Electrical feed for sub panel for Guest house
David
running into questions, especially with feeding the guest house.
Guest house is:
1200 sq ft with interior, and exteriour lights, cieling fans. kitchen,
bath, dishwasher, disposal, AC, heater, and about 20 outlets. Only
thing that is 240 volt is AC unit.
I'm saying we need at least 100 amps from the main (which is 200
amps).
Previous "Electrician" ran (before guest house was built) 3 - 6AWG
wires (2 hots, 1 neutral) to 2 - 50 amp breakers in main (which he
left HOT with black tape on the ends! Idiot!)
One guy says that 2 - 50 amp breakers is 100amp (on 2 phases) and we
should be ok
I say that we need 2 AWG wire (based on asking around) for 100 amp
240 volt service.
Besides "Call a real electrician" , can anyone offer advice on feeding
the sub panel.
Yes, it will be inspected.
David
Mark or Sue
news:utgilvs0j3e2g2no8...@4ax.com...
> I'm helping with the electrical in a newly built guest house. We are
> running into questions, especially with feeding the guest house.
>
> Guest house is:
> 1200 sq ft with interior, and exteriour lights, cieling fans. kitchen,
> bath, dishwasher, disposal, AC, heater, and about 20 outlets. Only
> thing that is 240 volt is AC unit.
>
> I'm saying we need at least 100 amps from the main (which is 200
> amps).
>
> Previous "Electrician" ran (before guest house was built) 3 - 6AWG
> wires (2 hots, 1 neutral) to 2 - 50 amp breakers in main (which he
> left HOT with black tape on the ends! Idiot!)
>
> One guy says that 2 - 50 amp breakers is 100amp (on 2 phases) and we
> should be ok
>
> I say that we need 2 AWG wire (based on asking around) for 100 amp
> 240 volt service.
A dwelling requires a minimum of 100A service per the NEC, so you're on the
right path. Two 50A breakers does not make 100A service! You are required to
have 100A @ 240V. Per table 310.15(b)(6), you can use #4 copper for a 100A
service if you use one of the specified wire types in the list (RHW, THWN,
SE, USE, some others). You would think that you would have to use #3 or #2
based on the normal ampacity tables, but you don't. Connect this wire to a
100A doubple pole breaker in your panel and you're set. As long as there are
no conductive metal paths between the main house and guest house (e.g. metal
gas or water pipes, coax cables, etc) then you can run 3 wires and bond the
neutral and ground at the guest house. If you have (or later want) metal
paths, you need a 4th grounding wire and you keep neutral and ground busses
separate at the guest house. The ground wire would have to be #8 copper for
a 100A service.
Don't forget that separate buildings require their own main disconnect and
grounding system at that building.
--
Mark
Kent, WA
John Grabowski
feeding the guesthouse is NOT a service. It is a subfeed. Unless the
guesthouse has its own meter and service drop from the utility company the
circuit breaker panel would be considered a subpanel. Therefore #4 wire
would be considered too small for a 100 feed to the guesthouse.
Depending on the distance from the main service and ratings of the terminals
(60 degrees or 75 degrees Celsius) a #3, #2, or #1 conductor size is
appropriate for 100 amps. A safe bet is to use the 60 degree column in
table 310.16.
4 wires must be installed. 2 current carrying conductors, 1 grounded
conductor (Neutral) and 1 grounding conductor (Bare or green). The neutral
and grounding conductor must be isolated from each other in the subpanel.
Although a # 8 grounding conductor is acceptable, I suggest a #6.
David, to get an idea of what is the proper size feed you can consult the
code book under Article 220 and Annex D. 100 amps is minimal, but you
should consider the actual load in case it should be larger. Although the
circuit breaker is 100 amps it is only rated at 80% continuous load. This
means that if you have a continuous load of over 80 amps the circuit breaker
will trip eventually. The code book defines continuous load as "A load
where the maximum current is expected to continue for 3 hours or more".
This is especially important if the guesthouse will be totally electric.
You didn't mention if the stove, oven, heating system, and water heater are
going to be gas or electric.
A red flag has gone up in my mind concerning the feed of the guest house.
