When I built my addition, complete with a subpanel, the inspector refused to approve my work because I had not separated the neutrals and the grounds into their own bus bars. I did it, with some grumbling, but never got an explanation beyond “That’s the code.”
I know it’s fine to combine them in a main panel, since neutrals are by definition at ground potential, but why does it matter on a subpanel? How could there be any risk of current leakage, or stray voltage of some kind, in a subpanel that isn’t present in a main?
IANE, but it’s redundancy in hopes of making a foolproof code.
In normal operation, the ground is not supposed to have any current going through it (as you knew.) If the neutral goes open between the main service and the sub panel, the current running through the ground could be a hazard to anyone trying to work on the ground with power on to the house. Separating the ground from the neutrals everywhere except at the main service forces the circuits to fail if the neutral opens.
Additionally, the main ground can be sized slightly smaller than the hot and neutral for some services. I’ll try to find a cite and post.
The danger in connecting them at a subpanel is that you introduce the possibility of ground loops, which equals ground current. If neutral and ground are tied together, some neutral current from the subpanel to the main panel will conduct on the ground wire (since it’s now in parallel with the neutral). Since current is flowing through the wire, a voltage is developed across it, which makes the voltage at one end different than the the other- it’s no longer ground at one end (the subpanel end). This voltage is usually small (1 volt or less), but can still cause problems. For example, it might spike to 10 volts or more during a motor start. Tie two computers together via serial cable, with each computer on each end of the ground, and now their grounds are 10 volts apart! Problems like this happen more often than you might think, due to bad grounding.
Also, now that current is flowing through the ground wire, if the neutral opens for some reason, the ground is taking all the neutral current- bad in itself because the ground often undersized. In addition, if someone lifts the ground wire, or it is opened up, it’s now hot, since the hot is trying to return through the ground as well as the neutral.
Anyway, the ground and neutral can only be tied together at one spot- the service entrance (usually the main panel). If you connect them anywhere else, you’re inviting trouble.
Note, this is from an old handyman’s reference based on the code in use in 1998 and may or may not be current NEC code; ask an electrician if you need specifics. Furthermore, the following is a reference to main services:
gauge of min. gauge
main wire of ground
------------ ----------
2 or smaller 8
1 or 1/0 6
2/0 or 3/0 4
Anything larger probably ain’t a house service.
My limited experiences as a homeowner are that inspectors answer all questions about code with the statement “Because it’s CODE.” I assume that this is because they do not want to argue code with anyone and couldn’t change it even if they wanted. My guess is that some people try to debate code with them and they avoid rhetorical questions to cut these arguments off at the pass.
The explanations about current through the ground if the neutral to the subpanel is broken make complete sense - thanks, folks.
This has nothing to do with GFI’s, btw - it would be code even without considering them. GFI’s only break their own individual circuits in response to a difference between hot and neutral current, indicating a current leak outside the circuit, as I understand them.
That’s why I mentioned my ex-hot tub. If it had been wired to 1990’s codes it would have worked with the GFI the inspector called for. As the wirirng was a mish-mash of 220V and 110V with neutrals and control wires being the ones the solvent-addled fiberglassers chose at the moment, it just never would work on a GFI.
I thought about re-wiring it. Luckily it blew up before I could get to it.
>> My limited experiences as a homeowner are that inspectors answer all questions about code with the statement “Because it’s CODE.” I assume that this is because they do not want to argue code with anyone and couldn’t change it even if they wanted. My guess is that some people try to debate code with them and they avoid rhetorical questions to cut these arguments off at the pass.
Well, of course. They enforce the code, they don’t make it. Just like a cop ticketing you for not stopping at a stop sign is not going to engage in a discussion about whether the sign is warranted or not.