that run on single-phase 240V, but if the single-phase motor
on the table saw has a service factor >1, a nominal 3 HP inverter will
not be quite sufficient. Does anyone know whether electronic VFD
inverters with single-phase input and 5HP capacity even exist? I
haven't seen one yet.
--
The address in the header is invalid for obvious reasons. Please
reconstruct the address from the information below (look for _).
Ralph Becker-Szendy _firstname_@lr _dot_ los-gatos _dot_ ca.us
Matthias Mühe
the big point in using an inverter on the TS is not the speed (except you
want to cut steel on your saw), but the inherent motor break in the
inverter. My saw was spinning for close to a minute with a heavy dado stack
on after switching it off. With the inverter, it comes to an instant stop.
Regards Matthias
<_firstname_@lr_dot_los-gatos_dot_ca.us> schrieb im Newsbeitrag
news:1078204646.761648@smirk...
Robert Bonomi
<_firstname_@lr_dot_los-gatos_dot_ca.us> wrote:
>What amperage breaker does one need for a 5HP table saw on a 240V
>single phase circuit?
5 HP is 3725 watts. At 240v, this is a 15.52A draw, assuming 100% efficiency.
at 80% efficiency, you've got a 19.4A draw @ 240V.
>My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
>at 19.8 or 20 A. Given that a circuit is supposed to be loaded 80%
>(look it up in the NEC sometime),
"No more than 80%", for permanently attached loads, yup.
> this means a 25 A breaker would be
>borderline sufficient, and that a 30A breaker should be generous.
Yup. 30A is "more than sufficient".
>With a slow breaker (not a fuse), there should be no problems with the
>startup surge of the motor tripping the breaker.
>
>The reason I'm asking is that I've recently read a few posts claiming
>that a 5HP tool needs a 40A or even 50A breaker.
At 120V, yup. not at 240.
They, and even bigger ones, do exist. You *don't* want to see the price tag,
however.
Joe McDonald
<_firstname_@lr_dot_los-gatos_dot_ca.us> wrote:
> What amperage breaker does one need for a 5HP table saw on a 240V
> single phase circuit?
>
> My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
> at 19.8 or 20 A. Given that a circuit is supposed to be loaded 80%
> (look it up in the NEC sometime), this means a 25 A breaker would be
> borderline sufficient, and that a 30A breaker should be generous.
> With a slow breaker (not a fuse), there should be no problems with the
> startup surge of the motor tripping the breaker.
Using Ohm's law this is a relatively simple question to answer...
1 horsepower = 745.7 watts
Power (watts) = I (amperage) x E (voltage)
When you have two values for the variables in the equation you can
figure out the third.
P 3728.5 (watts 1 HP x 5)
Therefore I = --- or I = ------
E 230 volts
230 volts = 16.21 amps
240 volts = 15.53 amps
Using the "80% rule", a 20 amp breaker should not exceed a continuous
load above 16 amps. It seems to me, that with the proper type of "slow"
breaker, you should be OK with a 20 amp breaker.
Joe
PS
I am not an electrician or an electrical engineer. In fact, I have
never even played one on TV.
Al Reid
efficiency.
P = (HP * 746) / (pf * eff)
So with an efficiency of 80% (.8) and a Power factor of 90% (.9), 1 horsepower = 1036 Watts
For the 5 HP example I = (5 * 746) / (.9* .8) / 230 = 22.5. amps
Using the 80 % rule on breaker sizing, 22.52/.8 = 28.23, so a 30 amp breaker will be required.
--
Al Reid
"It ain't what you don't know that gets you into trouble. It's what you know
for sure that just ain't so." --- Mark Twain
"Joe McDonald" <comfo...@work.com> wrote in message news:020320040720322031%comfo...@work.com...
Paul Kierstead
Joe McDonald <comfo...@work.com> wrote:
> Using Ohm's law this is a relatively simple question to answer...
>
> 1 horsepower = 745.7 watts
>
Alas, this assumes 100% efficiency, which saws are not. For example, my
2HP saw is rated to draw 9A @ 220 or 1980W, considerably more then the
1492W ideal rating. It is this that makes me shy from 2HP+ tools; don't
wanna run 220 all over the shop. Though a 2HP DC is calling me...
Paul K
Bernard Randall
was trying to run a 5HP 24" planer/thicknesser because the mass is so large
it takes that much longer to get up to speed. Your 20 Amp running load is
right, but the startup Amps are round 2 to 2 1/2 times that.