What effect will this additional load have on the 200 amp service at the
main house? A new load calculation should be done for the main service to
determine if it is adequate for the existing load as well as the new guest
house load. The owner may find his main breaker tripping frequently as a
result of the new demands to his existing 200 amp service.
John Grabowski
http://www.mrelectrician.tv
"Mark or Sue" <suet...@comcast.net> wrote in message
news:wge6b.371122$o%2.167724@sccrnsc02...
David
wrote:
Hey thanks Mark,
I was pretty sure I was right. I needed something black and white to
prove my point. I had considered the metal path between buildings, gas
is plastic, I think water is too, but I didn't think about the coax
cable. I think we should run a groung to be safe.
We have a groung rod at guest house. Does the panel at the guest house
have to be able to disconect main power from that panel? I had thought
that a sub panel could have a disconnect at either main or sub panel.
Thanks a lot,
David
David
I haven't done a formal calculation based on the NEC code about the
continous load. I had come up with about 75 amps, as a guestimation.
Stove, oven, water heater, and heating are all gas.
I am supposed to pick up a circuit panel this weekend, so I want to
get the right one. I may just get 125 amp. I don't think the cost
difference in that much.
It appears the the nuetral needs to be isolated from ground.
Everything else, I guess I'll ask the guy from Home Depot.
I'd rather error on the side of caution. I am pretty sure that the
owner will pay for the wire if we tell him that this is what is needed
(after a little sticker shock), but I'd rather not have to pull huge
cable 150' through an undergroung pipe if I didn't need one that big.
I don't know if the load requirement for the main house were every
calculated. The house was remodeled and the service was bumbed up to
200 amps (from 125 I think). The "electrician" running the job was a
retired Marine, in his 70s I think, and he took a lot of shortcuts.
There are a lot of stories about that guy.
(Like when he branched power from a lighting cicuit to workstations
at an office, and fried everything because it was not a 120 volt
circuit)
Anyway, we were a little worried about the 200 Amps. I'm sure the 3
AC units draw a lot. I think the kitchen equip is gas. But we thought
that since 200 amps was the max for a home, there isn't much we can do
about it anyway. Hopefully he only has guests in his guest house
during the cooler months.
Thanks a lot for your help
David
Mark or Sue
news:LBl6b.45055$Ay2.10...@news4.srv.hcvlny.cv.net...
> Mark, I slightly disagree with your response to this question. The line
> feeding the guesthouse is NOT a service. It is a subfeed. Unless the
> guesthouse has its own meter and service drop from the utility company the
> circuit breaker panel would be considered a subpanel. Therefore #4 wire
> would be considered too small for a 100 feed to the guesthouse.
I still can't agree. This area of the code seems to cause more contention
than anything else. The revision for 2002 should make it clear in this case.
Some inspectors I've talked to believe that you can use this table for ALL
residential services and feeders. There should be absolutely no question
that this applies to a detached dwelling, whether service conductor or
feeder. Here is the code:
310.15(B)(6) 120/240-Volt, 3-Wire, Single-phase Dwelling Services and
Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6),
shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance
conductors, service lateral conductors, and feeder conductors that serve as
the main power feeder to a dwelling unit and are installed in raceway or
cable with or without an equipment grounding conductor. For application of
this section, the main power feeder shall be the feeder(s) between the main
disconnect and the lighting and appliance branch-circuit panelboard(s). The
feeder conductors to a dwelling unit shall not be required to be larger than
their service-entrance conductors. The grounded conductor shall be permitted
to be smaller than the ungrounded conductors, provided the requirements of
215.2, 220.22, and 230.42 are met.
The referenced table allows #4 copper or #2 aluminum for 100A service.
> Depending on the distance from the main service and ratings of the
terminals
> (60 degrees or 75 degrees Celsius) a #3, #2, or #1 conductor size is
> appropriate for 100 amps. A safe bet is to use the 60 degree column in
> table 310.16.
Just about any panel and breaker you buy today will be rated for 75 degrees
at the screws. However, for long runs I do prefer to use the 60 degree
column to help with voltage drop issues.
