575V Transformer

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Kilowatt76

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Rock Hill, SC US
Hi,

We have to add a 15kva, 575V to 480V transformer to one of our standard machines and I'm fusing the primary and secondary. Here are the calculations I used:

15000/600 = 25 amps.

25*250% = 62.5 amps

I would have to go to the next fuse size which is 70 amp for the primary fuses.

The secondary would be as follows:

15000/480 = 31.25 amps

31.25*125% = 39.06 amps

I would have to go to the next fuse size which is 40 amp for the secondary fuses. I just wanted to double check and see if this is correct. 70 amp seems pretty how and now have have to get 100A disconnect instead of our normal 30A.

My next question is, what would the FLA be now for the machine with the added transformer. Does the transformer change my FLA?

Thanks,
Pete Ippolito
 
Kilowatt76 said:
Hi,

We have to add a 15kva, 575V to 480V transformer to one of our standard machines and I'm fusing the primary and secondary. Here are the calculations I used:

15000/600 = 25 amps.

25*250% = 62.5 amps

I would have to go to the next fuse size which is 70 amp for the primary fuses.

The secondary would be as follows:

15000/480 = 31.25 amps

31.25*125% = 39.06 amps

I would have to go to the next fuse size which is 40 amp for the secondary fuses. I just wanted to double check and see if this is correct. 70 amp seems pretty how and now have have to get 100A disconnect instead of our normal 30A.

My next question is, what would the FLA be now for the machine with the added transformer. Does the transformer change my FLA?

Thanks,
Pete Ippolito
Click to expand...
If your application is 3phase your calculations are missing applying the square root of 3 (1.732) to the voltage.

15000/(480x1.732)=18amps

I do not understand your last question. The FLA on your machine/motor is it’s rated current under operation and as seen on the nameplate will increase/decrease with different voltages.
 
IF you elect to use the 250% allowable rating on the primary, the "next size up" allowance does not apply to the primary.
On a 15kva 3 phase, I believe that would limit you to a 35 amp breaker.
 
Dsg319 said:
If your application is 3phase your calculations are missing applying the square root of 3 (1.732) to the voltage.

15000/(480x1.732)=18amps

I do not understand your last question. The FLA on your machine/motor is it’s rated current under operation and as seen on the nameplate will increase/decrease with different voltages.
Click to expand...
The machine without the transformer has an FLA of 13.95 AMPS, 3 ph, 480V. We are sending it to Canada and they require 600V and this is the reason for the added 600V X 480V 3 ph transformer. I just want to make sure I have the fuses sized correctly to UL508A/NEC.
 
Kilowatt76 said:
The machine without the transformer has an FLA of 13.95 AMPS, 3 ph, 480V. We are sending it to Canada and they require 600V and this is the reason for the added 600V X 480V 3 ph transformer. I just want to make sure I have the fuses sized correctly to UL508A/NEC.
Click to expand...
The machine should have a nameplate with the FLA for 600v most likely being less then the 13.95 amps at 480v.
 
575V is the utilization voltage in Canada, ie what motors would be rated for. The distribution voltage is 600V. I see that’s what you used, so that’s good.

I’m not sure, but I BELIEVE that Canada is the same as us in this regard; the 480V side must be EITHER a 480/277V grounded Wye or corner grounded 480V Delta secondary, OR you must provide Ground Fault Monitoring if it is any form of ungrounded secondary.
 
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