It depends on two things: First, is this an old pre-existing installation, or new? If new, then the neutrals and grounds need to be separated in the subpanel. If old, it could go either way. Second, are the wires to the subpanel run in metal conduit? If so, then the conduit MAY be used as the grounding connection between the main panel and the subpanel, instead of a separate ground wire. However, if there is not a solid metal grounding path from the subpanel back to the main panel then a fourth grounding wire MUST be installed. It is very dangerous to have the neutrals and grounds separated without a grounding path back to the main panel. The ground rod changes nothing.

 

Yikes. Thank you for the response!

The sub panel and garage wiring looks quite new, like it was all done within the last couple years perhaps. This is a 90 yo house by the way. The last owner was an electrician and I bet he did the work himself.

The 3 wires that feed the sub panel are not inside conduit--they exit the house from underneath the eaves and just drape through the air over to the garage. No covering of any kind, just 1 sheathed & painted 3-wire run. This is the only thing connecting the house to the garage. No plumbing or anything.

Am I correct in understanding that if a have someone replace my current 3-wire feed with a new 3-wire+Ground that goes back to the main panel everything should be safe and the sub panel can be left as-is? Should the ground rod be left alone?

 

A three wire feed to the detached garage requires the neutral and ground to be bonded.]
If you had a four wire feed to the detached garage then the neutral and ground would kept separate.

 

{A three wire feed to the detached garage requires the neutral and ground to be bonded.}

If you had a four wire feed to the detached garage then the neutral and ground would kept separate.

{If new, then the neutrals and grounds need to be separated in the subpanel}

Only if you have a four wire feed from the main panel. This is not the case here.

A three-wire feed with neutral and ground bonded at the subpanel is not allowed for a new installation and hasn't been for awhile now. I agree that if there is a 3-wire feed which was legitimately installed, the neutral and ground busses MUST be bonded at the subpanel - it's the only way to make a 3-wire feed safe at all. However, it's still not a legal installation if it was done in the last few code cycles. A 4-wire feed is the proper method.

 

If new, then the neutrals and grounds need to be separated in the subpanel

Only if you have a four wire feed from the main panel. This is not the case here.

 

Perhaps the electrician assumed lain: it was a 4 wire feed because everything else in the garage is basically new. I'm going to turn off the double breaker to the garage until I can get a 3+ground installed from the main. The present 3 wire feed looks really old so I'm guessing a 4-wire with separate neutral/grounds is the current practice anyway. I'll consult with a better electrician this time. Thanks guys

 

The OP has a three wire feed and a ground rod on a detached building. That requires the ground and neutral to be bonded. That was the question posed by the OP.
The inspector says they need to be bonded and he is correct not the electrician who says they should not be bonded.

 

These recommendations are for a "what if" situation. And in this case, the "what if" is actually a quite common problem!

That is if the neutral wire from the home becomes disconnected, but the other wires remain connected. And say someone has jumpered a ground to neutral at a grounded outlet [wired with a 2 wire and no ground romex].

Then what? [Hint: Imagine you are using a metal grounded tool with a 3 prong plug]

Or in other words, someone not following electrical code rules and an accidental disconnection of a wire (probably from not properly torquing the lugs)... Can get someone electrocuted.

These things have happened in the past. A LOT!

Note: The neutral wire could also be cut by a trencher, someone drilling a hole in a wall, etc. All sorts of stuff happens...

 

A three wire feeder requires the ground to neutral bond. The conductors do not need to be on the same bar.