> 4 wires must be installed. 2 current carrying conductors, 1 grounded
> conductor (Neutral) and 1 grounding conductor (Bare or green). The
neutral
> and grounding conductor must be isolated from each other in the subpanel.
> Although a # 8 grounding conductor is acceptable, I suggest a #6.
It is not required by the NEC that all 4 wires be installed unless there are
metallic conductive paths between the detached building and its not an
agricutural building. This is one of the few exceptions to keeping neutral
and ground separate after the main disconnect.
> David, to get an idea of what is the proper size feed you can consult the
> code book under Article 220 and Annex D. 100 amps is minimal, but you
> should consider the actual load in case it should be larger. Although the
> circuit breaker is 100 amps it is only rated at 80% continuous load. This
> means that if you have a continuous load of over 80 amps the circuit
breaker
> will trip eventually. The code book defines continuous load as "A load
> where the maximum current is expected to continue for 3 hours or more".
> This is especially important if the guesthouse will be totally electric.
> You didn't mention if the stove, oven, heating system, and water heater
are
> going to be gas or electric.
For residential service calculations, there are few circuits where the
continuous issue comes up. Most load values are already set for you based on
square footage, the type of circuit, or demand tables in the code. You'll
only get it if the nameplate has already accounted for it (such as HVAC MCA)
or if you have a fixed appliance that could run continuously.
> A red flag has gone up in my mind concerning the feed of the guest house.
> What effect will this additional load have on the 200 amp service at the
> main house? A new load calculation should be done for the main service to
> determine if it is adequate for the existing load as well as the new guest
> house load. The owner may find his main breaker tripping frequently as a
> result of the new demands to his existing 200 amp service.
You have a very good point here. He may need to upgrade to a 400A service
with a double lugged class 320 meter base. With these, you put two 200A
panels side by side and is what I've just installed in my own house.
--
Mark
Kent, WA
Mark or Sue
> I was pretty sure I was right. I needed something black and white to
> prove my point. I had considered the metal path between buildings, gas
> is plastic, I think water is too, but I didn't think about the coax
> cable. I think we should run a groung to be safe.
>
> We have a groung rod at guest house. Does the panel at the guest house
> have to be able to disconect main power from that panel? I had thought
> that a sub panel could have a disconnect at either main or sub panel.
If the subpanel was in the same building, then it doesn't require a main.
But since this supplies a separate building, you must have a local
disconnect and you have three choices. Either use a panel that has no more
than 6 breakers, use a panel that has a main breaker, or install a separate
100A fused disconnect. Some areas require that the disconnect be on the
outside of a dwelling. If this applies to you, you need either a raintight
panel or an outside disconnect. Hopefully, you can just go buy a 20 slot
panel with an integral 100A main breaker.
Because you have a 200A service, you can't run an unfused 100A tap to the
guest house. You could either tap your 200A house service and run a parallel
200A service to the guest house, or run a feeder that is protected at the
main panel to the guest house. You must have a disconnect at the guest house
in case it catches fire -- the firemen don't want to be running around
trying to find what panel feeds this building. If you're going to tap the
200A main service to feed both buildings, it becomes important to do the
service calculations because the meter base and service conductors won't be
protected from any combined overloads.
Finally, you said grounding rod. If you can't prove the grounding rod is 25
ohms or less to ground, then you need a second one at least 6' away and
bonded to the first.
--
Mark
Kent, WA
Gary Tait
You can get more than 200A service. If it were me, and rather doable,
I'd connect this as a sub building off the meter, and have a main
there for the main house and another for the guest house.
John Grabowski
interpreted in several ways. I am in New Jersey and the inspectors are
fussy about strict adherence to the code. Article 230.2 (A) through (D) is
what they use to quench the use of 310.15(B)(6) for secondary buildings. I
could go forth and back on this issue with an inspector here for hours. The
fact of the matter is that safety is paramount and reliance is essential.
When those two factors are considered the choice becomes clear.
Since I am commenting on a job that I have not seen, do not know the
geographical region of, and do not know the abilities of the person asking
the questions, my response is to go for the safest and most reliable
installation.
