Transformer Sizing Calculation help

[goodoboy]

hello,

My first time performing transformer and want to make sure I am not missing anything regarding code or sizing.

Feeding 480V 3 phase A/C unit from a 480V 3 phase power source.

I need transformer for a single phase 120V 8 amp load (general lights and electronics).

Calculations:

(8x120)/(1000) = .96kVA

So I select a 1kVA single phase transformer?

Questions:

1. Is there any concern with using the 480V 3 phase for the a/c unit for the transformer as well to feed the 120V 8amp single phase load?

Thanks

 

[goodoboy]

hello,

My first time performing transformer and want to make sure I am not missing anything regarding code or sizing.

Feeding 480V 3 phase A/C unit from a 480V 3 phase power source.

I need transformer for a single phase 120V 8 amp load (general lights and electronics).

Calculations:

(8x120)/(1000) = .96kVA

So I select a 1kVA single phase transformer?

Questions:

1. Is there any concern with using the 480V 3 phase for the a/c unit for the transformer as well to feed the 120V 8amp single phase load?

Thanks

Just for more clarification so you understand my case.

I am wring 480V from the MCC to the panel in field. Then the 480V 3 phase will power up an A/C unit and it will be connected to transformer to power the 120V 8 amp load.

Thanks for any help.

 

[kwired]

Make sure you have enough capacity in the circuit to supply both loads.

The circuit essentially becomes a feeder and you need to supply a branch circuit overcurrent device at some point for the AC unit, and in this case you likely need an overcurrent device for the transformer also.

 

[goodoboy]

Make sure you have enough capacity in the circuit to supply both loads.

The circuit essentially becomes a feeder and you need to supply a branch circuit overcurrent device at some point for the AC unit, and in this case you likely need an overcurrent device for the transformer also.

Thank you kwired,

What do you mean by capacity?

Yes, the 480V 3 phase is coming from a MCC that has a fuse and fuse disconnect. Then the 480V is termintad in a panel in the field to power the A/C unit and then connected to a transformer for the 120V 8 amp single phase load. The transfomer has 10amp fuses on the primary side. And the transformer is rated up to 25kVA.

Thanks for the help.

 

[kwired]

What do you mean by capacity?

An AC circuit only needs to be sized by the MCA and other data on the nameplate.

You now have additional load you are feeding. @ 480 volts your added load is only about 2 amps, lets say the AC unit needs a minimum ampacity of 19 amps, you now have raised the total load supplied to 21 amps, you could end up needing a different conductor size in some cases. Overcurrent protection will likely still work with an added 2 amps, but there could be instances where it needs changed also.

 

[goodoboy]

An AC circuit only needs to be sized by the MCA and other data on the nameplate.

You now have additional load you are feeding. @ 480 volts your added load is only about 2 amps, lets say the AC unit needs a minimum ampacity of 19 amps, you now have raised the total load supplied to 21 amps, you could end up needing a different conductor size in some cases. Overcurrent protection will likely still work with an added 2 amps, but there could be instances where it needs changed also.

Thanks for help,

The AC unit requires MCA of 15amps. So my conductor is sized based on this load and the voltage drop due to distance. I am also using 15amp fuse at the MCC for the circuit for overcurrent protection

My questions is do,

1. I am currently using 15amp fuses at the MCC feeder for the 480V 3 phase, and with this new load of 17amps (15 + 2 ), will I need increase my fuse size?

Thanks

 

[augie47]

If the AC unit calls for a maximum OCP device sized at 15 amps, you can not go above 15 amps.
IF you wish to increase the fuse size at the MCC to higher than 15, your conductor would need to have adequate amapcity and you would need to add 15 amp protection somewhere prior to the AC unit.
Depending on the units actual load and other factors, you might get by with adding the transformer and not increasing the fuse size.

 

[kwired]

What I have been getting at is you first need enough ampacity from the conductors, you then have two different utilization equipment that eventually will require different things like different overcurrent protection, individual protection, individual disconnecting means or other individual characteristics.