Bernard R
<_firstname_@lr_dot_los-gatos_dot_ca.us> wrote in message
news:1078204646.761648@smirk...
xrongor
"Paul Kierstead" <pmki...@hotmail.spam.off.com> wrote in message
news:pmkierst-7B863A...@nntp.bloor.is.net.cable.rogers.com...
you guys are starting to get there, but whats so hard about looking on the
motor for the current rating or calling the manufacturer and getting a power
factor curve for it if you insist on going that route?
randy
xrongor
all i mean is that if you want to start doing the math, there are many
factors involved. the load conditions on startup. the load conditions
during running. and none of these are simple to calculate but most motor
manufactures will provide charts.
but in any case, if you cant run your 5horse motor on a 30 amp circuit,
something is wrong.
randy
DonkeyHody
> What amperage breaker does one need for a 5HP table saw on a 240V
> single phase circuit?
>
> My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
> at 19.8 or 20 A. Given that a circuit is supposed to be loaded 80%
> (look it up in the NEC sometime), this means a 25 A breaker would be
> borderline sufficient, and that a 30A breaker should be generous.
> With a slow breaker (not a fuse), there should be no problems with the
> startup surge of the motor tripping the breaker.
>
> The reason I'm asking is that I've recently read a few posts claiming
> that a 5HP tool needs a 40A or even 50A breaker. There are a few
> reasons not to do that. First, it requires 8 gauge wire (for 40A) or
> 6 gauge wire (for 50A), which is considerably harder to work with.
> Second, it requires a different connector, which means less
> flexibility in moving tools around. For this reason, I had been
> planning to make all the 240 V outlets be twist-look 30A outlets in my
> soon-to-be-built shop. Third, it is unsafe to use too large a
> breaker: If something goes wrong, there is more current around to fry
> things or start a fire.
I just finished wrestling with that very issue. I became the
unexpected owner of a 5 HP Single Phase Unisaw and had to figure out
how to wire it. In a phone call to Delta's technical people, they
told me:
1. Delta's 5 HP motor draws 100 Amps for about 1.5 seconds at
startup.
2. A 30 Amp "slow trip" breaker will hold it, but they are more
expensive and hard to find. (Home Depot never heard of 'em.)
3. A 40 Amp breaker will hold it fine, and that is what Delta
recommends.
4. The 5 HP saw comes without a power cord because they expect it to
be hard-wired by someone who knows what he's doing.
I sought advice from a couple of Electrical Engineers at work. They
assured me that:
1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only
feeding an electric motor that is hard-wired, AND there are no other
loads on the circuit.
2. It's not unsafe to run such a setup, because the motor's internal
overload switch will protect the motor itself. The only other
catastrophe would be a direct short, and the #10 wire will carry
enough amps to trip the 40 Amp breaker before the wire overheats.
So, you'll need to ditch the idea of twist-lock plugs for a 5 HP
motor. Plan to use either a 30 Amp slow trip or a 40 Amp breaker,
along with whatever wire size makes you happy (as long as it's #10 or
bigger).
The information presented above is hearsay. I am not a licensed
electrician, nor an electrical engineer. Check local codes before
installation. Void where prohibited. Your mileage may vary. Free
advice is worth what you pay for it.
DonkeyHody
Even an old blind hog finds an acorn every now and then.
Al Reid
> I sought advice from a couple of Electrical Engineers at work. They
> assured me that:
> 1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only
> feeding an electric motor that is hard-wired, AND there are no other
> loads on the circuit.
Are you sure about that one. All of the references I am aware of limit the overcurrent device to 30A on a #10 wire.
Anyone know for sure?
--
Al Reid
Lew Hodgett
Somebody wrote:
> > I sought advice from a couple of Electrical Engineers at work. They
> > assured me that:
> > 1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only
> > feeding an electric motor that is hard-wired, AND there are no other
> > loads on the circuit.
"Al Reid" asks:
> Are you sure about that one. All of the references I am aware of limit
the overcurrent device to 30A on a #10 wire.
>
> Anyone know for sure?
It is all a function of the insulation used.
The function of the c'bkr is to protect the insulation on the wire, not the
wire itself and most definitely not the motor.
Don't have my code book handy to verify; however, the use of a #10 conductor
on a 5HP/240V/1PH motor even if legal, would at best be considered a bad
installation.