Judging by your responses you appear to be a professional in the trade. You
can make an honest assessment of each job that you look at and work on as
can I. Based on our experience and training and responsibility we are
qualified to make judgment calls. People who ask questions about home
projects they intend to tackle with limited resources and experience are not
always qualified to make an accurate assessment of their needs.
Additionally, I have found that many do-it-yourselfers don't understand the
ramifications that are a result of doing things the wrong way.
In David's reply to me he mentioned that the run was 150' of underground
pipe. He did not mention this in his initial posting. Is it metal pipe or
PVC? What size pipe? If it is metal then the neutral must be kept isolated
from the ground and the conduit could serve as the grounding conductor.
Does David know that the bonding jumper must not be installed (Or should be
removed) from the circuit breaker panel if there is metal conduit connecting
the panel? Does he know that a person could die as a result of not removing
that jumper or bonding screw? I have no way of knowing. I also have no way
of knowing if the pipe was installed using the correct fittings and straps
or if it is approved for electrical use.
I consider your point to be quite valid, however in this particular case I
stand by my original response. In this instance obtaining the job based on
the lowest price was not a factor. Cost of materials and labor are a
consideration for a contractor bidding on a job. David is already working
on the project and although he appears responsible I question why the owner
did not hire a professional to do the electrical work.
John G.
"Mark or Sue" <suet...@comcast.net> wrote in message
news:ySu6b.182706$2x.5...@rwcrnsc52.ops.asp.att.net...
Mark or Sue
I don't know how those NJ inspectors can use 230.2 against this -- that
references not having multiple services, but the wording of 310.15B6 says
services AND FEEDERS. But in the end, the rules are what the inspector says
they are and its best to just learn their interpretation and deal with it
unless the cost is prohibitive.
I'm not an elecrtician, just a homeowner who can't find decent electrical
help. In my work I do deal with engineering requirements and specifications
at times. Many people (especially managers) think you can write a complete
requirements document that would have no questions or inconsistencies. The
NEC is an obvious example of how this is impossible, and they roll the book
every 3 years on a well understood topic. This is not rocket science, but so
many things are nebulous. There is also the NEC committee who fails to use
precise words to mean what they say, but there are inspectors who are
reading every single word and beating people over the head with them.
If I was doing this feeder, I would be using #2 copper and 4 wires. In the
past when I have posted answers like this, people would chime in and point
out that this was not the legal minimum. So to avoid that I try to post the
legal minimum. I should have done a better job and also indicated how I
would do it in additional to the legal minimum.
--
Mark
Kent, WA
TURTLE
this is Turtle.
John G. , I just smell smoke here afterwards after reading all the post. I
do commercial electric work and don't know all the residentiual parts of the
NEC but I just use the Ugly Book Commercial and Seco manual and it will not
allow for what Mark says and you get over the limits with what you say. I
will say there is just too many gray area in this business to concider. When
i do a job it is Black and white and no gray at all. the ugly allows me 90
amp service with #4 RHH wire at best and I have to have #3 RHW / THW / THNN
/ SE-U to get 100 amp service. Now I don't want to go look up 150 feet away
of #3 in conduit under ground. I don't like argueing with the State Fire
Marshal for he can shut a $50,000.00 job down till we finshing argueing. I
stay in the Black and White Zone and don't let gray color get near for Gray
can get you into a arguement with a $500.00 a day fine to keep running or
cut the cooling system off to a school building. Damned if you do and Damned
if you don't. I don't even like the color gray for just this reason and if
you walk the line , you will fall off it.
TURTLE
Dave
I was hired to do the electrical because I had some experience running
electrical, and the owner, and contractor were trying to save money.
The feeder issue came up, and I felt like I was a bit over my head.
There had been talk about bring in a professional if we (me and a
partner) did not feel 100% confident in what we were doing. I'd rather
not do it at all if it isn't going to be done right. I try to error on
the side of caution.
The pipe has not been put in place yet. We were going to run an
appropriate sized PVC pipe based on the size wire we run (per NEC). I
definitely want to follow all the rules. I've been reading quite a bit
lately, including these post.
The wires in the house are done. I'm pretty confident that they are
done correctly. There was a feed ran for the house (by previous owner
and electrician), but it was only 6 AWG, and we were all sure that it
was undersized.