Transformers are a little different creature and rules can be a little different than for other equipment in general, they do require overcurrent protection and do require a disconnecting means, but are not exactly required to be on an individual branch circuit either, but the AC unit in just about all cases must definitely be on an individual branch circuit. That does not mean that there can not be a feeder between a distribution panel and a disconnect with required overcurrent protection that is located adjacent to the AC unit. In that case the branch circuit is fairly short in length and the rest is a feeder.

Air conditioning equipment often requires an overcurrent device with a higher rating than the ampacity of the branch circuit conductors. Quite often around 1.5 to 2.5 times the full load current in order to allow for motor starting, so I wouldn't expect to see a MCA of 15 and max OCPD of 15 on the same unit. I don't know if it is done but could see them marking one with MCA of 15 just because you need a 14AWG copper conductor minimum, even though MCA should really be based on actual loads and possibly say 11, 12 13.5 or whatever actual value is supposed to be instead of just calling it 15. In all likelihood a unit with MCA of 12 amps probably will call for a overcurrent device of 20 or 25 amps though.


Getting back to the OP - more details may be needed to verify all code compliance, but from a "will it work perspective" chances are you have enough conductor capacity to handle both loads, and you likely have high enough overcurrent protection that there will be no "nuisance tripping".

 

[goodoboy]

If the AC unit calls for a maximum OCP device sized at 15 amps, you can not go above 15 amps.
IF you wish to increase the fuse size at the MCC to higher than 15, your conductor would need to have adequate amapcity and you would need to add 15 amp protection somewhere prior to the AC unit.
Depending on the units actual load and other factors, you might get by with adding the transformer and not increasing the fuse size.

Thanks,

If I keep the 15amp fuse, what about the 17 amp (15amp = A/C unit and 2amp = transformer load). Are you saying that maybe the A/C unit will not run at 15amp during normal operation?

Thanks

 

[GoldDigger]

Are you saying that maybe the A/C unit will not run at 15amp during normal operation?

Thanks

Yes.
The 15 amp breaker (if that it the specified Max OCPD on the nameplate) will be sized to cover the motor starting situation and allow at least 25% extra for continuous operation if appropriate. The FLA of the motor will be something less than that, and the motor overloads that are part of the equipment will protect at the normal FLA of the motor.

A typical motor will specify a minimum circuit amperage required and a maximum overload protective device rating. The two will not necessarily be the same.

 

[goodoboy]

What I have been getting at is you first need enough ampacity from the conductors, you then have two different utilization equipment that eventually will require different things like different overcurrent protection, individual protection, individual disconnecting means or other individual characteristics.

Transformers are a little different creature and rules can be a little different than for other equipment in general, they do require overcurrent protection and do require a disconnecting means, but are not exactly required to be on an individual branch circuit either, but the AC unit in just about all cases must definitely be on an individual branch circuit. That does not mean that there can not be a feeder between a distribution panel and a disconnect with required overcurrent protection that is located adjacent to the AC unit. In that case the branch circuit is fairly short in length and the rest is a feeder.

Air conditioning equipment often requires an overcurrent device with a higher rating than the ampacity of the branch circuit conductors. Quite often around 1.5 to 2.5 times the full load current in order to allow for motor starting, so I wouldn't expect to see a MCA of 15 and max OCPD of 15 on the same unit. I don't know if it is done but could see them marking one with MCA of 15 just because you need a 14AWG copper conductor minimum, even though MCA should really be based on actual loads and possibly say 11, 12 13.5 or whatever actual value is supposed to be instead of just calling it 15. In all likelihood a unit with MCA of 12 amps probably will call for a overcurrent device of 20 or 25 amps though.


Getting back to the OP - more details may be needed to verify all code compliance, but from a "will it work perspective" chances are you have enough conductor capacity to handle both loads, and you likely have high enough overcurrent protection that there will be no "nuisance tripping".

Thanks,

I have to re-read the post and respond tonight. My brain is really fried right now from reading this concern throughout the day.