non service rated panel install

[Jim]

I am installing a 100 amp non service rated panel (main lug) and am hoping you can advise me on the proper copper conductor and emt conduit sizing?

The non service rated panel will be approximately 50 feet from my 200 amp service panel and its first use will be to feed a 30 amp water heater and a 20 amp outlet box in a workshop.

I will be installing a 100 amp double poll OCPD on the load side of the service panel and I have tried to figure out the different NEC ratings for amp and voltage drop I don't perform those calculations enough to make it easy.

I understand that I will need two non grounded conductors, one grounded conductor and one grounding conductor and I imagine I'll be using THHN for the conductor type.

Can you tell me the AWG size I would need for the conductors and what the correct emt conduit size would be?

I would really appreciate your opinion. Thanks for the help.

[suemarkp]

If this is part of a dwelling, then #4 copper may be permitted via 310.15(B)(6), but check with your inspector to make sure you can use that table for a subpanel. Otherwise you need #3 copper, but #2 copper is probably easier to find. If you want to use aluminum, increase by 2 sizes (i.e. #2 Al = #4 copper, #1 Al = #3 copper, etc). Your equipment grounding wire can be #8 copper or you could just use the EMT as the ground (but a wire is the better design).

Using 1.25" EMT or PVC conduit will work for most combinations of the above choices, especially if you use the N suffix wire (e.g. THHN).

[mdshunk]

I forgot to ask. Ohm, when did you change your name to Jim?

[Jim]

Mark, Thank you for this information and I have a follow up question.

My question is regarding the sizing of the grounding conductor used for this installation. You mentioned, in your reply, that using #8 wire, for my grounding conductor, should be sufficient and I'm assuming that is correct for EMT as well as PVC conduit material? I guess my question, after reading through NEC 250.66, is possibly one of physics.

If the #8 is suppose to create a low impedance path, in the case of a ungrounded wire causing a problem and that ungrounded conductor is hooked to a 100 amp OCPD, isn't there a "size" issue in respect to the #8's ability to handle 100 amp potential? My guess, in using my weak electrical physics ability, is that this works because the #8 only has to create enough of a low impedance path to trip the OCPD and this happens so quickly, minus the time interval designed into OCPD’s to handle motor startup, that the #8 never really has to carry the brunt of a sustained 100 amp circuit?

Thanks, -jim

[suemarkp]

Your guess is close. First, this is really a bonding conductor (or equipment grounding conductor), so you size it per 250.122 and not 250.66. The wire has to be large enough to conduct a fault (which can be thousands of amperes) so the breaker can trip. This duration of these faults is so short that the wire won't melt. The values in 250.122 need to be increased if the conductors of the feeder have been increased because of voltage drop concerns. But your feeder is close enough that the values in 250.122 are fine.

You really shouldn't have an overload (say using 150 amps off of that 100A feeder) that goes thorough the equipment grounding conductor. It is possible, but rare. Even if it happens, the #8 wire will get quite hot but it won't melt. However, its insulation could be degraded. Since EGCs aren't required to be insulated, this end up not being an issue anyway.

The values in 250.66 are for ground electrode conductors, and the size here is larger. I don't know why this table uses larger wires than 250.122, as lightning can seem to melt anything regardless of how large you make the conductors. Perhaps its because this table is kind of a dual use EGC/GEC table and the supplying utility can source more amps than can your feeder, so a larger size wire is specified in this case.

I'm not sure I follow your EMT / PCV conduit question. The size of the EGC is not dependent on conduit material. If you use any rigid metallic raceway (EMT, IMC, RMC), then it can be used as the EGC and a separate wire doesn't have to be pulled. But poor make-up of the couplings can cause a bad connection which greatly increases the impedance of the raceway rendering it ineffective. That is why I prefer to always run an EGC even if code says you don't have to.

[Jim]

Mark,

Thanks for your response to my second question and from it I gather I'm at least in the correct ballpark with my understanding of how the "safety" grounding bond does its job even though it is allowed to be a smaller conductor size.

The past of my question that wasn't very clear to you was really an attempt to see if you technically, per the NEC, need to have a larger bonding wire if you use a non-metalic conduit because it wouldn't per pasticipating in the fault current like a metalic conduit would.

-jim