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I have a small subpanel next to my main house panel with only 4 slots. I am out of breaker spaces, so I've routed 8 gauge wiring (2-120V split phase, 1-nuetral and 1 ground) to the new sub-panel. Very short run of less than 6 feet of wiring between the panels.

I have filled only two slots with a ganged 30 amp breaker in the sub-panel that I need for some testing equipment in my garage (both panels and in a detached garage). I will likely put 2 additional 15 amp breakers in the future in the sub-panel. I have tied all 8 awg wires into the main busses.

Neutral and ground are bonded, but I've run separate wires to the bonded bus. I believe this is a very conservative, safe installation, but is it code, or even good practice? The 8 gauge wiring is ultimately protected by the main disconnect breaker (200 amps) since I've tied to the buss directly, but the voltage drop in only 1-2 volts, so even a direct short should not overheat the wiring.

I don't have any spare slots in the main panel to install a ganged 50 amp breaker - would like to do that, but just don't have the space. I guess I could pull two existing 15 amp breakers, install the ganged 50 amp and reroute the existing services to the new 15 amp breakers in the sub-panel - and I can do that if the only safe installation.

Should I re-wire or not?

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  • but the voltage drop in only 1-2 volts, so even a direct short should not overheat the wiring. this is really a scary level of not understanding - in the case of a direct short, you'll be dropping 240V and those wires will literally be on fire.
    – Ecnerwal
    Sep 24, 2015 at 0:23

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So, you have 8 gauge wire potentially subject to a fault current of up to 200 amps without anything tripping. You also have the neutral and ground bonded in a sub-panel (despite running separate neutral and ground wires to it, so bonding in the sub-panel is a puzzling choice in itself, as well as being a violation.) [or else possibly you have written unclearly about what you have done with the ground and neutral?]

Wrong. Just....wrong.

Pull two breakers, insert a 50 A breaker, and move the 15A circuits to the sub. And unbond/isolate the neutral and ground in the sub. Get a different/larger sub-panel if you need additional circuits (or give a long hard look at a complete panel replacement with adequate spaces in the main panel - not all that expensive if the incoming service is adequate, but attached to a panel with few spaces.)

If your panel is listed for them breakers with two separate 15A circuits in a single space breaker are another option to make space.

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  • I certainly stand corrected - after a quick estimate of expected temperature rise combined with trip times vrs fault current of (Siemens Residential) common circuit breakers it becomes very obvious that the 8 awg wiring is woefully inadequate - and length is not the determinant! The breaker re-arrangement is a necessity. BTW, the "bonding" is in the main panel only, with the ground rod directly below the main panel.
    – Mark
    Sep 24, 2015 at 13:21
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Your installation is NOT code compliant.

If you want to tap directly off the 200 ampere bus bars, your conductors will have to be sized to carry 200 amperes. #8 is nowhere near large enough. You're going to have to figure out a way to install a double pole 50 ampere breaker to protect the 8 AWG feeders, or install 3/0 copper feeders instead.

I'm not even sure if you're allowed to tap directly off the ungrounded (hot) bus bars, I'd have to read through the code to find out. However, I suspect it may not be allowed.

The grounded (neutral) and grounding bus bars in the secondary panel must be separated. The grounded (neutral) bus should be isolated, while the grounding bar and the enclosure should be bonded.

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