Breaker sizing for 750VA transformer

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reaper63

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Alright, so I have a 750VA transformer in a control panel that I designed and built. Everything works great, except that every now and then when the customer turns on the system it pops the primary side breaker on the control transformer. The panel has a 125 amp main breaker installed that is operated from a remote handle.

VPrimary = 480V
VA Primary = 750VA
Calculated IPrimary = 1.5625 amps (not a 3 phase xfmr)
VSecondary = 120V

Sizing would be 1.5625 x 1.25 = 1.9531 (2 amp breaker size)
I'm using an A-B supplementary breaker, 2 pole, 2 amp, B-curve.

This is all correct, yes?
Any ideas?
 
reaper63 said:
Alright, so I have a 750VA transformer in a control panel that I designed and built. Everything works great, except that every now and then when the customer turns on the system it pops the primary side breaker on the control transformer. The panel has a 125 amp main breaker installed that is operated from a remote handle.

VPrimary = 480V
VA Primary = 750VA
Calculated IPrimary = 1.5625 amps (not a 3 phase xfmr)
VSecondary = 120V

Sizing would be 1.5625 x 1.25 = 1.9531 (2 amp breaker size)
I'm using an A-B supplementary breaker, 2 pole, 2 amp, B-curve.

This is all correct, yes?
Any ideas?
Click to expand...

I would use Table 450.3(B) I think. 1.56 x 300% = a 5 amp, not 2 amp, primary side fuse or breaker.

Ben
 
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