OK so here are my questions. Can I run a subpanel from a subpanel? Subpanel (A)(existing subpanel) has a three wire feed from the main ran underground through metal conduit and ground and neutrals are connected, I am guessing because this met code not long ago so I guess my question is do I need to run a 4th wire from main panel to subpanel(A) before running 4 service entrance wires to subpanel (B)(new subpanel) or use the conduit as a ground and seperate ground and neutral bus bars Also subpanel(B) will be about 250 ft away will that cause voltage drop?
Running a sub panel from sub panel
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jaggedben
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Generally speaking, yes. You could put a million subpanels in series, and by itself that wouldn't be a code violation.
The metal conduit can act as the equipment ground so a 4th wire is not required. (But you may want to test the resistance to make sure it hasn't rusted away, or actually transitions to PVC somewhere, or something like that.)
Yes, you most likely need to separate the grounds and neutrals. But if it turns out that the metal conduit is not continuous and won't act as a ground, then read the Exception to 250.32(B) very carefully.
Voltage drop is a function of current as well. One would need to know the projected load on both panels and the size of the wires feeding the existing panel to address the question properly. But yes, at 250ft the effect should be considered.
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Thank you. The current wire size is #6 to existing panel and I was going to use #2 to the new panel the existing panel runs a pump for an in ground pool and a couple lights. The new panel is actually a 100 amp main box that I want to use for a subpanel and all it will run are lights in a garage. I had a whole meter and service installed for the garage when I had it built but I never use it so I don't want to pay the extra charges every month from the electric company
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Thank you. The current wire size is #6 to existing panel and I was going to use #2 to the new panel the existing panel runs a pump for an in ground pool and a couple lights. The new panel is actually a 100 amp main box that I want to use for a subpanel and all it will run are lights in a garage. I had a whole meter and service installed for the garage when I had it built but I never use it so I don't want to pay the extra charges every month from the electric company
jaggedben
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- Northern California
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- Solar and Energy Storage Installer
Well you can't connect more power through the existing sub than it's already set up for. Assuming the existing #6 AL is power through something like a 40A breaker, that's the most you're going to get and there's no point in having a 100A feed downstream. There's probably not much point in installing #2 AL either, although voltage drop issues might call for having it larger than the #6 feeding it. You'll have to do the calculations. Also depending on how much power that pool pump can draw, it's possible you may not have a large enough feed to the existing panel to do this.
jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
That may be okay. If your utility happens to provide 15 minute usage data (if they did, it would probably only be online), then that could help you figure out if the existing subfeed would be adequate for the garage needs.
Although probably not required, if you think the original conduit run is in good condition I would recommend making that feeder a 4 wire if possible. Since grounding of pool equipment is so important anything you do to assure a good ground now and in the future when the conduit may deteriorate would be a positive step.
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