240V with no neutral? Can I build a rig?

[TheZer]

GFI does not care how many wires you have involved. It can be a hot and a neutral (standard 120V), two hots (240V w/o neutral), two hots and a neutral (240V w/ neutral), or 13 hots and 7 neutrals (?).

All the GFI does is watch the current flowing in and out of the system across all of the wires present. If the sum of current going in matches the sum of current coming back out, then the GFI knows that all is well and power stays on.

If it ever sees a different between current in and current out, then it will cut the power.

If your neutral isn't connected to anything, the GFI will still work. Nothing will be flowing in or out on the neutral line, and the GFI will just be ensuring that the two hot lines have equal and opposite currents.

BUT....

The "test" button on the breaker MIGHT be making use of the neutral for it's functionality, so you would need the neutral connected (pigtail to panel) in order for that button to work.

Question: If the ground is tied into the pigtail neutral, and a short to ground occurs from the hot leg that the neutral is paired with, wouldn't the short go through the neutral wire of the gfci and defeat the gfci protection.

1. Current going through hot lead in one direction --->
2. Short to ground occurs, and the ground is tied to the neutral in the spa panel.
3. An increase in current now going through hot lead -------->
4. Because neutral is tied to ground, increase in current also going through neutral in other direction <--------
5. Net sum of current is zero. ( amps ---> + amps <--- = 0)
6. Gfci does not trip

 

[Walker]

Question: If the ground is tied into the pigtail neutral, and a short to ground occurs from the hot leg that the neutral is paired with, wouldn't the short go through the neutral wire of the gfci and defeat the gfci protection.

1. Current going through hot lead in one direction --->
2. Short to ground occurs, and the ground is tied to the neutral in the spa panel.
3. An increase in current now going through hot lead -------->
4. Because neutral is tied to ground, increase in current also going through neutral in other direction <--------
5. Net sum of current is zero. ( amps ---> + amps <--- = 0)
6. Gfci does not trip

I am a little confused by the question, or maybe I have the wrong set-up in my head.

There is a spa panel with a GFI breaker in it. On the "brewery" side of this breaker, there is a cord feeding the brewery with 240V w/o a neutral. Two hots and ground.

Inside the spa panel, the brewery hots are connected to the breaker. The brewery ground is connected to the common ground in the spa panel. Nothing is connected to GFI breaker's neutral on the brewery side.

On the "source" of the GFI breaker, the two hots are coming in from a non-GFI source and then there is a ground coming in. The pigtail on the source side of the GFI breaker is tied to the common ground.

Like this:

So, if a short to ground occurs in the brewery, then there will be current coming flowing on the green ground line, bypassing the GFI breaker. This would trip the GFI.

I suppose the real question might be about what this kind of wiring means at the source. The GFI would work in the above case, but I don't know if tying the gfi "source" neutral to the spa ground is OK under code. At SOME point in the system, neutral and ground ARE tied together. For me, I believe this is inside of the panel on the wall in my garage, so what I have drawn would work just fine for me.

 

[Walker]

this is the kind of thing I would like to hear from some of the electricians on the board. (note my sig.)

I am an electrical engineer, but this kind of stuff is the domain of an electrician. Different job, different knowledge, different skill set.

 

[jflimbo]

Well I just finished wiring and testing my spa panel. I wired exactly as Deaf explained tying together the ground and neutral pig tail, also as Walker's diagram above shows. I also tried without bringing the ground together with the pig tail. The end result is the gfci appears to work either way. However, if the pig tail and ground are not brought together, the gfci will not test with it's test button. But either way it will trip if you short one hot to ground at the kettle outlet.

 

[DeafSmith]

Well I just finished wiring and testing my spa panel. I wired exactly as Deaf explained tying together the ground and neutral pig tail, also as Walker's diagram above shows. I also tried without bringing the ground together with the pig tail. The end result is the gfci appears to work either way. However, if the pig tail and ground are not brought together, the gfci will not test with it's test button. But either way it will trip if you short one hot to ground at the kettle outlet.

If you are shorting hot to ground, it'll trip due to current overload (50 amps). The real GFCI test, aside from the test button, would be to connect a large value resistor from hot to ground. I think these are set to trip at differential current greater than 5 ma, so a 10K resistor should easily trip it.

 

[BrewBeemer]

DeafSmith,
I have a brand new in metal box a 50 Amp Fiji S53B GFI with a inlet plug and receptacle 3 pin with side ground.
This a 3 phase unit the tag shows 120/240 VAC 10.KW or 480 VAC 7.5 KW.
I was wondering if it could be used as single phase 240 VAC for the brewery
with 120 pump tap?
Electrical liquidator had a pallet of 50 offered to me at $10 each I took only one.
Back pain kicked in time for my needed short nap.

 

[DeafSmith]

DeafSmith,
I have a brand new in metal box a 50 Amp Fiji S53B GFI with a inlet plug and receptacle 3 pin with side ground.
This a 3 phase unit the tag shows 120/240 VAC 10.KW or 480 VAC 7.5 KW.
I was wondering if it could be used as single phase 240 VAC for the brewery
with 120 pump tap?
Electrical liquidator had a pallet of 50 offered to me at $10 each I took only one.
Back pain kicked in time for my needed short nap.

I can't think of any obvious reason why it wouldn't work if you just didn't wire up one of the phases. If it's just looking for total current through the GFI to be zero, it shouldn't care whether it's single phase or three phase. One thing to check is what the trip current is - for human protection this is supposed to be 5 ma, but some gfi's are designed to protect equipment and have higher trip currents, so would not protect you. If you can't find a rating, you could hook it up and try tripping it with different sized resistors to find the current at which it trips.

EDIT: I did think of a possible problem - if the GFI has internal circuits that run on low voltage DC derived from the 3-phase inputs, then it might or might not work, depending on how that voltage is generated internal to the GFI. If they just tap off one phase and neutral, or between two phases, then it should work, assuming the phase(s) tapped aren't the one(s) you don't have connected - if they use all three phases to generate their internal supply voltage, then you're probably SOL.

 

[TheZer]

WOW, i missed the point of this thread. I see that the neutral (white) in Walker's diagram is NOT connected to source.

Sorry

 

[jflimbo]

If you are shorting hot to ground, it'll trip due to current overload (50 amps). The real GFCI test, aside from the test button, would be to connect a large value resistor from hot to ground. I think these are set to trip at differential current greater than 5 ma, so a 10K resistor should easily trip it.

DeafSmith,
What I left out was that the spa panel is down stream of the dryer 30amp breaker ( I'm using the HD panel just for the GFCI). When I short one hot to ground the 50 amp pops. I assumed it was because the GFCI caught the fault before the circuit breaker because the 30 amp breaker remained closed.
But I agree with you this really isn't a test of the GFCI. It could be the 50amp breaker is just faster than the 30 amp.