120/208 generator as stand-by for 120/240 residential service

[kingpb]

For situations where you are feeding, say, an apartment sub panel fed from a 208Y/120 service.
But I think we need clarification from on the proper way to express this. Is it 120/208 or 120/208Y? I'll sure Jim Dungar will have the answer.

According to ANSI C84.1, Note (d) the correct way is 120/208Y for a single phase, 3 wire system.

 

[GoldDigger]

According to ANSI C84.1, Note (d) the correct way is 120/208Y for a single phase, 3 wire system.

It is not exactly a single phase system. L1 and L2 will be 120 degrees apart. The fact that you do not install any multiphase loads (single phase L1 or L2 to N and L1 to L2 loads only) does not make it, IMHO a single phase system. Others may differ on this point.
It is certainly not a three phase system either though because of the missing phase.

One critical way in which it differs from a single phase 120/240 3 wire system is that in 120/208Y the neutral in an MWBC will generally be current carrying, even with balanced L to N loads.

 

[kwired]

It is not exactly a single phase system. L1 and L2 will be 120 degrees apart. The fact that you do not install any multiphase loads (single phase L1 or L2 to N and L1 to L2 loads only) does not make it, IMHO a single phase system. Others may differ on this point.
It is certainly not a three phase system either though because of the missing phase.

One critical way in which it differs from a single phase 120/240 3 wire system is that in 120/208Y the neutral in an MWBC will generally be current carrying, even with balanced L to N loads.

Seems kind of redundant to me to requirem the "Y" to be in there. Only way I know of to have 208/120 or 120/208 would be for the source to be wye or an electronically derived equivalent, if someone mentions either voltage combination whether we have single or three phase it automatically registers in my mind as originating at a three phase source. And though it is possible to have a so called "open wye" it just doesn't really happen. If the load is there you will be subscribing to full three phase service, if limited three phase load is there you will get open delta derived service. If you only need two ungrounded and a grounding conductor and are the only customer on the transformer you will get it from a single phase transformer in almost all instances.

Most any other instance where you have two ungrounded and a grounded conductor that is 208/120 or 120/208 will be feeders originating from a source that is three phase, and technically I guess in the case of "open wye" the source still has to be three phase or it is not really possible to get the 120 degree phase angle.

Someone go ahead and shoot that down

 

[kingpb]

I'd say y'all need to take it up with ANSI/IEEE.

But, until they change the standard there is nothing you can say that will keep me from using the prescribed designation.

It is, what it is!

 

[kwired]

I'd say y'all need to take it up with ANSI/IEEE.

But, until they change the standard there is nothing you can say that will keep me from using the prescribed designation.

It is, what it is!

Nothing wrong with taking a vow to use the correct terms, I try to do so myself as much as possible. But you still need to take what others say carefully as not everyone knows what is the correct term in all cases, some may know but were taught wrong at one time and is just a bad habit to use what they were once taught. If someone has 120 and 208 grouped together, most of us usually have about a 90%+ chance of correctly understanding what they have there, and any other information given may help straighten out that 10% uncertainty.

 

[Birken Vogt]

It is not exactly a single phase system. L1 and L2 will be 120 degrees apart. The fact that you do not install any multiphase loads (single phase L1 or L2 to N and L1 to L2 loads only) does not make it, IMHO a single phase system. Others may differ on this point.
It is certainly not a three phase system either though because of the missing phase.

It (could) be a three phase system, if you installed an open delta isolation transformer setup to feed a three phase load....

Any time you have phase displacement other than 180?, you can get three phase if you really want to.

But we know what you meant.

 

[JoeStillman]

Another consideration: is the generator a 208/120V generator, or is it a '12 lead' generator connected for 208/120V? The installation electrician may be able to configure the generator for straight 120/240V single phase operation.

-Jon

This is your best option.

A 12-lead alternator can be reconnected for 120/240V in a double delta configuration for approximately 50% of the kVA output. However, the available kW is determined by the engine, so you will still have 45 kW available. Ordinarily, the engine doesn't have enough power to overload the alternator unless the power factor is very low (say 80%), but in this case you will be able to overpower the alternator if the load is too high. You will need to change the output breaker.

The 150A ATS is big enough for the kVA, just use 2 of its poles. But it's voltage relay in the generator start circuit may be 3-phase, so it would have to be replaced. It may work fine in manual mode, or, if the voltage relay is 3-phase, it could prevent you from transferring to a single-phase source.

IMHO, 26 kVA is plenty for a typical house, air conditioning, well pump and all -- just dont try to bake a turkey.

If you can't reconnect the alternator as single phase, you should pass on this opportunity. 208V is not enough for your well pump and you don't want to place that at risk.