14/3 wiring for 2 circuits????
Jim Franklin
I just replaced/upgraded the service panel and meter in my house, and
when reconnectiong the existing original circuits I found out they used
14/3 wire to do two circuits- black for the outlets, red for the
overhead/permanent lights, and they share a common neutral. Was this
standard practice 50 years ago? It'd seem to me that if they needed 2
wires to handle the load of the house, they should use two neutrals as
well, no?
jim
John S. Freitag
Well, no not really.
In theory the neutral only darries the difference in current between the 2
hot wires so each one never sees more than the maximum load of a single
circuit. That is the worst case where circuit 1 has full load and circuit 2
has zero load.
I say "in theory" because the non linear switching power supplies in
computers has kchanged things a bit. In a home, unless you have a whole
raft of computers working the theory works out OK.
J.Freitag
anon...@my.net
Jim Franklin wrote:
>
> I just replaced/upgraded the service panel and meter in my house, and
> when reconnectiong the existing original circuits I found out they used
> 14/3 wire to do two circuits- black for the outlets, red for the
> overhead/permanent lights, and they share a common neutral. Was this
> standard practice 50 years ago? It'd seem to me that if they needed 2
> wires to handle the load of the house, they should use two neutrals as
> well, no?
>
> jim
second hot leg was "B" amps, the current in the neutral would be
A minus B.
However, and this is very important: the two hot legs MUST NOT be
connected to the same breaker panel buss bar. Said another way, there
MUST be 220 volts measured between the breaker feeding the first hot leg
and the breaker feeding the second hot leg. If you somehow get both legs
connected to the same panel buss, and the voltage measured between the
two hot legs is zero or nearly so, the neutral will, instead of carrying
A-B, will carry A+B, which will be a real problem.
A 3-wire circuit was and still is legal. Just be careful when you
rearrange breakers in the panel, and be especially sure to make up all
neutral connections very well.
Chris Lewis
In article <3516F7...@cisco.com>, Jim Franklin <jam...@cisco.com> wrote:
>I just replaced/upgraded the service panel and meter in my house, and
>when reconnectiong the existing original circuits I found out they used
>14/3 wire to do two circuits- black for the outlets, red for the
>overhead/permanent lights, and they share a common neutral. Was this
>standard practice 50 years ago?
It still is quite standard. Mandatory in some cases. Besides, your
stove and electric dryer are wired the same way...
>It'd seem to me that if they needed 2
>wires to handle the load of the house, they should use two neutrals as
>well, no?
As the two hots are 180 degrees out of phase, the neutral only carries
the difference current. Just make sure that the two hots are from opposite
sides of the main. And preferably on a dual breaker.
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In article <3516F7...@cisco.com> Jim Franklin <jam...@cisco.com> writes:
>From: Jim Franklin <jam...@cisco.com>
>Subject: 14/3 wiring for 2 circuits????
>Date: Mon, 23 Mar 1998 16:01:49 -0800
>I just replaced/upgraded the service panel and meter in my house, and
>when reconnectiong the existing original circuits I found out they used
>14/3 wire to do two circuits- black for the outlets, red for the
>overhead/permanent lights, and they share a common neutral. Was this
>standard practice 50 years ago? It'd seem to me that if they needed 2
>wires to handle the load of the house, they should use two neutrals as
>well, no?
>jim
I have a pair of circuits fed from one 14/3 cable like that, too, bringing
electricity to the upstairs level of my 30 year old home. My
understanding is that it's fine if 240 V is coming on the red and black, each
of which then carries 120 V with respect to the white neutral. In both of our
homes, it sounds like the circuits then split into normal 120 V lines.
In other words, the red and black are 180 degrees out of phase, so the neutral
current will be the DIFFERENCE of their two currents, not the sum. When both
red and black are carrying 15 amps, the neutral current is zero. This
copper-saving strategy has been used to supply a dishwasher and disposer in
one published diagram I recall seeing awhile ago.
But it becomes hazardous when amateurs (like our previous homes'
owners, perhaps) add half height circuit breakers to expand capacity but
unwittingly move the red or black line's breaker (or both) so that they're
on the same 'stab' in the breaker panel, on the same side of the house supply.
Now there is no voltage between red and black, and there is 120 V between
each of them and neutral white, and the neutral current is the SUM of the
currents in red and black. Picture some cold day when a space heater happens
to be plugged into each of the two lines, maybe drawing 12 amps apiece,
leading to 24 amps on the neutral which is safe for only up to 15 amps. No
breaker will be tripped, because the breakers are only on the hot side (red
and black). But the white wire inside the wall will heat up excessively and
start a fire.
I believe that the simple solution is to move one of the breakers to the
opposite stab, but then you'll want to make a sign for the junction box where
the two lines divide to warn about 240 V being present between red and black
and 'WARNING: MULTIPLE FEEDS' and also specify which breakers to
shut off for work in this box.
I'm not an electrician! I'll also welcome informed commentary.
(Please delete spam-suppressing '_AT_' from e-mail address to reply directly.)
Dennis Webber
Jim Franklin wrote:
>
> I just replaced/upgraded the service panel and meter in my house, and
> when reconnectiong the existing original circuits I found out they used
> 14/3 wire to do two circuits- black for the outlets, red for the
> overhead/permanent lights, and they share a common neutral. Was this
> standard practice 50 years ago? It'd seem to me that if they needed 2
> wires to handle the load of the house, they should use two neutrals as
> well, no?
It's a permitted method. Only
requirements are that it comes from a
single panel board and it uses a common
trip breaker if serving more than one
device on the same yoke (I.E. a
receptacle).
See Articles 210-4, 240-20(b)(1) &
300-13(b).
As to the two neutrals, remember that in
a multiwire branch circuit, the neutral
only carries the unbalanced load. (In a
single wire branch circuit, there is a
grounded conductor but no neutral.)