Generator Voltage and Normal Service Voltage, Do they ha

[jschultz]

I have a job I am currently designing. The em service is a separate feed from the utility. THe feed is a 208/120v single phase.

It seems the generator manufacturers make 240/120V single phase or 208/120V three phase Gensets.

Would any of you mix a 240/120V single phase genset with a normal service of 208/120V. They are both connected to the same ATS. The panelboard is an old plug fuse panelboard that is marked as 208V. So I am not sure if it is actually rated for 240V and the 208 was jsut the service voltage marking.

Personally, I do not think it is a good idea to mix the two voltages. It leads to confusion I believe. I also believe the panelboard should be replaced. There is a utility vault that has 480 and 208 3 phase in it. So I cannot get a 240 volt service without major expense. I can get a new 208/120v 3 phase service tho.
The problem is the owner got a quote from an electrician to provide a solution. Then they hired us to do permit drawings. My design is quite a bit more than the electricians. His generator I feel was undersized to begin with, because the chicago code only allows you to have a max of 80% load on an em ltg generator. But that all depends on what load you assume, which is why my other question about VA/SF for em ltg was asked.
Please give me your opinion and if you have a code section about mixing voltages, I would love to hear it.

 

[wanderer20001us]

Re: Generator Voltage and Normal Service Voltage, Do they ha

I think it would be hard to find an ATS control panel that would accept both voltages and still give you adequate pick-up and drop-out ranges for voltage sensing.

Then you have the problems associated with equipment being subjected to significant voltage differences between the generator and normal service (more than 10%).

Long story short, IMO the two system voltages need to match.

 

[ed downey]

Re: Generator Voltage and Normal Service Voltage, Do they ha

Couldn't You Provide A 3 Phase 120/208V Generator With A Distribution Panel Attached Then Feed The Transfer Switch With A 2 Pole Circuit Breaker For Your 120/208V 1 Phase Loads. This May Cost A Little More But It Is The Only Option I See With The Items You've Listed.
-Ed

 

[steve66]

Re: Generator Voltage and Normal Service Voltage, Do they ha

I'm not sure I like the idea of only having two phases loaded. It may cause problems if monthly testing is required (i.e. you have to have the generator loaded to some percent of rated KVA).

The genset manufacturer may be able to modify the voltage regulator to give the voltage you need. Or they may be able to give custom voltages. For a small generator, I would consider a single phase 208-240 transformer, but that reduces reliability slightly.

Steve

 

[jschultz]

Re: Generator Voltage and Normal Service Voltage, Do they ha

I ended up figuring out why I did not want the two voltages and it was the same reason wanderer stated. I am going to stay with my original design of 208/120V 3phase since I believe it is the only solution that makes sense. The owner will have to pay more for the installation than the original quote by the electrician, but hey that is why they hired us to engineer the job, so that it works. Less money for something that does not work the first time is not less money in the end usually.
I am not going to throw in some extra stuff to try and make it work, it will probably end up being more expensive to try and make it work with adding equipment. I knew there was a reason why I did not want to do it, I was just coming up with blanks previously. Talking to the generator rep about pricing is what eventually made me think about the voltage difference thing. But i am glad to know that someone from the board came up with it also. Thanks all of you that replied.

[ April 07, 2004, 12:38 PM: Message edited by: jschultz ]