On Sat, 06 Sep 2003 13:49:31 GMT, "John Grabowski"
wrote:

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



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