Feeding a single-phase panel bus with 120-V

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oldsparky52

Senior Member
There was a discussion today as to whether it is permissible to install a subpanel and feed it with 120-V (feed the bus with 120). The neutral bar might or might not be hooked up. The purpose would be to use 2-pole breakers to break the neutral of a 120-V circuit (in lieu of a switched neutral breaker).

Nobody thought it would pass inspection, but everyone agreed it would do what is intended (break the neutral along with the hot).

We figured the AHJ would say the panel is designed to have 2 phase conductors on the bus, but is that a reason to not allow it?

What are your thoughts on whether it should be allowed or not.
 

JFletcher

Senior Member
Location
Williamsburg, VA
Off the top of my head, it seems okay to me as long as no AFCI or GFCI breakers are involved, and the calculated load doesnt exceed what the main panel can handle. Why you'd want to do it is another matter...
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Other than possibly usage of equipment, it will not violate anything if the breakers have internal common trip.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
There was a discussion today as to whether it is permissible to install a subpanel and feed it with 120-V (feed the bus with 120). The neutral bar might or might not be hooked up. The purpose would be to use 2-pole breakers to break the neutral of a 120-V circuit (in lieu of a switched neutral breaker).

Nobody thought it would pass inspection, but everyone agreed it would do what is intended (break the neutral along with the hot).

We figured the AHJ would say the panel is designed to have 2 phase conductors on the bus, but is that a reason to not allow it?

What are your thoughts on whether it should be allowed or not.

The only objection might be if the lugs are labeled L1 and L2, or something else that indicates they are line voltage. You want to hook N to one of these lugs. I don't recall off hand if the lugs are labeled or not.
 

kwired

Electron manager
Location
NE Nebraska
The only objection might be if the lugs are labeled L1 and L2, or something else that indicates they are line voltage. You want to hook N to one of these lugs. I don't recall off hand if the lugs are labeled or not.

Does L1/L2 designations automatically mean both are ungrounded?

Not all grounded conductors are "neutral" either. Take a corner ground system or a two wire secondary for example.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I see it as a potential problem for some future person, as in ASSuming the 2 pole breakers mean it is 240V, or ASSuming adding a single pole breaker will give you 120V and tying the N of that circuit to ground instead of noticing that the other bus is N. Sure, a qualified electrician SHOULD understand whatever signage you put into it, but not everyone who works on panels is a qualified electrician, unless maybe this is an industrial site with work rules and enforcement.

Why are you so focused on breaking the neutral on everything in that panel?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Does L1/L2 designations automatically mean both are ungrounded?

I don't know. part of why I brought it up.

Not all grounded conductors are "neutral" either. Take a corner ground system or a two wire secondary for example.

In this case it would probably be a N though, although I suppose it could be fed from an SDS 2 wire xfmr. Seems unlikely though.

It would certainly work. Whether there is an issue that might cause it to not pass inspection is something else.
 

kwired

Electron manager
Location
NE Nebraska
Why are you so focused on breaking the neutral on everything in that panel?

Seems he has something that requires opening the neutral as he would need switched neutral breakers if he doesn't do what he is proposing here. Possibly circuits to 120 volt fuel dispensers or something of that nature.
 
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