120/240 volt 3 phase 4 wire systems load calculation

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Jimmy4645

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Was wondering if you guys can help me determine the demand load amps?


I have a residential multi-family building, 60 residential units at 120/240 volts, single phase totaling 180,000VA on the A,C phases since the B phase is the highleg. All the residential units are fed from a 120/240 volt, single phase meter bank.

For the common area: I have 10,000VA of lights and receptacles on A,C phase. I have a 30HP elevator motor totalling 33,000VA spread out on all three phases. And ten 5KW 240 Volt,single phase wall heaters spread out on all three phases. AB,BC,AC and so on..

What would be the total demand amps if my voltage is 120/240volt 3 phase, 4 wire?

Thanks
 
Jimmy4645 said:
Was wondering if you guys can help me determine the demand load amps?
What would be the total demand amps if my voltage is 120/240volt 3 phase, 4 wire?

Thanks
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You can not calculate the load as 3 phase with this type of service. You may have the basic apt load determined in 220.84. The majority of the load is single phase and connected to A-C. The elevator and heat are connected to A-C-B per your post. Most designers will add the load on A-C to the loads connected to A-C-B so that most of the load ends up on A-C. IMO only the 3phase loads are to be connected to phase B.

danickstr
The phases are not balanced with this type of service. Depending on the utility, you may only have 2 transformers installed instead of 3.
 
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bob said:
IMO only the 3phase loads are to be connected to phase B.
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Actually, any 1ph 240v load w/out a neutral connection can be supplied from any two phases, even across the open side of an open Delta (within ampacity limits, of course.)

You must still use orange insulation or marking everywhere that conductor is accessible.
 
Right or Wrong??

Right or Wrong??

Yes. I de-rated the residential load per NEC to get my VA for the meter bank. .

I'm having a Hugh debate with another engineer in the office. He's telling me that I can take all my loads whether it's single phase (phase-to-neutral), phase-to-phase, three phase loads and divide it by (240 times square root of three) = 415.. Which I think is wrong because most of the load is on the AC phases not on ABC.

My way: is you take all the residential single phase loads and common area loads (lightings & receptacles) and divide it by 240. Then take all my three phase and phase-to-phase loads and divide it by 415. Then you add the amperage together, which gives you your minimum service size @ 120/240V 3phase,4wire.

If you guys agree with me. Is there any literature out there that can back my theory up? Internet or book?

Thanks
 
LarryFine said:
Actually, any 1ph 240v load w/out a neutral connection can be supplied from any two phases, even across the open side of an open Delta (within ampacity limits, of course.)
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Sure but that would require bring 3 phase to each tenant.

You must still use orange insulation or marking everywhere that conductor is accessible.
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As I have seen you say ...... 'don't generalize' ..... many areas use other colors then orange. :smile:
 
Jimmy4645 said:
My way: is you take all the residential single phase loads and common area loads (lightings & receptacles) and divide it by 240. Then take all my three phase and phase-to-phase loads and divide it by 415. Then you add the amperage together, which gives you your minimum service size @ 120/240V 3phase,4wire.
Thanks
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All loads with a neutral are connected to A-C. Any 240 volt load can be connected to AB AC or BC as Larry pointed out but that would require all 3 phase in each apartment per Iwire. It is very likely that the AC loads are much greater that the ABC loads and the loading is unbalanced.
You can not use you associates method with unbalanced loads. Your method takes care of that problem.
 
I would only bring single phase to the residential units A,C,N phases only. Bringing three phases to the apartments is not an option and not allowed here.

Three phase A,B,C,N phases would be used for the common (public) area
 
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Jimmy4645 said:
I would only bring single phase to the residential units A,C,N phases only. Bringing three phases to the apartments is not an option and not allowed here.

Three phase A,B,C,N phases would be used for the common (public) area
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I was just generalizing. :cool:
 
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