Job Forman

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ketoe

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I need to size a Transformer for a building that has 480 volt 3 phase AC condenser and air handler, both have a nameplate rating of 8.4 amps but the building has a 120/208 3 phase 150 amp panelboard, Ac guys set wrong units so now we have to change circuits for units, panel has plenty of spares , only using about 6- 20 amp circuits, please advise.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
At 8.4 amps each you are looking at a total load of about 15kva but with motors involved I'd most likely go with a 30 kva (next standard size).
 

ketoe

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At 8.4 amps each you are looking at a total load of about 15kva but with motors involved I'd most likely go with a 30 kva (next standard size).

the 8.4 amps includes the moter, these are small ac units, I am more concerned about overcurrent protection for the two units with adding a small transformer, what are my options, can I go to J box with 480 circuits and feed the ac"s and use the overcurrent protection of the transformer? Or on the secondary side go to fusable disconnects
 
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iwire

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Massachusetts
the 8.4 amps includes the moter, these are small ac units,

I am sure Gus understood that, what he is saying is that when supplying a motor load with a transformer you do not want to cut it close or you may have trouble getting the motor to start.

The inrush / starting current is much higher than the marked motor load and with a small transformer that can cause a lot of voltage drop, so much so the motor won't start.

I am more concerned about overcurrent protection for the two units with adding a small transformer, what are my options, can I go to J box with 480 circuits and feed the ac"s and use the overcurrent protection of the transformer? Or on the secondary side go to fusable disconnects

You are going to have to set up an entire SDS, along with the over current protection requirements of article 240 for conductors and article 450 for transformers you need a grounding electrode system.

Are you going to use a delta to delta transformer with a 480 volt corner grounded output or are you going to use a delta to wye transformer and just ground the neutral?
 
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