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277-120 buck transmormer

1216 Views 4 Replies 4 Participants Last post by  pfactor2, 
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277-120 transformer

I'm installing a stand-alone irrigation service for one 5HP centrifugal force motor, plus a irrigation clock. The service is 277/480 so I will be installing a 1KVA transformer, 277 primary 120 secondary for the clock. The branch circuit OLOC protection before transformer will be a fusible safety switch. Question is do a need OLOC protection on the secondary side to the clock.
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pfactor2 said:
I'm installing a stand-alone irrigation service for one 5HP centrifugal force motor, plus a irrigation clock. The service is 277/480 so I will be installing a 1KVA transformer, 277 primary 120 secondary for the clock. The branch circuit OLOC protection before transformer will be a fusible safety switch. Question is do a need OLOC protection on the secondary side to the clock.
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If your transformer is single phase side grounded (120 only) or delta delta you do not need secondary protection as long as the primary OCPD rating matches the secondary side per volt amp ratio.

In English terms if a secondary side protection device needs to be 100 amps at 240 volts you can omit it if the primary side OCPD is 50amps at 480 volts.
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Bad Electrician
And your post header says buck, this is a step down not a buck transformer.
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NEC 450.3(B)
In this case, a 1 KVA transformer has a 480 volt primary rating of 2.08 amps and a secondary rating, at 120 volt, of 8.33 amps. So, if your primary fusing is 3 amps (2.08 x 1.25 = 2.6, round up to next standard size ), you would not need secondary fusing to be code compliant.
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P
Its only one 277 phase for the clock. Secondary is 2.5amps x 120 = .3KVA
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