Electrical feed for sub panel for Guest house

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.
Besides "Call a real electrician" , can anyone offer advice on feeding the sub panel.
Yes, it will be inspected.
David
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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
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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

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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 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
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You can get more than 200A service. If it were me, and rather doable, I'd connect this as a sub building off the meter, and have a main there for the main house and another for the guest house.
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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.
-- Mark Kent, WA
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wrote:

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,
David
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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.
-- Mark Kent, WA
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