The difference between the I^2*R heat losses of that #10 vs the lower heat
losses of a #8 would soon consume any material savings made during
installation.
After that, the power company would be laughing all the way to the bank.
Personally, I'd use #6 wire /w/ a 2P-40A c'bkr for a 5HP/240V/1PH motor.
SFWIW, have designed and sold the equipment to build more than a few
electrical distribution systems during my career.
HTH
--
Lew
S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: <http://home.earthlink.net/~lewhodgett> for Pictures
John Crea
John
Al Reid
"Lew Hodgett" <lewho...@earthlink.net> wrote in message
news:dH61c.18278$aT1....@newsread1.news.pas.earthlink.net...
>
> Somebody wrote:
> > > I sought advice from a couple of Electrical Engineers at work. They
> > > assured me that:
> > > 1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only
> > > feeding an electric motor that is hard-wired, AND there are no other
> > > loads on the circuit.
>
>
> "Al Reid" asks:
>
> > Are you sure about that one. All of the references I am aware of limit
> the overcurrent device to 30A on a #10 wire.
> >
> > Anyone know for sure?
>
> It is all a function of the insulation used.
No, it's not. Theampacity may be higher than 30 A, but code limits the
evercurrent device to 30A. See the tables in section 310. Section 310
states that #14 = 15A, #12=20A and #10=30A. I agree that the rated
ampacities may, however, be greater than those limits.
My question is, can anyone cite the section of the NEC that allows a larger
overprotection that is stated in section 310?
I have never ran across that exception and I have been an Electrical
Engineer for 24 years working in, among other areas, electrical construction
engineering. I am still looking at the code book.
DonkeyHody
didn't look it up myself; but I'm satisfied those guys knew what they
were talking about. I'm comfortable with it. But then, I'm known for
taking risks and living on the ragged edge. I used to hang off
cliffs on ropes and ride motorcycles really fast. I don't have a
guard on my tablesaw blade. And I used PVC pipe for airline. Don't
do what I do. Do what makes YOU comfortable.
DonkeyHody
If ignorance is bliss, why aren't more people happy?
Charlie Self
>> > 1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only
>> > feeding an electric motor that is hard-wired, AND there are no other
>> > loads on the circuit.
>>
>> Are you sure about that one. All of the references I am aware of limit the
>overcurrent device to 30A on a #10 wire.
>>
>
>See the disclaimer attached to my post. No, I'm not sure, because I
>didn't look it up myself; but I'm satisfied those guys knew what they
>were talking about. I'm comfortable with it. But then, I'm known for
>taking risks and living on the ragged edge. I used to hang off
>cliffs on ropes and ride motorcycles really fast. I don't have a
>guard on my tablesaw blade. And I used PVC pipe for airline. Don't
>do what I do. Do what makes YOU comfortable.
Comfort is not the point. If you run an undersized wire, you're risking
problems with the system that might become critical. If that system is part of
your house wiring, failure could be fatal. I very much doubt the NEC allows a
40 amp breaker on 10 gauge wire, though many odd things are possible. I don't
have even my old copies here to check, but your best source for a check will be
your local building inspection office. After all, they have to pass your
installation.
Another point: most electrical engineers of my acquaintance know very little
about NEC and building codes in general. They have no need to know, unless
their speciality is in some area of construction.
I've seen guys who can do a superb job of designing transport wiring systems
for production lines that I would let wire the last motorcycle I had...and that
was a bike without a headlight or taillight.
Charlie Self
"There is nothing wrong with America that cannot be cured with what is right in
America." William J. Clinton
Dave Balderstone
<charl...@aol.comnotforme> wrote:
> If you run an undersized wire, you're risking
> problems with the system that might become critical. If that system is part of
> your house wiring, failure could be fatal.
I'm anything but an electrician, but I learned early that the breaker
is there to protect the wire, 'cause when wires fail houses burn.
djb
--
Is it time to change my sig line yet?
Frank B. Mayberry
uses the full voltage motor starter (GPE). I have the saw connected to a 30
A breaker. The saw starts right up and has never tripped the breaker when
starting or in use. While waiting for delivery of the saw, I talked to
Delta technical support. They said I would need a 40 Ampere Slo-Blow Fuse.
But as noted in earlier email, there is no such beast at Home Depot. Once I
had the saw, I looked at rating of the motor starter. I was rated for 18-25
A devices. And I did verify that the motor was rate at 5 HP.
