1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
(OP)
Looked at a facility that has 208Y/120 power throughout.
Their automatic genset is a 120/240 1Φ set that only powers selected circuits.
The way this is accomplished is by interrupting or transferring phases A and B before they hit the subpanel. Phase C is going straight from the utility-supplied breaker to the subpanel without interruption.
All loads on this circuit are 1Φ, so no risk of single-phasing any 3Φ motors.
But it occurs to me that under certain conditions, a heavy heating element from A-C or B-C could sufficiently energize C (reference neutral) to cause a hazard to personnel on line.
When utility returns but time has not elapsed for retransfer, voltages A-C and B-C could be wild and fluctuating.
Voltage of 208V equipment connected A-C or B-C would remain >120V during the duration of the outage.
I don't know where to look for the code violation here, but there must be one somewhere.
Their automatic genset is a 120/240 1Φ set that only powers selected circuits.
The way this is accomplished is by interrupting or transferring phases A and B before they hit the subpanel. Phase C is going straight from the utility-supplied breaker to the subpanel without interruption.
All loads on this circuit are 1Φ, so no risk of single-phasing any 3Φ motors.
But it occurs to me that under certain conditions, a heavy heating element from A-C or B-C could sufficiently energize C (reference neutral) to cause a hazard to personnel on line.
When utility returns but time has not elapsed for retransfer, voltages A-C and B-C could be wild and fluctuating.
Voltage of 208V equipment connected A-C or B-C would remain >120V during the duration of the outage.
I don't know where to look for the code violation here, but there must be one somewhere.





RE: 1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
I don't believe that the code allows a panel to be fed from two sources let alone two sources that may be at differing frequencies.
How large is the genset? When I was doing a lot of generator work, it was common for sets above about 15 KVA to be re-connected and re-rated three phase sets.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
I am going to work on them to see if we could possibly change the subpanel feeder circuits to single phase.
But it would be nice to have a code citation.
RE: 1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
If you can gain access to the NEC in .pdf format do a search on "transfer".
The AHJ may support your position.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
I found nothing else in the code under "transfer" that seems to apply.
RE: 1ph Genset and Transfer Switch Switching 2 Legs of 208Y/120
Bill
--------------------
"Why not the best?"
Jimmy Carter