Risk of bonding neutrals and grounds in sub-panel

Question

I am replacing my complete electrical service entrance (weatherhead, meter box, breaker panel in the house). The meter housing I have chosen is a meter/main combo that will have a 200 AMP breaker. So the meter/main combo is essentially my main panel. The neutral and ground will be bonded in this meter/main combo main panel. Then I will run 4 wires (hot, hot, neutral, ground) to the interior breaker panel which will in essesnce be a sub-panel. I will not bond the neutral and ground in this interior sub-panel.

Now I am able to install my entire new service entrance while the old service entrance is still working. Then the power company will come and switch the service drop from the old service entrance to the new. So I won't be out of power very long at all. Now when the service drop is switched, all my house circuits will still be connected to the old fuse box, which is now dead. I would like to then TEMPORARILY make this old box a sub-panel of the my new sub-panel to give me time to move the circuits from the old box to the new when I am able to do so. But the problem is the neutrals and equipment grounds are bonded in this old fusebox and they share the same bus bar. The fuse box is so old, and is rusty inside, so I would very much not like to try to separate the grounds and neutrals unless I really need to do so. Also, this box does not have an earth ground of any sort. I found the wire, but it went to a plastic water pipe. I know the grounds and neutrals should always be separated in a sub-panel. But I don't even know if I can do so due to the age of the box. So with all of that said, what risk is there to making this old fuse box a subpanel for a short time even though the neutrals and grounds are bonded?

Answer 1

Prior to the 99 code sub panels were wired the same as main so there are possibly 10’s of millions of homes that still have the neutral-ground bond on the sub. So is there a risk? Sure about the same as getting out of bed each day. Would I do exactly what you propose, yep have on many occasions when updating a service. I am not working for a company that dose residential any more but I can say I have changed out panels and I had the old main panels isolated by final inspection. The nice thing with the new disconnect prior to your old main is you can kill power while working on it, the only real risk is with a bolted short the parallel ground path can energize frame work, just like all the tens of million homes with 3 wire services.

Answer 2

I commented earlier and this might not be a true "answer", but it'll be too long for a comment. Years ago, I replaced an ancient fuse type panel for my father-in-law. Due to a remodel and space requirements the new panel was going to be about 15' away from the old one.

Because the existing wiring couldn't reach the new panel, I ran all the cables needed from the new to the old, put them in proper cable clamps in the old panel, but didn't connect anything as the old panel was still in use. Once all that was done, we called the POCO and had them swap the service to the new panel. Since most everything was already in place, it didn't take long to disconnect the circuits in the old panel, "gut it" (remove all components) and use it as a junction box and reconnect the old circuits to the new cable runs. Maybe took an hour or two. We had already added circuits for the kitchen and clothes washer to make it more code legal, so those were live as soon as soon as the new panel was powered up, so we could plug the fridge into the new outlets right away.

Legal stuff: In my father-in-laws jurisdiction, it's OK for the POCO to transfer power prior to inspection, at least at that time it was. We did pull a permit and it was inspected and passed. Supposedly my father-in-law did the work (wink wink), and the inspector mentioned he was surprised how everything was done correctly. And NO, I wasn't paid for it BC i'm not licensed, only paid in gratitude!

Anyway, for the OP, a lot depends upon topology of the existing layout. My suggestion may or may not be practical in your situation.

Answer 3

There is some risk. I think it is very low, but I am in no way qualified to determine whether it is an acceptable risk or not.

However, there may be another (partial) solution. Assuming that:

• You have at least one reasonably large (30A or more) circuit in the old panel that you can temporarily do without (e.g., water heater, oven, dryer or air conditioner)
• Most of your loads to be transferred that you don't want to be without for very long - e.g., refrigerator, at least one countertop receptacle in the kitchen, at least one lighting circuit - are 15A or 20A circuits with relatively low, and controllable, actual load

then you can do the following:

• Install the new panel
• Connect the largest available 240V/120V circuit in the old panel as the feed into the new panel
• Move key circuits to the new panel
• On the day of the power company meter installation, before the power company gets there, connect the new feed from the meter/main to the new panel in place of the feed from the old panel
• Power company does their work

You now have key circuits transferred with minimal downtime. Move the rest at your own pace based on priority.

Note that the feed circuit from the old panel must include a proper neutral. So if you use your water heater circuit (normally 240V = 2 hots, not 240V/120V hot/hot/neutral) then you can't just use the existing cable - you need to wire it up (temporarily) with a proper hot/hot/neutral/ground.

This works best if you have a seasonal circuit - e.g., an air conditioner that you don't need in the winter (which is not always the case - if you have an air handler that runs both furnace and air conditioner then that's not a good choice unless you have really mild weather at the time) - or an electric oven if you can survive on the microwave (plugged into an ordinary 15A or 20A circuit that would be in the initial move) for a day or two.

Also keep in mind that a 30A 240V circuit is really 60A 120V if balanced well, so you can run a lot of stuff, except the big 240V loads, from one circuit-as-feed.

This method would require a "main lug/breaker" swap. There is a slight twist to this which might help decrease your downtime even more:

• Connect the new feed to the new panel (main lugs).
• Leave the meter main breaker off. (Well, you'd have it off during power company work anyway, but just clarifying here that you want it off the whole time until "done").
• Connect the 30A (or whatever) from the old panel to a backfed breaker in the new panel. In many (but not necessarily all) panels, a backfed breaker is really the same as any other breaker, except (a) bolted down for safety and (b) can't be GFCI/AFCI (which is fine here - you actually wouldn't want that anyway). It is commonly used with generator interlocks or similar things and also sometimes to feed power in to a panel that doesn't have a main breaker as feeding through a regular breaker lets that breaker act as the main breaker for the panel.

When the power company shows up, you turn off that breaker and the feed breaker in the old panel and once the power company is done you turn on the meter main breaker and (assuming your new panel has one) the main breaker of the new panel.

Warning: There might be a neutral issue with doing this. I think it is OK because until the power company does their thing, the neutral from the meter main won't be active anyway and as soon as the power company is done you can just disconnect the 30A feed (hot & neutral).

Answer 4

Well, it's not Code, but it's a better plan than what you have now.

I would temporarily temporarily fit a neutral bar to the old fuse box, and by "fit" I mean "not fit".

Get an accessory ground bar, move all the neutrals to it, and then wrap it ludicrously with electrical tape. So that it is isolated from the metal frame of the box.

I would also temporarily jumper the old grounding electrode to a competent ground rod of some kind. Because your house's grounds not being bonded anywhere, in any way to earth is much more disturbing than a pre-2008 combined neutral-ground bond in a sub panel.

Answer 5

With ground and neutral bonded, current can travel on both ground and neutral back to the main panel. If the load becomes unbalanced and ground and neutral are bonded, the current will flow through anything bonded to the sub-panel (enclosure, ground wire, piping, etc.) and back to the main panel. Obvious shock hazard!

https://imhomeinspections.com/why-grounds-and-neutrals-should-be-separated-on-a-sub-panel/