"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