Undersizing breakers?

[kjroller]

I have a 25kva transformer 480 to 240 single phase. On the secondary it feeds 1awg wire to a main 125 amp breaker. When I do the calc for OCPD 25000÷480=104 ×1.25= 130 I get 130 amps so 150 amp is next biggest .Why is it ok or not ok to undersize the breaker to 125 amps instead of going to 150 amps?

Is this because the 125% rule is for sizing the primary side ?

 

[kjroller]

I have a 25kva transformer 480 to 240 single phase. On the secondary it feeds 1awg wire to a main 125 amp breaker. When I do the calc for OCPD 25000÷480=104 ×1.25= 130 I get 130 amps so 150 amp is next biggest .Why is it ok or not ok to undersize the breaker to 125 amps instead of going to 150 amps?

Is this because the 125% rule is for sizing the primary side ?

I believe I answered my own question with Article 240.21 (C) (6), 2

It states "secondary conductors shall terminate on a breaker or set of fuses that limit the load current to not mor than the conductor ampacity that is permitted by 310.14

 

[wwhitney]

The secondary conductors on a transformer only have to be sized for the calculated load. And if there are no continuous loads, there's no 125% factor in that calculation.

So a 125A breaker on the secondary is fine, as it will support up to 100A of continuous loads (would would be 24 kVA on the transformer, you would just be slightly underutilizing the transformer if all your loads are continuous) or up to 125A of non-continuous loads, more than the 104A of load the transformer can supply.

Cheers, Wayne

 

[kjroller]

The secondary conductors on a transformer only have to be sized for the calculated load. And if there are no continuous loads, there's no 125% factor in that calculation.

So a 125A breaker on the secondary is fine, as it will support up to 100A of continuous loads (would would be 24 kVA on the transformer, you would just be slightly underutilizing the transformer if all your loads are continuous) or up to 125A of non-continuous loads, more than the 104A of load the transformer can supply.

Cheers, Wayne

So lets say there is only one continuous load (There is many however for this example lets say there is only one) if I do the calculations I need to use the 125% to calculate the conductors

 

[wwhitney]

So lets say there is only one continuous load (There is many however for this example lets say there is only one) if I do the calculations I need to use the 125% to calculate the conductors

Correct. If you have 100A (or less) of continuous load, you can use a 125A secondary breaker. If you have 101-104A of continuous load, you'd need to use a 150A secondary breaker. If you have more than 104A of continuous load, your transformer is too small.

Cheers, Wayne

 

[kjroller]

Correct. If you have 100A (or less) of continuous load, you can use a 125A secondary breaker. If you have 101-104A of continuous load, you'd need to use a 150A secondary breaker. If you have more than 104A of continuous load, your transformer is too small.

Cheers, Wayne

So if I had to use the 150 amp breaker I would have to upsize the wire as well so the overcurrent device is less amps then what the wire is rated for?

 

[infinity]

So if I had to use the 150 amp breaker I would have to upsize the wire as well so the overcurrent device is less amps then what the wire is rated for?

If the secondary conductors terminate into the 150 amp breaker then the conductors must be rated for at least 150 amps.

 

[kjroller]

The secondary conductors on a transformer only have to be sized for the calculated load. And if there are no continuous loads, there's no 125% factor in that calculation.

So a 125A breaker on the secondary is fine, as it will support up to 100A of continuous loads (would would be 24 kVA on the transformer, you would just be slightly underutilizing the transformer if all your loads are continuous) or up to 125A of non-continuous loads, more than the 104A of load the transformer can supply.

Cheers, Wayne

One more question if the load would be more then the transformer could supply why do you ever use the 1.25 multiplier on the secondary side wouldn't you just size the wire directly to the amps from the kva calculation being 104amps or is it smart to leave the breaker loaded at 80%? I understand why you would for the wire but if it cant ever reach that amperage on the overcurrent device what is the point?

 

[wwhitney]

One more question if the load would be more then the transformer could supply why do you ever use the 1.25 multiplier on the secondary side wouldn't you just size the wire directly to the amps from the kva calculation being 104amps or is it smart to leave the breaker loaded at 80%?

The 1.25 multiplier for continuous loads is (solely) a reflection of the limitations of OCPD. The OCPD is tested as part of its listing in free air to hold continuously at its rating at 40C ambient. But when the OCPD is in an enclosure, which is how we use it, it may not hold continuously at its rating, due to impaired heat dissipation compared to free air. [Unless additional testing of the OCPD is done inside a designated enclosure, and if that combination passes, the combination is 100% rated, so no multiplier applies.]

However the transformer rating is to my understanding a continuous rating. So your 25 kVA transformer can supply 104A at 240V continuously. Because of the breaker limitation, doing that will require a 125A (up to 100A continuous) or 150A (101-104A continuous) breaker on the secondary.

Cheers, Wayne

 

[jim dungar]

So if I had to use the 150 amp breaker I would have to upsize the wire as well so the overcurrent device is less amps then what the wire is rated for?

The conductors and OCPD should be sized the same. There is no NEC requirement for 'wire' to be larger than its overcurrent device.

 

[kjroller]

I have a 25kva transformer 480 to 240 single phase. On the secondary it feeds 1awg wire to a main 125 amp breaker. When I do the calc for OCPD 25000÷480=104 ×1.25= 130 I get 130 amps so 150 amp is next biggest .Why is it ok or not ok to undersize the breaker to 125 amps instead of going to 150 amps?

Is this because the 125% rule is for sizing the primary side ?

25000/240 is 104 my bad