[richl56]

I have a 200amp main panel in my basement. I want to put a 100amp subpanel in my garage to service 2- 2pole 20amp circuits for 2 a/c condensers and 2- single pole 20a convenience circuits for the garage. My questions are 1 what size feeder wire would I need for the subpanel. (THHN in conduit) and what size conduit. Also in my main panel is it allowable to have both bare ground wires and neutral wires coexist on the same neutral bar and the ground bar.also, because the main panel houses my main disconnect should my neutral bar be grounded. And last question should the neutral bar of the subpanel be isolated from the panel box as to be unbonded. If so then I should need a bonded ground bar. Is this correct

[Missouri Bound]

How far from the main panel is the garage sub panel going to be? That helps determine wire size. You will need 4 wires to the sub panel. The neutral and ground are allowed together only at the main, the sub panel will need separate bars and the neutral not to be bonded to the panel. If the garage is not attached, you will need a separate grounding rod. Conduit size will depend on which wire size you wind up with... but it won't be less than 1" at any rate.

[Home_Inspector]

First to answer your questions one at a time. Depending on the amount of amps needed at any given time and the size of the breaker from the main panel will determine what size wire you will need. If you plan on using this garage as a shop and will be using a lot of draw you will probably need #4 AWG copper wire or #2 AWG aluminum wire with a 100 Amp double pole breaker in the main panel, unless you are talking hundreds of feet, the service wires shouldn't need to be oversized. I estimate 6 volts dropped using #4 copper at 100 feet, which would be 1/2 the total circuit, so you would have approximately 114 volts load at the end of the circuit. Using #2 AWG aluminum would be approximately the same at that distance. If you opt for using the aluminum wire be sure to use antioxident on exposed aluminum at each panel. I would also recommend THWN, UMC, or UFB wire, if the THHN wire you are referring to isn't dual rated (THHN/THWN) as many are. THHN wire is suitable for dry locations only. The THWN wire is rated for use in wet locations and will fare much better than the THHN wire once the conduit gets water in it, and it will! Even better would be to use UF-B, UMC or UG wire which is rated and designed to be used either alone underground UF-B and UMC or within conduit UF-B or UG, but since you will be needing such a large conductor then service wire or UMC (underground metal conduit) wire would probably be your best bet, as I don't think that UFB comes in a #4 AWG configuration. The conduit should be large enough to allow you to pull the wires through without too much issue at the elbows. As per many codes the conduit shouldn't have more than 360 degrees of bends total. The UMC or UFB cable would eliminate the pain of pulling the wire through the conduit and the UF-B cable doesn't require a conduit at all, but as always check local codes. I would still use conduit where the wire comes from underground and enters the garage and at the home is any cable is expoxed, to prevent weedeaters, squirrels and other things from damaging the insullation and sheathing on the wire if you use UF-B. If you don't plan on using the garage as a shop and won't have much demand on the sub then #6 AWG copper with a 60 Amp breaker would probably be fine and UFB could be used. Hopefully this isn't going to be a long run as the wiring wouldn't be as much as a consideration as would be the lines running to and from the A/C condensers! If there is a large distance with several elbows and fittings used the refrigerant lines diameters may need to be adjusted to maintain optimal efficiency and large distances may also cause the equipment to work harder and result in a reduced life expectancy and increased electric bills.
The main service panel you are branching from should have the neutral and ground bonded, however, any subpanels should NOT be bonded and the neutral and ground wires should be isolated. If you are using 4 wires to the subpanel (2 hot, 1 neutral and 1 ground)adding a ground rod isn't required, unless by your local code and considerations for the distance factor, but certainly wouldn't hurt. I realize that the date you posted this was over a year ago but I posted this in case someone else stumbles upon this and has the same question or similar situation. I hope you didn't go with the THHN wire!

[stanfortyman]

If you are using 4 wires to the subpanel (2 hot, 1 neutral and 1 ground)adding a ground rod isn't required, unless by your local code and considerations for the distance factor, but certainly wouldn't hurt.

This is absolutely untrue.

A grounding electrode is definitely required for a detached structure with a sub-panel, regardless of 3 or 4 wire feeder, or the distance (not sure what the distance has to do with it :confused:). This is not a local code thing. It is an NEC thing and I have never heard of any amendments to relax this rule.

A grounding electrode has nothing to do with the equipment ground run with a feeder.

[Home_Inspector]

My apologies you are correct. I misread the code specifying whether a single circuit or a two circuit system requires a grounding electrode on a detached structure. If this garage were on a single (1 hot, 1 neutral and 1 ground)circuit it would not require an additional grounding electrode. Article 250-32-Exception
But this garage would require a grounding electrode as it is a two circuit system.
I just ran a single circuit to my garage and was told I wouldn't require an additional ground rod and incorrectly applied that to this circumstance. Thanks for the correction. Still not sure what a subpanel would have to do with it. Can you clarify. Thanks!

