120/240 volt service 208/120 back up genset

[Saturn_Europa]

I work for an international NGO as an electrician.

We are building a small hospital in Haiti. The city power to the site is 120/240 single phase. Its the same thing you would see at 99.5% of any residential service in the US. There is NO 3 phase equipment. We will of course need back up power.

We are having a difficult time finding a 65 kva 120/240 volt single phase generator. It is much easier to source a 208/120 volt 3 phase generator. We have been discussing this as a work around.

I would use 3 phase panels in the hospital.

We use 4 pole transfer switches. On the city power side of the transfer switch you would jump L2 - L3 to power the missing phase. There is no three phase equipment.

When purchasing equipment for the hospital we would make sure that it is dual rated for 208 volts and 240 volts.

1: Are there any huge safety issues with doing this? I would of course still balance the loads on the city power L1 and L2.

2: Are 3 phase generators more efficient than single phase generators as far fuel consumption.

3. How bad is it to run a 3 phase generator on single phase? I am assuming it would cause lots of vibration and stress on the fly wheel.

 

[kwired]

I work for an international NGO as an electrician.

We are building a small hospital in Haiti. The city power to the site is 120/240 single phase. Its the same thing you would see at 99.5% of any residential service in the US. There is NO 3 phase equipment. We will of course need back up power.

We are having a difficult time finding a 65 kva 120/240 volt single phase generator. It is much easier to source a 208/120 volt 3 phase generator. We have been discussing this as a work around.

I would use 3 phase panels in the hospital.

We use 4 pole transfer switches. On the city power side of the transfer switch you would jump L2 - L3 to power the missing phase. There is no three phase equipment.

When purchasing equipment for the hospital we would make sure that it is dual rated for 208 volts and 240 volts.

1: Are there any huge safety issues with doing this? I would of course still balance the loads on the city power L1 and L2.

2: Are 3 phase generators more efficient than single phase generators as far fuel consumption.

3. How bad is it to run a 3 phase generator on single phase? I am assuming it would cause lots of vibration and stress on the fly wheel.

If you use three phase panels you would need to utilize only the two busses that also connect to utility power or else anything on third bus only works while on standby.

At very least you need to increase generator capacity and only utilize two of the three phases. That won't hurt a thing, just will be less efficient fuel wise because of the increased size. (you will only be utilizing 2/3 capacity of the unit max)

 

[hillbilly1]

I think using three phase panels would be the wrong approach, I would set three single phase panels, balance the load between them relatively close, each with their own single phase transfer switch. If automatic, you may have an isssues with the voltage sense, as it may not transfer seeing only 208. (I have done it with 208 single phase utility input with a 240 volt generator, Generac’s transferswitch didn’t care) If manual. It wouldn’t be a problem. Use a wireway, or a panel board off the generator to split the three phase feed into three single phase feeds with single phase disconnects.

 

[EC Dan]

I think using three phase panels would be the wrong approach, I would set three single phase panels, balance the load between them relatively close, each with their own single phase transfer switch. If automatic, you may have an isssues with the voltage sense, as it may not transfer seeing only 208. (I have done it with 208 single phase utility input with a 240 volt generator, Generac’s transferswitch didn’t care) If manual. It wouldn’t be a problem. Use a wireway, or a panel board off the generator to split the three phase feed into three single phase feeds with single phase disconnects.

That's what I was thinking as well. Would a set of three autotransformers work here to boost each panel L-L voltage to 240 V or would it result in a L-N voltage higher than 120 V?

 

[hillbilly1]

That's what I was thinking as well. Would a set of three autotransformers work here to boost each panel L-L voltage to 240 V or would it result in a L-N voltage higher than 120 V?

Yes, it would result in a high line to neutral voltage, so I wouldn’t go that route. If line to line voltage did matter, three 208 to 120/240 volt single phase transformers could be used, but would be expensive. Your best hope is that the equipment will accept 208 and 240. Another issue may be if they have a UPS or UPC, it will definitely hate the 208 if it is set up for 240, unless they are smaller 120 volt units.

 

[kwired]

That's what I was thinking as well. Would a set of three autotransformers work here to boost each panel L-L voltage to 240 V or would it result in a L-N voltage higher than 120 V?

Will result in ~138/240 volts.

IF there is only 120 volt loads involved no need to boost line to line volts. Loads will all still see 120. Can not reduce neutral like you possibly can with 120/240. Same load on both ungrounded lines results in same amount of current also on the neutral with the 120 volt phase angle.

If there is line to line loads, it will depend if they are acceptable to operate at 208 and at 240, along with whether a diminished capacity at 208 is acceptable for specific loads.