To pie or not to pie

[Mike McCallister]

Trying to determine required ampacity for 3 phase, 240 volt service

70% of loads are 1 phase 240 volt A/C equip. remainder is 110 volt recepts and lights.

Plan to use a 3 phase load center to feed all 240 volt A/C equip. and a 1 phase load center for all 120 volt circuits.
Feed load centers from MDP with 1-3 pole breaker for A/C panel and 1- 2 pole breaker for lighting and recept. loads

I thought I could calculate load for 3 phase A/C panel as follows

KVA 68,000
.............. = amps or ................. = 163.58 amps

240 volts X 1.732 415.68


engineer insist that all loads on 3 phase delta connected sevice must be calculated as if it was 1 phase, since it is an unbalanced system.

KVA 68,000
....... = amps or .................. = 282.33 amps

240 volts 240

 

[kingpb]

Please confirm that your 3-phase system is a 240V/120V Delta system, the 110V you mention does not compute correctly.

Also, what are the ratings of the loads that you are connecting?

Not understanding what your 68,000 number is representing.

 

[Mike McCallister]

Yes 3 phase system is 240/120 delta
36 pieces of A/C equip. 30- 5.5 amp, 5- 11 amp, 1-24 amp
68,000 is connected VA

 

[Smart $]

If all your loads are 1?, I don't believe there is any major advantage to having a 3? panel and a 1? panel (other than appearance). Just install a 200A 3? panel and be done with it (unless there's something you're not telling us).

Edit, didn't see your last post until after posting mine (I was away from my 'puter in post mode). Going to need a fair amount of breaker space.. 36 2-pole is 72 spaces, not counting the 120V spaces. You could install a 200A MCB panel with feed thru lugs and a 200A MLO panel and then sub a 100A 120/240 1? 3W panel of that.

 

[Smart $]

Yes 3 phase system is 240/120 delta
36 pieces of A/C equip. 30- 5.5 amp, 5- 11 amp, 1-24 amp
68,000 is connected VA

With the number of 1? 240V loads you have, you can balance out the system by leaving the A to C loads lighter to account for the 120V loads.