GFCI feeding a subpanel

[Bill Fitch]

Hello!
I just pulled the trigger on a new G0490 jointer and have just a few days to get some 240V hooked up before it arrives.
My workshop is in my garage, and I would really like to use GFCI breakers, but they are VERY expensive, that is, except for 60A breakers. I suspect that since hot tubs need GFCI, the market for 60A is such that their prices are not horrible.
I am going to install a sub-panel for some 240V outlets. If I feed the sub-panel from my existing panel with a 60A GFCI breaker, will that give me protection on everything in the sub-panel? Is this legal?
Thanks for your help.

[Rob Russell]

If the subpanel is fed with a GFCI breaker, then everything run off of the subpanel will be GFCI-protected. That's perfectly fine.

[Kirk Poore]

If the subpanel is fed with a GFCI breaker, then everything run off of the subpanel will be GFCI-protected. That's perfectly fine.

Except, of course, that you might have a hard time finding the source of the problem if the GFCI trips. Also, if you have your lights wired into the same subpanel, they'll go out when it trips and that will make diagnosis tougher too.

Kirk

[Bill Fitch]

I did not think about debugging, so that is a good point. No lights will be fed from this sub-panel, so I won't have to worry about that. I suppose if things get bizarre, I can shuffle around the GFCI and non-GFCI breakers until I could localize the problem.

I will be meeting with the inspector shortly to pull my permit. I will see what he has to say.

Bill

[glenn bradley]

Bill, I am interested in this as i GFCI protected my 110 outlets but not my 220. The 110's get plugged and unplugged and used by whatever comes along. The 220 are used for fixed machines but this, of course, does not mean they will never have a fault. Wondering what the inspector says about GFCI for 220 and hope you will share ;-)

[Jim Rimmer]

I am by no means an electrical expert so I was curious as to why you need GFCI protection for your machines. Is there a water problem in your shop? Does GFCI provide surge protection? If it's for surges, is there a less trouble prone method?

[Bill Fitch]

Jim,
I don't think GFCI is a requirement for 240V branch circuits other than for something specific like a hot tub. I just think it is a good safety measure. Things just happen in a garage, cords get yanked or run over, things get dropped, you name it. It is not that unlikely a short could happen that would energize the metal surface of a tool. Standing on concrete give a pretty good path from a hot tool to ground through your body.

Not that I am paranoid, but if I can get GFCI protection for the cost of a single breaker I will do it. If not, I probably won't. The 20A 2 pole GFCI breakers for my panel that I would need for each circuit are better than $100 a piece. Yikes, they add up fast.

Bill

[Bill Brady]

You will have trouble with nussance trips if the load on the hot lines is not equal. The GFCI monitors the neutral current and if the swing is to big it will trip out.

[Rob Russell]

You will have trouble with nussance trips if the load on the hot lines is not equal. The GFCI monitors the neutral current and if the swing is to big it will trip out.

By definition, a 240v, single-phase machine isn't going to use the neutral so there's nothing that the GFCI breaker should see.

[Tom Veatch]

...I am going to install a sub-panel for some 240V outlets. If I feed the sub-panel from my existing panel with a 60A GFCI breaker, will that give me protection on everything in the sub-panel? ...

I confess that I don't know how a 240v GFCI breaker works. But I see a potential problem if a 240v 2-pole GFCI breaker is used to feed both 240 and 120 loads. If anyone could direct my attention to a source that explains the operation of a 240/120v GFCI, it would be appreciated.

If the 240v GFCI monitors/compares the current in each leg (hot/hot currents equal) like a 120v GFCI (hot/neutral currents equal), as soon as you turn on any 120v load, the breaker will trip because the current in the two hot legs will be unbalanced and look like a ground fault to the breaker. That could work well for a 240v circuit (no 120v loads) because, in that case, an imbalance would indicate a ground fault just as an imbalance in the hot/neutral currents indicates a ground fault in a 120v circuit.

Only way I see that a 240/120v GFCI could work would be to monitor all three wires and verify that the neutral current is equal to the difference between the currents in the hot legs. That is much more complex (expensive) than needed for a 240v circuit. If it monitored only the neutral current as implied in another post in this thread, it couldn't possibly detect a leakage to ground in either a 120 or a 240 load.

[Bill Fitch]

Tom,
A 2-pole GFCI works by sensing the sum of the currents of all 3 wires (2 hot, 1 neutral). No matter how you load the circuit, this sum should remain 0 unless the current has another path. The trip point is very small, such as 5mA or so, so it does not take much to cause the breaker to trip.

This really is just like the GFCI on a 120V circuit, the only difference is it is a sum of 2 wires instead of 3.

Even though my initial wiring will only have 2 wire recepticals (6-20r), I still plan to run the neutral to each box and tie it off. If ever I need all 3 wires, they will be there. In any case, the hope is the GFCI will operate as intended.

There is a good description here:
http://static.schneider-electric.us/...840-058-03.pdf

They show both a 240V application and 120V applications with the same breaker.

I think I will speak to the electrical inspector tomorrow and will get his take on this. I will post what I learn.

Bill

[Tom Veatch]

Thanks for the education, Bill. That makes sense.

[glenn bradley]

Yeah, thanks Bill. That was quite clear even for my foggy brain.

[Bill Fitch]

All,
I just returned with my electrical permit. I spoke with the inspector about using a GFCI breaker in my service panel that will feed my new feeder panel. He said that is OK. That is the good news.

The bad news for me, I have Square-D HOM series service panel. The biggest GFCI breaker I could find for that is 50A. I am not sure that is enough. I either spring for the GFCI breakers in my feeder panel (about $100 a pop), or I keep my total draw below 50A and use a single GFCI breaker in the service panel. Neither of these feel right to me, I think I need another solution.

One thought comes to mind, even though I normally would not need a main breaker on a feeder panel, could I get another brand of feeder panel with a main that could be GFCI? Then I could get past the 50A limit. Great, more research! Anyone know about whether GFCI main breakers even exist?

Bill