Greg O
> single phase circuit?
>
> My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
> at 19.8 or 20 A.
I have a true 5 HP motor that is rated at 25 amps @ 230 volt. I did a quick
check through a Grainger catalog and found only one 5 HP motor under 20 amps
@ 230 volt. Most were 22-23 amps.
Greg
Greg O
"xrongor" <nos...@spammyspam.com> wrote in message
news:PU31c.100625$Xp.434279@attbi_s54...
their 5 HP Unisaw. It may run on a 30 amp circuit, but I would not bet on
it!
Greg
DonkeyHody
conductor
> on a 5HP/240V/1PH motor even if legal, would at best be considered a bad
> installation.
>
> The difference between the I^2*R heat losses of that #10 vs the lower heat
> losses of a #8 would soon consume any material savings made during
> installation.
>
> After that, the power company would be laughing all the way to the bank.
>
> Personally, I'd use #6 wire /w/ a 2P-40A c'bkr for a 5HP/240V/1PH motor.
>
Lew,
The plate on my 5 HP motor indicates that it draws 20 Amps at full
load. If it was a motor that operated for extended periods at full
load, or on a long wire, I'd agree with your thinking on the I squared
R loss. But my table saw will rarely load the 5 HP motor, and the run
of #10 wire is less than 20 feet. Since I have a pretty good feel for
the tradeoffs involved, I elected to use the smaller wire, not because
of cost, but because it's easier to pull. Others may certainly choose
another path.
Nova
> What amperage breaker does one need for a 5HP table saw on a 240V
> single phase circuit?
A web search shows that a typical 5 HP 230V motor draws about 24.5 amps.
You could probably get away with a 30 amp breaker.
--
Jack Novak
Buffalo, NY - USA
(Remove "SPAM" from email address to reply)
xrongor
"Greg O" <goo...@cableone.net> wrote in message
news:104ahgk...@corp.supernews.com...
well, i cant vouch for that particular motor, but imho a 40 amp slow blow
fuse on a 5hp table saw motor is kind of like saying, ok, once the thing
catches on fire and explodes and the blade has spun out of control and the
power wires are actually touching and welding together, we better stop
supplying current to it <g>
i would bet that saw has internal protection that blows long before that
fuse does.
randy
DonkeyHody
> > > Are you sure about that one. All of the references I am aware of limit
> the overcurrent device to 30A on a #10 wire.
> > >
> > > Anyone know for sure?
> >
> > It is all a function of the insulation used.
>
> No, it's not. Theampacity may be higher than 30 A, but code limits the
> evercurrent device to 30A. See the tables in section 310. Section 310
> states that #14 = 15A, #12=20A and #10=30A. I agree that the rated
> ampacities may, however, be greater than those limits.
>
> My question is, can anyone cite the section of the NEC that allows a larger
> overprotection that is stated in section 310?
>
> I have never ran across that exception and I have been an Electrical
> Engineer for 24 years working in, among other areas, electrical construction
> engineering. I am still looking at the code book.
Well shucks. Y'all made me get out of my chair and go look it up.
Check out Section 430, Part D - Motor Banch-Circuit Short-Circuit and
Ground-Fault Protection.
Paragraph 430-52
(b) All Motors. "The motor branch-circuit and ground-fault protective
device shall be capable of carrying the starting current of the
motor."
(c) Rating or Setting.
(1) "A protective device that has a rating or setting not
exceeding the value calculated according to the values given in Table
430-152 shall be used."
EXCEPTION NO. 1 (Paraphrased)
Exception 1 allows you to go to the next size up if you are between
sizes.
EXCEPTION NO. 2 (Quote)
"Where the rating specified in Table 430-152 as modified by Exception
1, is not sufficient for the starting current of the motor:"
"(a) The rating of a nontime-delay fuse or a time-delay Class CC fuse
shall be permitted to be increased, but shall in no case exceed 400%
of the full-load current.
"(b) The rating of a time-delay (dual element)fuse shall be permitted
to be increased but shall in no case exceed 225% of the full-load
current.
"(c) The rating of an inverse-time circuit breaker shall be permitted
to be increased but shall in no case exceed 400 percent for full-load
currents of 100 amperes or less or 300 percent for full-load currents
greater than 100 amperes."
