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3 phase power, fuses, and load balance

max_well

Member
looking to get some input from you electric heads out there..
I've got an electrical question concerning 3 phase power. I have an industrial space with a 3 phase service, each line of which is connected through 70 amp fuses above our meter located in the electrical utility room for the whole building . The panel that contains the main breaker is locked (with local electric company lock), so I can't see what the actual service capacity is. The wire that feeds the breaker box in my room is #4 THW, which is rated to carry 85 amps. The run to the main is ~120 feet from my box. I have been planning a 12KW light show, cooled using 2 big (2.5 ton)mini splits, that draw max current of 14.2 amps each at 240v. By my calculations, I can best split and balance the load from lights and a/c to give 65 amps on legs 1 and 3, and 30 amps on leg 2 (all at 240 v), and then have the balance of the available power single pole on leg 2 to run fans, pumps, and other 120v devices (up to 40 amps at 120v, to stay below the 70 amp fuse limit ).
My question is: Does this sound like it is cutting things too close to the fuse ratings? I know that fuses can safely run above their rating for short amounts of time without issue (such that they won't pop when the a/c starts up, which requires lots of extra current ), but is it safe to run near their capacity? I will not have regular access to the utility room where the fuses are located. Would it be safe to change them out for 80 amp fuses considering the wire is rated to carry 85amps on each leg? or do I need to down size everything? any thoughts are welcome and appreciated!
cheers
Max
 

Newd

Active member
Hi Max,

Please don't change the fuses out for larger ones. When electricians install services, they are to code and code is based on safety. It sounds like you're electrically savvy and you've done some research. 85 amps for 4/0 sounds high (I think it's closer to 60). The rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. Balancing the loads is smart and good practice, but I would lower the consumption.

Be safe,

Newd
 

WasntMe

Member
how long is the sub-service panel 4 gauge run?

Inquiry with you Electric company as to your service capacity. "You are planning a potential remodel and need the info for your GC and electrical system designer."
 
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