Doing an estimate for a customer and want to make sure I'm on the right page and see if there are any other options. Customer wants a generator installed, however from the generator to where the transfer switch will go is 250'. It's a 24KW generator with a 100 AMP breaker. My calculations have brought me to need 1/0 for 100 amps at that distance and keeping it at 3% voltage drop. Using THWN. This would be running in EMT through attic, through a workshop and outside. I've never ran long runs like this as my jobs have usually been small residential homes. I'll take any help I can get, and take any suggestions that you may have. One question that I have is does the 1/0 (If I am correct on my calculation) need to run all the way to the generator, or can I change the size at some point, if so, where? how would I calculate that?
Wire size calculation from generator to transfer switch
- Thread starter ricksan76
- Start date
- Status
- Occupation
- Licensed Electrician
Voltage drop is based on the actual load not the size of the breaker. 250' is not that far and unless you have some unusually heavy loads the generator won't see any where near 100A, even if it did #3 copper keeps you right about 5% voltage drop which is more than reasonable since most anything can function no problem with 10% voltage drop. I would be quite comfortable running #3Cu. If I was going to run 1/0 it would be aluminum.
Logan2112
Member
- Location
- Klein Products
- Occupation
- Journeyman Electrician
I have a question for you Sir Dave. If someone was going to install a back-up generator (600amp) with an ATS(600amp) but needed to feed ( 3 ), 200 amp panels, (single phase with main breakers) Would you need a Main Dist. Panel with breakers in it between the panels and the ATS or would a fusible Disconnect suffice? Or would you need three ATSs one for each panel? Note the panels are in different locations and will have to be fed from a central location. There will also be MMSs installed for each panel's AC units.
- Occupation
- Licensed Electrician
How are the panels fed or designed to be fed now?
Logan2112
Member
- Location
- Klein Products
- Occupation
- Journeyman Electrician
At the moment they are Fed from 2 different 600 amp Disconnects which are supplied by the Outside Utility service. They will need to be taken out and re-piped into a central location because some of the panels in those disconnects will not be applied to the back-up power plan. At the moment I believe there are approx three or four panels landed in each disconnect.
Makes perfect sense. Thank you. I appreciate that. I've always been stuck at keeping things at 3%. I appreciate your time.
Logan2112
Member
- Location
- Klein Products
- Occupation
- Journeyman Electrician
At the moment they are Fed from 2 different 600 amp Disconnects which are supplied by the Outside Utility service. They will need to be taken out and re-piped into a central location because some of the panels in those disconnects will not be applied to the back-up power plan. At the moment I believe there are approx three or four panels landed in each disconnect.
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
If I recall right the total voltage drop can be 5% total. I think it's 3% for branch circuits and 2% for feeders although I never can remember which one is which.
Could you not run aluminum SER cable? It would save a bunch of $$$$ especially the way wire costs are up????
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
100 amps breaker, 5 seconds max clearing time= 800 amps.
120/800= 0.15 loop ohms
Me personaly I would go with #3 copper phase and neutral #4 copper ground will do the job for 250 feet.
120/800= 0.15 loop ohms
Me personaly I would go with #3 copper phase and neutral #4 copper ground will do the job for 250 feet.
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