120VAC 2 wire panelboard?
- Thread starter bozo
- Start date
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I have done it a few times. I don't know of any code stating you cannot do this.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
According to Schneider Electric's FAQ site, "Square D does not offer a 120V load center above 2 spaces."
Dennis, how do you tie the two busses together?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The last one I did I did not tie them together but you could certainly make a splice in the panel and feed both sides. You have to careful not to overload the feeder on these setups.
Thanks for the input. Appreciate the help.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
And that is what the feeder overcurrent device is for.
Most cases when doing something like this you are using a panel with at least 70 amp rating so unless you are using a feeder rated more than that it should not be a problem. You can not supply 2 - 100 amp buses from a 200 amp feeder, or something similar in nature to that.
Nothing wrong that I can see with using one bus for ungrounded the other for grounded conductor and use two pole breakers for all the circuits. Put the bonding jumper in the neutral assembly and use for equipment grounding only. Could (more likely will) be problems if you needed to use GFCI or AFCI breakers though.
Thanks for the input. Is there some advantage to that method (grounded conductor to one bus with 2 pole breakers)? Otherwise think I would prefer to utilize both busses for ungrounded with single pole breakers.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I have never done that before, but it just crossed my mind when I was posting that this idea may work, and I don't really see any problem with it other then the mentioned need for GFCI or AFCI breakers - then it is a problem just to make them work properly. Most times when I have ever used 120 volt only was when it was a separately derived system and maybe only 2 or 3 KVA max @ 120 volt. Number of branch circuits also limited to only 2-4 circuits max as much more than that and the transformer is probably not large enough anyway unless there is for some reason many circuits with pretty light loads. When I have done this there is usually a need for one 15 or 20 amp 125 volt circuit for maintenance and convenience purposes, and maybe a need for some 120 volt special equipment - often just light load for control equipment, or other electronic devices and no real power loading. If much real power loading is involved it is going to be 120/240 system if I have much to say about it.
Thanks for the reply. Haven't investigated how an AFCI works, but if my understanding of how a GFCI works (small differential transformer that detects an imbalance in current between the two conductors) guess I don't understand why at least a GFCI wouldn't function correctly.
Thanks again.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Where you going to hook the neutral lead with either the GFCI or the AFCI is the biggest problem as the neutral in the described situation is coming from one of the busses and not from a terminal bar.
John120/240
Senior Member
- Location
- Olathe, Kansas
Install the neutral to the neutral bus & use every other breaker space. If you were to land
neutral on the other hot bus & use two pole breakers isn't that a violation ?
The only situation I know of where you break the neutral connection is with fuel pumps.
Thanks for the reply. In the configuration you are apparently referring to (one bus ungrounded, the other neutral, with 2 pole breakers) I would think you could utilize a 2 pole GFCI without even wiring the neutral connection (unless it is somehow necessary for the GFCI trip mechanism). It would seem that the diff. transformer wouldn't care if the voltage is 240 or 120.
As for the configuration I suggested (both panel busses ungrounded with single pole breakers and neutral bar wired normally) I would think both the GFCI and AFCI breakers would function normally. Don't see how the zero potential between the two busses (only difference from a normal 120/240 config.) would have any impact on their operation.
Thanks again.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
A dp GFCI needs the neutral from the breaker to operate the electronics. It doesn't need the neutral load if the load is straight 240V but it does need the neutral line connected
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
There is no violation in breaking the neutral as long as you simultaneously break all other lines. A breaker does this. A fused disconnect does this with the operation of the disconnect but if the fuse in the neutral would open there is no gurantee the other fuses will open - therefore you can not use a fuse in the neutral.
That is correct.
Thanks for the info. So in a panel with bus A ungrounded and bus B grounded (as well as the neutral bar) the 2 pole GFCI would only function correctly when placed in spaces where the hot side of the electronics is on the ungrounded bus. Makes sense. Thanks again.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Actually it needs the neutral only for the electronics of the test circuit. A 2-pole GFCI breaker tripping is not dependent on the neutral pig-tail (it is powered by the voltage difference between L1 and L2).
Isn't the test circuit nothing more than a manual shunt between two of the circuit conductors that bypasses the diff. transformer in order to simulate a ground fault and actuate the trip mechanism?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I do not know the actual workings of the test circuit. My answer came directly from a manufacturer's FAQ page.
Very simplistically, what you describe is good for a 1-pole device, but in a 2-pole you need to check both L1->pig-tail and L2->pig-tail, while not 'backfeeding' through the other leg.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I can't tell you exactly how GFCI circuit breakers work, but 120 volt GFCI receptacles typically apply a test load that will draw the necessary 4-6 mA to trip from line side of one conductor to load side of other conductor. The result puts test current through only one side of circuit through the sensing CT and unbalanced current is sensed since both lines of the test current do not pass through the CT. That is also why the test button works even if there is no equipment ground present at the receptacle. I would guess a GFCI breaker performs the test in similar fashion.
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