The commentary in the handbook states: (Quote)
"It should be noted that it is not necessary to size the
branch-circuit conductors to the percentages (150 to 300) permitted
for the branch-circuit short circuit and ground-fault protective
devices." . . .
"During a short-circuit or phase-to-ground condition, the extremely
high current causes the protective fuses or circuit breakers to open
the circuit. Excess current flow caused by an overload condition
passes through the overload protective device at the motor controller,
thereby causing the device to open . . ."
DonkeyHody
Even a dumb old country boy gets it right once in a while.
Al Reid
"DonkeyHody" <mrm...@bellsouth.net> wrote in message news:f71a46f.04030...@posting.google.com...
Well, now you caught me without my code book but that seems to imply that it is a dedicated, hardwired motor, being supplied by a
motor controller that incorporates both instantaneous (like an MCP) and overload protection AND that it pertains to the wiring
between the motor controller and the motor.
I could be wrong, but I doubt you could convince a residential electrical inspector to pass an installation with #10 wire being
protected by a 40A breaker.
--
Al Reid
DonkeyHody
> motor controller that incorporates both instantaneous (like an MCP) and overload protection AND that it pertains to the wiring
> between the motor controller and the motor.
>
> I could be wrong, but I doubt you could convince a residential electrical inspector to pass an installation with #10 wire being
> protected by a 40A breaker.
Al,
That section is specifically NOT about the factory installed wire
between the factory installed motor controller and the motor. I went
to the trouble to look up the reference for you and even took the time
to quote chaper and verse. I'm not going to type the whole chapter
for you. If you want to argue context, at least go to the trouble to
read the chapter so you'll know what your're talking about.
Now I remember why I mostly just lurk here.
DonkeyHody
We should be careful to get out of an experience only the wisdom that
is in it - and stop there; lest we be like the cat that sits down on a
hot stove-lid. She will never sit down on a hot stove-lid again---and
that is well; but also she will never sit down on a cold one anymore.
Mark Twain
Al Reid
other NFPA publications, interpretation is everything. And I have found that there is no universal interpretation of all aspects of
the code.
I was merely expressing my doubts as to the wisdom of violating the 30A on a #10 wire rule in section 310. Most inspectors get hung
up un that one and usually don't budge. Perhaps, my designs are too conservative, however, I have never had one rejected by a PE
review before installation or by an inspector during/after installation.
Also, I started my comment by stating that I did not have my code book in front of me.
--
Al Reid
"It ain't what you don't know that gets you into trouble. It's what you know
for sure that just ain't so." --- Mark Twain
"DonkeyHody" <mrm...@bellsouth.net> wrote in message news:f71a46f.04030...@posting.google.com...
John848
that is fact:
1) a 30A breaker connected to and protecting #10 wire will give (by your NEC
code) 30 x 80% = 24 amps of continues current
2) You must get the FLA off the nameplate of the motor. Motors are ALL built
with different efficiciencies and housing structures - therefore the only
accurate current draw is that which was tested at the factory on their
design model. Motors, at startup typically drew about 6 x FLA at startup,
but today motors can easily hit 12 x FLA at startup because of lighter
housing, thinner internal wiring etc - in otherwords the motors have been
cheapened up.
3) If you motor nameplate, and it read 22 amps FLA - then you'd have 2 amps
left over and you are good to go. If the motor nameplate on the other hand
said 26 amps FLA you need to revisit the breaker/wire combination. It will
still work but likely it will trip, maybe not for hours (only the breaker
trip curve could answer that question) - the code can't stop you from
attaching a 35 amp load to a 30 amp breaker combo - it just won't stay live
very long.
The code derates by 20% to give you the easily calculated values that allow
you to enjoy continuous power on a circuit. The derating comes from the fact
that circuit breakers are thermal devices (BTW so are fuses) in that they
have as part of their protection system, a bimetallic element that works on
heat principles. Once you put a breaker in an enclosure, heat can build up
and trip the breaker. Enclosure are build with minimum cubic inches of
"cooling space" and in conjunction with the 20% derating rule offer you a
product that can supply current on a continuous basis. Put that product in
the boiler room of a ship, where the ambient temp is high and the breaker
will trip sooner. Same applies to a "hot" woodworking shop - this is why the
20% derating. If you want to spend the money, you can buy 100% rated
breakers which are certified as such and you can draw 100% of the current.
These breakers start at 400Amp and go to 6000Amp.
