"John Grabowski" wrote in message
. 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