subpanel considerations


Old 10-11-04, 06:15 AM
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Hi all! Some interesting things going on here, mind if I jump in and ask a couple of questions?

First, I just got done wireing a subpanel as part of an addition. The job was not inspected because the owner was basically an older, opinionated man who didn't want the city telling him what to do. Anyway...

The house was older and had two wire electrical through-out (no bare ground). When we added the sub-panel, the contractor wanted to add a grounding rod to the sub-panel because he was unsure how good the ground at the main panel was. We inspected it at the job site and there was a bare wire from the main panel to some older unused copper pipes, but we added the ground at the sub-panel anyway, "just to be safe."

Are there any concerns with this configuration? I looked but I couldn't find any references to grounded sub-panels in my reference books. The sub-panel was otherwise wired properly, just like a regular sub-panel (grounds isolated from neutrals). We just ran a bare wire to earth ground at the sub-panel is all.

Second, how does one go about calculating voltage drops in long feeder wires? The run here was about 80 feet. Not too bad but I was kind curious how one would actually calculate voltage drops, and what was considered acceptable in terms of a voltage drop, and when to start increasing the wire diameter.

Third, there was no attic space in this older house (literally none! the ceiling sheet rock was nailed right to the rafters for a vaulted look), and it was on a slab so we ran the conduit over the top of the roof. It was darn hot up there. How does one go about figuring acceptable temperatures inside of a conduit? I know the wire has a rating but how could one guestimate if this rating might be exceeded?

Thanks for any help you can provide!
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Old 10-11-04, 06:27 AM
Join Date: Sep 2000
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gameboy, I'm going to split this topic off in a separate thread, because it has many differences from the current topic.

If the addition is attached, then your "just to be safe" grounding rod is a serious code violation, creates a hazard, and is a really, really, really bad idea. One structure needs to have just one point of electrode grounding, and it needs to be at the same point as where the neutral and equipment grounds are bonded. It was a good idea to add a grounding rod, but it should have been connected to the main panel, not the subpanel.

As far as calculating voltage drops, there are many tables and many web sites for doing this. It's a pretty standard calculation. However, 80 feet for a 240-volt subpanel feeder does not normally require an increase in wire size.
Old 10-11-04, 06:41 AM
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Thanks, I was kinda concerned about it. Anything in particular to be worried about? It was the contractor who wanted the extra ground, not me, I was just the worker. The jobs not done yet (there was some extra remodeling inside) so we could still remove that ground on the sub-panel.

I'm worried about things like ground-loops and such like. The addition is attached, but isolated in the sense that it isn't connected to thre rest of the house at all. I could use a little bit of ammo when I talk to the contractor about it.
Old 10-11-04, 06:54 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
If your contractor has a copy of the NEC, tell him to read article 250. If he doesn't have a copy of the NEC, then he shouldn't be directing this operation. He should let somebody familiar with article 250 do it. Grounding isn't a place to make things up, and much of it can be counterintuitive. Even licensed electricians mess this up from time to time.

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