Sub Panel neutral bonding (2025 Update)

by cide the seb has to be "floating" - the neutral and ground bonded pnly at one point DOWNSTREAM OF THE MAIN DISCONNECT. The main panel gets "bonded" - nothuing else does - including a connected generator

What's the question again ? seriously ... John T.

If you only have 3 wires in the feeder going to the shop you need to bond the neutral and ground out there too or there is no path for fault current. You also need a ground electrode out there. Current code requires a 4 wire feeder and isolating neutral and ground in sub panels but if the wire was already there when the 1996 code was adopted the 3 wire feeder is grandfathered in. Ground loops are not an issue but carrying circuit current in the grounding conductor is, That is why you do need another ground electrode. Essentially you are creating another service in the second building and creating a new ground reference there. In the electrical biz, you can't have too many ground electrodes but they all need to be bonded together.

Until you get a local ground electrode out there I would be careful bonding anything. It is best to drive 2 rods 6 feet or more apart. That is about all you are going to get out of a rod electrode. Rods are not really that good though. The best is probably a concrete encased electrode, the rebar in your foundation. Next would be a ground ring, a buried wire all the way around the building. Back in the olden days we had the best electrode around when all of the underground water pipe was metal and everyone had a metal to metal connection all the way back to the water plant. There were cases where the neutrals were totally gone and nobody noticed because they gut a path through all of that water pipe back to somebody who had a good neutral.

True but the code allowed you to use the neutral for a ground until the 1996 cycle was adopted and there are plenty of 3 wire feeders, dryers and ranges out there. In the case of a second building you always need a ground electrode and even on those old ranges and dryers, the manufacturer's instructions suggested a supplemental grounding wire. In those days it went to a cold water pipe. Now that it is becoming rare to find metal pipe, that doesn't really work.

His main question appeared to be whether the neutral and ground need to be bonded in the MAIN panel if they are already bonded at the meter which is right next to it?

Even connecting to the ground wire back to the panel is then only as good as whatever grounds that......

I was kinda ASS U ME ing that we were talking a 4 wire cable and we were talking about bonding the neutral WIRE to the ground WIRE.

The only way anyone up here would have a 3 wire feed to an outbuilding would be if it was origionally set up for a 120 volt panel

- and then one wire would be bare - - - unless it was over 50 years old (at least). switched neutral transfer would be uncommon, i would assume. Only REQUIRED for a bonded neutral generator equipped with a GFI outlet.

On my generator I removed the bond jumper so I can connect it to the house and made a "bonding plug" that allows me to bond the neutral by simply putting the bonding plug into one of the 120 volt outlets. The plug has a jumper from neutral tothe safety ground.

That would be MY suggestion as well.

That is open to debate. A strict reading of the NEC says the PoCo should not have bonded the neutral in their can but since they do not have to follow the NEC, the language was tweaked a little (again 96 as I recall) so it is all "service equipment" and you still rebond the neutral in the service disconnect enclosure where the code says the "Main Bonding Jumper" is supposed to be. That is usually just a green screw on the neutral bus or a strap. The hitch is that the MBJ "shall remain accessible" and inside the meter can isn't. Sanity prevailed. Meter cans are usually made so the can is bonded to the neutral anyway. There is no way to avoid it.

Some inspectors require that there be no metallic paths between the meter can (a PVC sleeve) and that you only bring 3 wires in. Then we did not care what they did in the can, the MBJ was clearly in the service disconnect enclosure.

The grounding electrode conductor can land anywhere from the service point (usually the mast head on an overhead drop) to the service disconnect enclosure. That is how they can land it in the meter can. If you have an underground service lateral, the service point is usually the transformer connection at the street. Yes you usually own that wire in the yard. You typically do not own an overhead drop.

With a 3 wire feeder you are REQUIRED to bond the EGC (ground) to the neutral. The earth is NEVER a fault path. You are also required to provide a ground electrode system. If you drive a rod and if you can't demonstrate that it has < 25 ohms to ground, you need the second rod more than 6' from the first one. That has been code since Thomas Edison was around.

Whatever makes you feel safer but there are 10s of millions of circuits and feeders running like that safely for the last 100 years or so. The main justification in the code change was simply consistency in the code, not any particular tombstone count. You can look at the 96 ROP if you like. Phil Simmonds wrote the proposal and his words were "the war is over" We don't need to save copper anymore, in reference to ranges and dryers. (although the 3 wire feeder was around long before WWII)

Two ground rods unless you can show me a grounding electrode survey by an engineering firm. (Just drive 2 rods)

when the short happens, a large fault current will flow and blow the breaker. For a LARGE fault current to flow, the ground must be low resistance. So you want a BOND WIRE between the neutral and ground in the system. This provides a path for the fault current to flow sufficient to blow the breaker if there is a ground fault. Often a ground rod alone is too high resistance.

You can get a VFD that is single phase in and 3 phase out

I see, well that solves that.

So here's another puzzling question. In that thread about solar panels that has now run amok, Danny brought up the issue of preventing backfeeding into the grid. AFAIK, that's built into the inverters, but how do they do it? It would seem like the chicken and the egg problem. You need to detect the grid power going off, but you are connected directly to it and also powering it. So, how do they detect it? Obviously before connecting they must first monitor the voltage, freq and phase and sync to it, but once you connect, how do you then tell the other grid sources are gone? I guess you could look for voltage drop, since with the grid down you'd expect 99.9% there will be big voltage decrease. But is that sufficient? And what do they really do inside those inverters?

Hypothetically, suppose there are 6 houses on a utility line that have solar and that segment is connected to the whole distribution system. The sun is shining, those houses are using significantly less than the panels put out. I cut the line to the rest of the grid, leaving just those 6 connected. What happens? Is the "grid" down? How do those inverters detect it? Do they? What happens?

fault.

We got a pitch on these things a while ago. Basically a grid tie inverter is clocked from the grid. No grid, no output. They run at a voltage that can be potentially slightly higher than the grid so if there is any power left over, it flows backward into the grid. A smart meter will see that and meter based on your local tariff agreement. The old style would just run backward if net flow was out instead of in. If you netted more use than you fed back in, no harm, just a lower bill. If you showed a negative bill I guess you would be talking to the PoCo. That is how people used to steal power, plug the meter in upside down it runs backward so there is a chance you would be starting in the fraud department if they did not know about your solar. The "plug in" collectors plug in just like your toaster and will boost the power on that side of your service. These things are usually too small to ever feed back into the grid if you have anything running at all.