• We will be performing upgrades on the forums and server over the weekend. The forums may be unavailable multiple times for up to an hour each. Thank you for your patience and understanding as we work to make the forums even better.

Calculating the Kw of a XFMR for a panel schedule.

Merry Christmas

Hale5761

Member
Location
Virginia
Occupation
Electrical Superintendent
I'm doing a loose look at a temporary service in which I have a 150 Kva and a 300 Kva XFMR both 480V feeds. I need to put the circuits in a panel schedule but not sure how much Kw to carry for them. The amps off the chart seem high.
 

JoeStillman

Senior Member
Location
West Chester, PA
What will the transformers feed? If it's only temporary service, just use the loads that are on the transformers. For permanent installations, I always try to allow upstream capacity for the transformers to be loaded all the way up in the future.
 

Hale5761

Member
Location
Virginia
Occupation
Electrical Superintendent
They both feed panels. The 300 Kva XFMR has a load of just under 500 amps, and the 150 has a 200-amp load. Also consider I was very conservative on my loads. For example, construction trailers with a 70-amp panel I allotted 33 amps, I know that's probably twice what I need. What I did was take the load of each panel, add 25% and carry that back as my XFMR load.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
I'm doing a loose look at a temporary service in which I have a 150 Kva and a 300 Kva XFMR both 480V feeds. I need to put the circuits in a panel schedule but not sure how much Kw to carry for them. The amps off the chart seem high.
150kVA is ~180A @ 480/277V; 300kVA is about 360A.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have no idea where you came up with your numbers but I kind of agree that the NEC load calculations are very conservative so for a temporary service I would not be opposed to something more realistic.

Since you did not say I am guessing that the transformers are 480 V primary and 208-120 secondary.

A construction trailer allotted 33 A at 208 V 3 phase is 12 kW which seems more than adequate to run an electric heater or A/C, a coffee pot, and some lights and receptacles. Maybe not suitable for the three EVs they plan to charge up from it though.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Why would you run 480 to a construction trailer?
I have no idea, but 300kVA and 150kVA at 208/120V is about 1250A total; that seems like a lot of current for a construction trailer. But I don't work in construction, so I dunno about that, either.
 
Last edited:
Top