When I talk to the owner and other people on the project, I will
propose a 100 amp panel, with neutral isolated from the ground, #2 AWG
for 2 hots and 1 neutral, and #6 AWG for ground from main panel. We
will specify the size of pipe needed, after I check the NEC code book
( I have a 2002 copy on the PC). We will also tie the ground into the
grounding rod at the quest house.
I am hoping that the 200amps at the main don't become a problem. We
had actually been talking about it earlier. We will let the owner know
about the potential problem and decide on a future coarse of action.
The more I work with electrical, the more I appreciate the people
that do this all the time. I greatly appreciate all your responses. I
will keep researching the requirements until we are done with the
project.
Thanks a lot,
David (From California)
volts500
"Dave" <pix...@earthlink.net> wrote in message
news:ivpnlvc4oghh76ppt...@4ax.com...
> Hi John,
>
> I was hired to do the electrical because I had some experience running
> electrical, and the owner, and contractor were trying to save money.
> The feeder issue came up, and I felt like I was a bit over my head.
> There had been talk about bring in a professional if we (me and a
> partner) did not feel 100% confident in what we were doing. I'd rather
> not do it at all if it isn't going to be done right. I try to error on
> the side of caution.
If that's really true, then back out of the deal.......or at least check
your local laws. While many local areas do allow home_owners_ to do their
own electrical work if the homeowner is also going to live in the house
_and_ perform the work, the vast majority of localities do _not_ allow
unlicensed people to do electrical work on other people's property........in
fact, it may be a criminal offense. In my area it's a second degree
misdemeanor..........with a separate offense for _each_ day of work
performed.
Should you decide to forge ahead, at least do a service load demand
calculation for the main house service.......one should never add that much
load to an existing service without making the calculation. If it is going
to be inspected, as you said, it won't pass if the main service is
overloaded (by calculation)........don't think that the inspector won't ask
to see the calculation. Also, most inspectors will want to see the conduit
in the trench _before_ it gets backfilled. Same goes for any wiring done
inside the guest house.......if the drywall is already up, somebody will be
tearing that out too.
Not trying to be negative, just trying to save both you and the homeowner
some major headaches.
Jeff Cochran
>I was hired to do the electrical because I had some experience running
>electrical, and the owner, and contractor were trying to save money.
Locally, the homeowner would be fined, the contractor would have his
licensed pulled and you wouldn't have a problem (since I assume by the
above you're not a licensed electrician). In any case, the permit
would be voided, and the homeowner would face the strictest possible
scrutiny. Unfortunately, locally the remedy for unlicensed workmen is
to pull their license, which kind of makes no sense, although the GC
also gets his suspended for using an unlicensed sub.
> When I talk to the owner and other people on the project, I will
>propose a 100 amp panel, with neutral isolated from the ground, #2 AWG
>for 2 hots and 1 neutral, and #6 AWG for ground from main panel. We
>will specify the size of pipe needed, after I check the NEC code book
>( I have a 2002 copy on the PC). We will also tie the ground into the
>grounding rod at the quest house.
Locally you also have the option of a second service entrance for
about the same costs when you realize you don't need to run a line.
Other jurisdictions allow a single entrance per residential property,
so check your options with your local building department.
Jeff
Dave
voltage wiring (which I've been doing for some time, I often preach
the dangers of using someone inexperienced.
I diidn't make the deal, and the others involved still plan
on moving forward. I was originally brought on as just labor, but I
was asked to research the feed issue because they know that I would do
the research, that I am detail oriented, and that I have a decent
understanding of technical issues.
I would rather just let someone else do the feed, rather then take a
chance on missing something, but if the others involved are still
wanting to move forward, then I will try to cover all the details so
that it is done the best we can.
Locally the contractor laws are that an independent can do up to $500
for of work per project without a contractors license (guess they
figure you cant do to much damage at that amount). But like I said, my
understanding is that I was originally brought on as labor, so I would
think that the work is covered by the GC's license.
Drywal is not up yet. Trench is not dug yet. No pipe yet. Panel should
be in place this weekend.
Thanks for all your input.
David