Sizing a Service...When do you go up in Size?

S

smithjo30

Member
Location
Wisconsin
Occupation
Electrical Engineer
Hello all,

I am in charge of sizing a service for a Fire Station that also has an EMS department in it. Currently the building runs on 600 AMP 3 Phase 120/208V. The calculation i did with our new upgrades we are introducing to the build, brings the calculation to 732 AMPS. Keep in mind this is everything on at the same time. Heat, A/C etc...

I compared Heat and A/C (HVAC), and dropped one of the loads, as in theory, you shouldn't be running both. In doing so, that brought the calculation down to 641 AMPS. That still also has everything else on at the same time, which is highly unlikely as well. I'm really trying to figure out what is the most realistic calculations or AMP Draw they would really have that would justify the increase in service size. Is there any other calculations I should be looking at before I make the confirmation that they do in fact need to go to a bigger system size of 800 AMPS? I have a call out to the utility company for their 15 minute interval power logs.

Also, if i have a 600 AMP panel, should it only be filled to 80% (480 AMPS)?

Any suggestions on how I can make my math as realistic as possible?

Thanks all!
 
First thing is that after the load calculation is performed and you find the minimum service size it doesn't matter if loads are going to be on at the same time. The calculated load is the number that you use. If it's 600 amps then you need a 600 amp service. If it's 650 amps then the 600 amp service is too small. There is no 80% factor either. The calculated load number that is derived from the parameters of Article 220 is all you need.
 
You need to do a standard load calculation, or you can look at your existing demand for the last year if your power company can provide 15 minute interval load data. Real usage is typically less than calculated. If you use real, you still need to add 25% to it and your new load to see what the new service requirement is.

So visit article 220 and look for the standard and 220.87 existing demand based calculation choices.
 
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