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.

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" wrote in message
news:wge6b.371122$o%2.167724@sccrnsc02...
"David" wrote in message
...
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