120 volt subpanel?

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pete m. said:
Are the panels and breakers listed to be connected in this manner? If not you would have a violation of 110.3(B)

Pete
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You may be right on this but I have not been able to find specific documentation:(. As for the panes they are general NQODs with 2 pole type QO breakers.
 
Apparently instead of a 240/120 residential system the EE is wiring this place as a 120/60 system. Then where you'd normally have 240V receptacles used he's got 120V receptacles.

I think the EE out-clevered himself. There are some special circuit designs for low noise environments such as microphones. They use 120/60 with the ground attached in the center. This guy attached the ground on a phase leg which defeats the whole purpose of using 120/60.

There are whole discussions and arguments about 120V circuit design regarding noise, harmonics, and voltage-ground that are argued by supporters of 120/60 systems. Other than voltage-ground I don't know how proven any of those arguments may be. But this guy hasn't done that. The ground is not centered but still at one end of the phase. To achieve what I believe he thinks he achieved the X2-ground has to be lifted and reattached as neutral-ground.

My understanding is this is considered desirable for microphones. But it would make lamp shells hot at 60V. But only if done properly :)
 
240.22(1) says that in order for an over current device to open a grounded conductor, the device has to open all circuits and be designed so that no pole can operate independently.

The last part may be the 'gotcha'. Most 2 pole breakers can be used to protect a single pole. I really don't know any that can't, so there must be special breakers designed not to, and if you don't have those breakers, it seems to me that would be a violation.
 
K8MHZ said:
My concern would be that the breakers may not operate properly with the neutral opening simultaneously with the hot.

I think it would be better to swing the neutral from the breaker to a solidly connected neutral bar.
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I could be wrong but I do not think it would operate any differently, a phase to neutral fault would be like a phase to phase on a 240 volt and a phase to ground would be the same as 2 pole protecting a MWBC with only a fault on one pole. The breakers are 2 pole QOs with internal common trip on them.

Fixing this would take only an easy day but the owner is opinionated and would rather not spend the money unless its a real danger:slaphead:
 
The violations I see according to the drawing is the lack of primary and secondary protection at the transformer. The other question that comes to mind that others have mentioned is the panel listed for use in this way??

The use of 2 poles breakers in the manner shown is needed for many grounded circuits in many classified areas. (must simultaneously disconnect the grounded conductor). Most 2 pole breakers are listed for 120/240.
 
WIMaster said:
The violations I see according to the drawing is the lack of primary and secondary protection at the transformer. The other question that comes to mind that others have mentioned is the panel listed for use in this way??

The use of 2 poles breakers in the manner shown is needed for many grounded circuits in many classified areas. (must simultaneously disconnect the grounded conductor). Most 2 pole breakers are listed for 120/240.
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The transformers have primary protection, I omitted it for simplicity. The 25kva units are protected by a fused disconnect with 50amp 600v fuses.
According to the NEC 240.21, a 2 wire secondary does not need protection because the primary gives this the needed protection:

(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer
having a 2-wire (single-voltage) secondary, or a
three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary, shall be permitted to be
protected by overcurrent protection provided on the primary
(supply) side of the transformer, provided this protection is in
accordance with 450.3 and does not exceed the value determined
by multiplying the secondary conductor ampacity by
the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other
than delta-delta, 3-wire) transformer secondary conductors
are not considered to be protected by the primary overcurrent
protective device.

The way I see it is a current of 50amps on the primary gives a secondary current of 200amps at 120 on the secondary 3/0 copper conductors. The panel buss is 225 amps rated so I think this part is safe. Calculated load comes out as about 135 amps worst case scenario.

As for the panel listing Im clueless I can not find any documents saying yes or no.
 
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