Art. 700 Generator

Status
Not open for further replies.
S

SpiderMT

Member
I had a job kinda dropped in my lap yesterday that involves swapping out an existing transfer switch and tying it into the new generator, the EC who was going to do the job had a family emergency and had to leave town.

I picked up a set of plans from the GC and head down to the site to check actual conditions and see what materials we'll need, the swap was supposed to be today. Anyway, I get down there, open the plans and start checking things out. The plans single line state the service is 120/240V 1PH, the generator spec'ed out say the same but the service and existing transfer switch is 208Y/120V.

I call the GC and tell him about the discrepancies and it goes on up to the EE on the job. I get a call back today that EE says it is okay to use that generator, he'll draw something up.He wants to swap out the 3PH loadcenter for 1PH, all the branch circuits are 120V, and use what he spec'ed wrong.

I don't like the idea of feeding a panel used for emergency standby in an elderly care facility with a 240V generator that is utility fed with 208V. I also see possible transfer issues with the switch swapping back over to utility power if it only sees 208V on the normal side and is running 240V on the emergency side. I can't find anything in Art. 700 to prevent this install (I don't think), anybody else have any ideas where this may be prevented by code?
 
It could be fine.

If the ATS has the ability to be adjusted for 208 on one side and 240 on the other

If any utilization equipment that now runs on 208 and will be supplied with 240 at other times is rated for both voltages.

An electric dryer or range is often rated to run on either, however 208 volt motors are not usually rated to run on 240.
 
Thanks for the reply. I'll have to check and see if the ATS can be adjusted to work with both voltages. There are no 208V loads on the panel so there shouldn't be an issue with that.
 
I agree with you it would make much more sense to have ordered the correct generator but if that is a done deal you could always offer to install (as an extra) a pair of buck boost transformers set up to buck the voltage down between the generator and the transfer switch.

This may be required if the transfer switch can not be set up to deal with the 240/120 supply. Many ATS that have been set up for 208 would see the 240 as generator over voltage and not transfer the load onto the generator.

A lot depends on the sophistication and capabilities of the existing transfer switch.
 
That was part of my concern whether it would transfer at all, or transfer back with the differnce in voltages. I hadn't considered the buck/boost, that may be an option. The transfer switch that will be installed is set up for the generator, 120/240V, not 208V. The existing transfer switch is old and it is time to retire it.
I believe I should be getting something from the engineer today so I will see what he ultimately comes up with and go from there. Thanks for your help.
 
iwire said:
---you could always offer to install (as an extra) a pair of buck boost transformers set up to buck the voltage down between the generator and the transfer switch.---
Click to expand...
Bob -
I'm not real clear on this. Are you suggesting the two b-b xfm to be on the gen output - dropping the gen output from 120/240 to 104/208?

cf
 
Cold Fusion said:
Bob -
I'm not real clear on this. Are you suggesting the two b-b xfm to be on the gen output - dropping the gen output from 120/240 to 104/208?

cf
Click to expand...

I thought there was a way using two auto transformers to derive 208/120 from 240/120.

I certainly could be mistaken.
 
iwire said:
I thought there was a way using two auto transformers to derive 208/120 from 240/120.---
Click to expand...
None I know of - but that could easily be only one of the many things I don't know of.

Another thought:
I have seen small generators (~25kw) that are 12 lead. They can be connected 120/240, 1ph; 208/120, 3ph; 240D. I haven't seen many, but that's probably because I don't deal much with small gens.

Spyder -
Take a look at the gen nameplate. Maybe you (and the customer) will get lucky - you would be a hero.

cf
 
Yeah but the trans switch is spec'd out for the gen. Your proble might be the other way...it not wanting to back to utility at all because of the lower voltage. I bet the trans switch can be setup to work.

Do you have the new trans switch yet?
 
Cold Fusion said:
Another thought:
I have seen small generators (~25kw) that are 12 lead. They can be connected 120/240, 1ph; 208/120, 3ph; 240D. I haven't seen many, but that's probably because I don't deal much with small gens.
Click to expand...

I ran into one of these recently while doing a 'favor job'

DoubleDelta003.jpg


DoubleDelta002.jpg
 
I will look to see if it is a 12 wire genny or not. The nameplate state 120/240 1PH so I kind of doubt it. If I do get that lucky I am buying a lottery ticket on my way home.

The ATS is onsite. I am not sure what the range for over/under voltage is on the ATS. It would have to be set transfer to Normal at 15% under voltage for it to work, I think.
 
Just my two cents. If the transfer switch is not a closed transition switch, it should'nt be a problem. If it is then the voltage acceptance range can be changed or if the generator has voltage range adjustment, it can be adjusted.

Rick
 
It is not closed transition, so no problem there. I received a drawing from the engineer a little while ago. He wants me to add an isolated lug block into the existing panel fed from the transfer switch and feed Phases A and C from one leg and Phase B from the second leg. I don't see why it won't work this way now, just not the way I would do it if given the choice. The engineer will have to sign off on, and take resposibility for this install as a field change on an OSHPD job so if it doesn't work as designed, it is on him. I'm just an installer now.

I appreciate all the advice.

Spider
 
Status
Not open for further replies.
Top