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power at exterior of remote building, but not supplying building #90349 11/16/04 11:48 AM
Joined: Sep 2004
Posts: 138
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royta Offline OP
Member
A homeowner has an air compressor he would like installed on the backside of a remote building near his house. I will install an 8x8x4 can containing a relay, with the relay control switch located in the garage. There is already a 50A 240V circuit running to the building, but it is supplying the AC unit, and not the building itself. There are no electrical items associated with the remote building whatsoever. There is no room for another breaker in the main panel, so running a second circuit to the air compressor is not an option, at least not from the main panel itself. I'm trying to do this as economical as possible for the homeowner, and obviously meet code at the same time.

Should I:
A) Turn the 50A AC circuit into a sub-panel feed with sub-panel located on exterior of remote building and drive a ground rod?

B) Turn the 50A AC circuit into a sub-panel feed with sub-panel located immedietely to the left or right of main panel. Install one conduit, containing two circuits, to junction box located on exterior of remote building. Branch out to disconnects for each unit. Wait, wouldn't I still need a ground rod, as the Exception to 250.32(A) would not apply, would it?

Is there a scenario where I won't have to drive a ground rod?

Thanks.

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Re: power at exterior of remote building, but not supplying building #90350 11/16/04 01:21 PM
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gfretwell Offline
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Is 50a enough to run both of them?
You really need the nameplate ratings and 430.53 to decide.
If so tap the branch circuit to the disconnect for the A/C and install another fused disconnect for the compressor, sized to the manufacturers recomendation (I doubt it is 50a). It is still a single branch circuit IMHO so you still don't need a ground.


Greg Fretwell
Re: power at exterior of remote building, but not supplying building #90351 11/16/04 04:36 PM
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iwire Offline
Moderator
Greg the moment you install the fused disconnect it becomes a feeder.

royta

I do not see anyway out of connecting a grounding electrode.

I would install a small breaker enclosure mounted at the units to serve as the OCP and disconnects

I would verify what the feeder wire size is and see if 50 amps is the max. OCP allowed.

Use the correct conductor rating and round up to the next standard breaker size. This may allow you to go larger than the 50 amp OCP now in place.

If it happens to be 6 Copper 75 C wire you may be able to use a 70 amp breaker.

Look at the ACs tag, if it needed a 50 amp breaker the minimum circuit ampacity is likely to be in the high 20s to low 30s.

How big is the new unit?

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: power at exterior of remote building, but not supplying building #90352 11/16/04 05:49 PM
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Posts: 138
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royta Offline OP
Member
The feeder is actually AL 6-3 SE Type R, in a 3/4" conduit. At 75C, it's limited to an even 50A. Perhaps I should pull it out and install #6 THHN. You mentioned using the next breaker size, so would that be 70A (65A @ 75C), or 80A (75A @ 90C)? The breaker will more than likely be limited to 75C.

I forgot the rating on the AC unit's (condensor) name plate, but IIRC, it was only pulling about 17A when running. I forgot the rating of the air compressor. I'm obviously going to have to go back and check it out one more time.

It's probably a good idea about the sub-panel though. This way I can install a 1-pole 15A breaker for the relay control.

[This message has been edited by royta (edited 11-16-2004).]

Re: power at exterior of remote building, but not supplying building #90353 11/16/04 10:13 PM
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gfretwell Offline
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Yup you are right about "feeder" ... sorry.
That still gets back to whether you are "serving" a building or just 2 pieces of outdoor equipment.
The added flexibility is certainly a selling point for the sub panel.


Greg Fretwell
Re: power at exterior of remote building, but not supplying building #90354 11/17/04 01:28 AM
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royta Offline OP
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I'm only "serving" two pieces of outdoor equipment, not the building they are located near. I believe the sub-panel at that particular location is actually kind of worthless. Of course, this will be a surface mount panel, and will be much easier to run conduit out of to another location in their yard.

Re: power at exterior of remote building, but not supplying building #90355 11/17/04 04:04 AM
Joined: Jan 2003
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iwire Offline
Moderator
Unless you nail the panel to a tree [Linked Image] whatever it is mounted to is a structure and the rules for grounding electrodes apply.

Even a simple 4"x4" post stuck in the ground with a panel mounted to it would be a "structure".

Here is the entire NEC defintion of structure.

Quote
Structure. That which is built or constructed.

It sounds to that the 6 AWG AL will be adequate for the proposed load.

If you do change to 6 AWG CU the largest standard breaker you can use is a 70.

If the AC unit has a minimum circuit ampacity rating of say 25 amps that still leaves 25 amps for the compressor.

I am guessing that someone sized the wire to the AC based on the breaker not the load, this is good for you. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: power at exterior of remote building, but not supplying building #90356 11/17/04 08:10 AM
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Posts: 138
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royta Offline OP
Member
Quote
If you do change to 6 AWG CU the largest standard breaker you can use is a 70.
Thanks for the reminder. The 90C row is used for derating purposes, that is unless I can actually find devices that are 90C rated.

Thanks for the reminder about the structures and ground rods. I guess you remember what you practice, and unfortunately, the last time I dealt with ground rods, was in 1997 or 1998. The company I worked for was hired to install about 5 or 6 RV power pedestals at a guy's house, so all of his Prevost RV buddies could hook up. Most of the work we did was is in the petroleum industry. I've never installed a sub-panel in a structure other than where the main was located. I could use a little education on grounding, and on sub-panels for that matter.

Thanks for the help on the simple questions everyone. I really appreciate it.

[This message has been edited by royta (edited 11-17-2004).]

Re: power at exterior of remote building, but not supplying building #90357 11/17/04 09:05 AM
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Posts: 138
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royta Offline OP
Member
I have another question that refers to only the AC condensor unit currently installed, not tapping the disco side of the disco. It is for my knowledge only and has nothing to do with the planned installation of the air compressor.

If the original installer used a fused disconnect for the AC unit, the wire, and associated OCPD, feeding the disconnect would be considered a feeder. It wouldn't necessarily be a sub-panel, but it would be a sub-OCPD type item (even though it's only feeding a single appliance), and a ground rod would be necessary. Since it is only a non-fused disconnect, it is a branch circuit, and no ground rod is necessary. Am I correct?

Does this mean that a spa or pool sub-panel that is not mounted to the structure where the main panel is located, requires a ground rod to be driven? Actually, the pool sub-panel would get it's grounding system from the rebar in the pool, but a spa sub-panel (no pool) would need the ground rod.

Thanks.

[This message has been edited by royta (edited 11-17-2004).]

Re: power at exterior of remote building, but not supplying building #90358 11/17/04 12:13 PM
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gfretwell Offline
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A swimming pool is not a grounding electrode.

I also wonder about this whole disconnect/feeder/structure question. If these are feeders then we need to drive a rod at every A/C condenser if it has an internal disconnect/breaker.


Greg Fretwell
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