[stanfortyman]

My apologies you are correct. I misread the code specifying whether a single circuit or a two circuit system requires a grounding electrode on a detached structure. If this garage were on a single (1 hot, 1 neutral and 1 ground)circuit it would not require an additional grounding electrode. Article 250-32-Exception

Actually 225.32 is disconnecting means location. What edition are you using?
This is in 225.30 and allows a multi-wire (shared neutral) branch circuit (hot, hot, neutral, ground) in this instance. For this application it is considered one circuit, although it is actually two.

Still not sure what a subpanel would have to do with it. Can you clarify. Thanks!

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).
(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

Exception: A grounding electrode shall not be required where only a single branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non–current-carrying metal parts of equipment.

[Home_Inspector]

OK, I'm obviously not a certified electrician, but reading the above exception confuses me. The way I read it is that there wouldn't be a ground rod needed if there was a single circuit (1 hot, 1 neutral and 1 ground) used. On my garage, it has very little usage for electricity, I was told since I was running 30 Amps from the distribution panel using 10-2 w/ground I wouldn't need a ground rod. They did require me to use a sub panel because I needed a disconnect. 4 receptacles with gfci and 2 without (because it would have a refrigerator connected to it and a garage door opener). I ran the lighting (2) fluorescent shop lights and the other 2 receptacles on the gfci and the 5th and 6th receptacles on a non gfci circuit. On the subpanel I only use one bar and 2-20 Amp breakers are connected to that bar. I was told I would need the subpanel also because I needed a disconnect for those 2 separate circuits inside the garage. I had planned on just using a junction box and wiring gfci receptacle as the third receptacle in the system and allow it to serve the other receptacles as well, having 2 non gfci (the first 2 on the run) and the other 4 gfci protected all on one circuit, but was told I couldn't do that because a disconnect was required.

I am a little confused as to why an additional ground rod would be needed when the bare ground wire that goes to the garage is connected to the grounding bar in the main panel inside that is directly in line with the ground rod's used for the home's main panel. I had assumed it may a distance factor but cannot find anything related to maximum distance in the NEC as far as grounding goes. So I'm a little confused as to what the difference would be between using a ground wire going to the panel and then to the ground rods used for the main panel or using a separate ground rod as required per NEC. Would both not adequately dissepate a short circuit properly and throw the breaker?

The primary reason I posted to this thread was because the person was going to use THHN wire in conduit going outside to that garage and that could be a problem and it hadn't been addressed. I realize that the post was old but some may come here seeking advice and see that old post and follow it. I should have read the code about the grounding electrode before commenting on that, but thought (incorrectly) that since I wasn't required to have one it wouldn't be needed here.

[tkrussell]

I am going to confuse you even more.

Your system, if located in the USA, requires a ground rod at the garage. Since a panel was installed, you now do not have a single circuit serving the garage.

The exception Stan cited is for a single circuit, or multiwire circuit, that enters the building, goes through a disconnect switch only, which can be a single pole switch, and continues onto the loads being served.

The installation of the two circuit panel now makes the exception cited not applicable, in your case. If you were told no rod was needed because you only ran a #10-2, was that decision based on the fact of the two circuit panel?

The purpose of the ground rod is for lightning protection of the panel.

Good catch on the use of THHN, and it is not allowed outdoors. Straight THHN is rare, most is all dual rated THHN/THWN. But good to pint that out.

[Home_Inspector]

You are right I'm more confused than ever. LOL
No, I was told only that I would be required to have a disconnect and decided to add the panel for another 20 amp breaker in the future if needed and to have separate disconnects for each circuit. I read that exception in the code and cited it here because the way I interpreted it is that it was dependent on the wire that "supplied" (entered) the garage and not the actual number of circuits that branched within the garage itself. I should have just used a simple single throw disconnect and wired it as I had originally intended.
I am confused as to why a subpanel would require lightning protection and a disconnect switch box wouldn't. I realize that you don't make the rules but am here to learn and contribute when I can. I may just replace that panel with a disconnect switch and use that Square D subpanel to replace the FPE subpanel with stab lok breakers on my air handler in the house we just purchased. Before I totally hijack this thread I will start another regarding the questions and issues I am having with my just purchased homes wiring and the FPE box... Thanks for the info and clarifications!

[tkrussell]

A disconnect switch is just that, a switch.

A panel is a bit more involved with multiple circuits, circuit breakers, bare bus in proximity to each other. A lightning strike is not something desirable to collect in a panelboard, and the ground rod is intended to help dissipate any large amount of voltage as quickly as possible.

When you asked for advice and explained to someone you intended to have one circuit, the advice for not needing a ground rod was correct as long as you had one disconnect for that one circuit.

Once you added the 2 circuit panel, you changed the system, and the exception was now not applicable.