Anyway, I digress - don't try to re-engineer some that has been designed for
you and enforced by the NEC:
12A circuit = 15A breaker + #14 wire
16A circuit = 20A breaker + #12 wire
24A circuit = 30A breaker + #10 wire
32A circuit = 40A breaker + #8 wire
Once you pick the right size "service" - make sure you have the proper
device to protect your motor - the upstream breaker has nothing to do with
that - but that a lot more typing and I'm not good at that.
Regards
John
<_firstname_@lr_dot_los-gatos_dot_ca.us> wrote in message
news:1078204646.761648@smirk...
> What amperage breaker does one need for a 5HP table saw on a 240V
> single phase circuit?
>
> My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
Ripsaw
I also have a 24" 5hp delta x5 planner that I also connect through my dryer outlet and also have never had an issue.
--
For full context, visit https://www.homeownershub.com/woodworking/amperage-for-5hp-table-saw-218022-.htm
k...@notreal.com
<9dd8514b1cf2e999...@example.com> wrote:
>I have the exact same Unisaw. I connect my saw to my 30 amp dryer connector via a 30 amp twist lock converter and I have never experienced any issue.
>I also have a 24" 5hp delta x5 planner that I also connect through my dryer outlet and also have never had an issue.
Do you feel better after trolling?
Bob Davis
> In article <1078204646.761648@smirk>,
> > What amperage breaker does one need for a 5HP table saw on a 240V
> > single phase circuit?
> >
> > My logic: Typically, single phase 5HP 240V (or 230V) motors are rated
> > at 19.8 or 20 A. Given that a circuit is supposed to be loaded 80%
> > (look it up in the NEC sometime), this means a 25 A breaker would be
> > borderline sufficient, and that a 30A breaker should be generous.
> > With a slow breaker (not a fuse), there should be no problems with the
> > startup surge of the motor tripping the breaker.
> 1 horsepower = 745.7 watts
> When you have two values for the variables in the equation you can
> figure out the third.
> P 3728.5 (watts 1 HP x 5)
> Therefore I = --- or I = ------
> E 230 volts
>
> 230 volts = 16.21 amps
> 240 volts = 15.53 amps
>
> Using the "80% rule", a 20 amp breaker should not exceed a continuous
> load above 16 amps. It seems to me, that with the proper type of "slow"
> breaker, you should be OK with a 20 amp breaker.
>
> Joe
>
> PS
> I am not an electrician or an electrical engineer. In fact, I have
> never even played one on TV.
Ohm's law is for DC. It does not apply to AC.
Joe Gwinn
wires and components).
Motors and inductors and capacitors are a different kettle of fish. An
ordinary ohmmeter measures DC resistance, which for an inductor is the
resistance of the copper wire winding, but AC current and voltage are
connected by AC reactance., which will be far larger than the DC
resistance at the usual operating frequency of that inductor.
Joe Gwinn
k...@notreal.com
wrote:
"work". If the motor were 100% efficient, it would look like a pure
resistor. They aren't, of course, but the more efficient they are,
the closer to a resistor they appear.
80% efficiency for fully loaded small motors isn't too far off but
probably a little high. They get worse when lightly loaded but for
figuring circuit size, the fully loaded numbers are used. A safety
factor of another 20% is a good idea (if not code). Motors can be
overloaded. ;-)
Clare Snyder
most "twinned" breakers are (at leat all Square D QO breakers are ). I
ended up using a twinned breaker for my central vac (120 volt 12 amp)
because it quite occaisionally tripped the single standard 15 amp
breaker on start-up and a single high magnetic breaker was not
readilly available locally and cost more to order in than my local
supplier charged for the twinned breaker. Hasn't tripped since (about
3 years) - and it tripped on 3 different single breakers - - -
Clare Snyder
wheather inductive or capacitive. With resistive loads it's the same
as with DC - read the resistance on your ohm-meter. With "inductive
loads" the reactance will always be higher than the resistance (draws
less current)
Bob Davis
Bob Davis
Scott Lurndal
>
>What is a real world example of purely resistive load supplied by AC?
k...@notreal.com
temperature dependent).
J. Clarke
Markem618
for when the temp is to cold.
Bob Davis
Clare Snyder
incandescent lamp. So that covers an electric water heater - and
heat-chaser cables - and electric in-floor heating cables - even an
electric blanket. (Yes there IS a small inductive element if it has a
coiled heat element - but virtually none if carbon fiber)