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
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.
Besides "Call a real electrician" , can anyone offer advice on feeding
the sub panel.
Yes, it will be inspected.
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, 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
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.
On Sat, 06 Sep 2003 13:49:31 GMT, "John Grabowski"
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
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
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.
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.
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.
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.
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.
Hey thanks Mark,
I was pretty sure I was right. I needed something black and white to
prove my point. I had considered the metal path between buildings, gas
is plastic, I think water is too, but I didn't think about the coax
cable. I think we should run a groung to be safe.
We have a groung rod at guest house. Does the panel at the guest house
have to be able to disconect main power from that panel? I had thought
that a sub panel could have a disconnect at either main or sub panel.
Thanks a lot,
If the subpanel was in the same building, then it doesn't require a main.
But since this supplies a separate building, you must have a local
disconnect and you have three choices. Either use a panel that has no more
than 6 breakers, use a panel that has a main breaker, or install a separate
100A fused disconnect. Some areas require that the disconnect be on the
outside of a dwelling. If this applies to you, you need either a raintight
panel or an outside disconnect. Hopefully, you can just go buy a 20 slot
panel with an integral 100A main breaker.
Because you have a 200A service, you can't run an unfused 100A tap to the
guest house. You could either tap your 200A house service and run a parallel
200A service to the guest house, or run a feeder that is protected at the
main panel to the guest house. You must have a disconnect at the guest house
in case it catches fire -- the firemen don't want to be running around
trying to find what panel feeds this building. If you're going to tap the
200A main service to feed both buildings, it becomes important to do the
service calculations because the meter base and service conductors won't be
protected from any combined overloads.
Finally, you said grounding rod. If you can't prove the grounding rod is 25
ohms or less to ground, then you need a second one at least 6' away and
bonded to the first.
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