Tranformer and Breaker

[hl53us]

I have a job where the engineer is calling for a 37.5KVA transformer: primary 480VAC, stepping down to 120/240v to power up a portable office trailer. I believe the office trailers drawings only about 30 amps max.

It's calling out for a 2-pole, 175A, 480VAC, 65KAIC in the main panel to the transformer. Isn't this too high an amperage. Are there any safety issues?

 

[wbdvt]

At 240V, 37.5kVA transformer full load amps is ~156A. Next standard breaker size is 175A

 

[hl53us]

I thought as far as protecting the primary, the calculation is as follows:

37.5KVA x 1.25 = 48.675, next size up 50a

 

[david luchini]

I thought as far as protecting the primary, the calculation is as follows:

37.5KVA x 1.25 = 48.675, next size up 50a

37.5kVA/480=78.125 x 2.5 = 195.3.

A 175A primary breaker is OK per T450.3(B)

 

[hl53us]

ok, another question. Based on the existing main panel (GE), the max amp 2-pole breaker is 100amp. Would 100amp breaker be sufficient?

 

[david luchini]

ok, another question. Based on the existing main panel (GE), the max amp 2-pole breaker is 100amp. Would 100amp breaker be sufficient?

100A main breaker would be sufficient for a 30A load. Code would allow a breaker up to 200A on the secondary.

 

[kwired]

Why a 37.5 kVA for a 30 amp load? Something existing? Maybe a unit that will be available later in the project for permanent loads?

For this temporary use smallest overcurent device that will hold during energizing of the unit is sufficient, primary will only draw 15 amps at full applied load plus any inefficiencies and reactive current (because of power factor) I bet it works fine on at least 50 amp primary device, maybe even less.

 

[templdl]

Has the transformer's inrush current when the transformer is first energized even being considered here? Basing the pri OCPD on load or cable size isn't going to change what that transformer magnetizing current is.
Being limited to a 100a breaker frame in a panel can be a bit depressing though. A 100at breaker will have a inst trip of 1000aa, maybe ip to 1200a. The question is will the breaker nuisance trip or not. Expect the inrush for a transformer of this size should be about 10x the pri FLA.

 

[don_resqcapt19]

Around here an office trailer will pull very close to the rating of the main breaker in the winter time because of the electric heat and poor insulation in the trailer.

 

[iceworm]

Has the transformer's inrush current when the transformer is first energized even being considered here? Basing the pri OCPD on load or cable size isn't going to change what that transformer magnetizing current is.
Being limited to a 100a breaker frame in a panel can be a bit depressing though. A 100at breaker will have a inst trip of 1000aa, maybe ip to 1200a. The question is will the breaker nuisance trip or not. Expect the inrush for a transformer of this size should be about 10x the pri FLA.

tempdl -
I have not seen any papers on single phase xfm inrush.. I have not seen any testing on single phase xfm inrush. Do you have any data, or seen any testing, or reports of any testing - other than anecdotal?

ice

 

[iceworm]

... 37.5KVA transformer: primary 480VAC, stepping down to 120/240v ...
It's calling out for a 2-pole, 175A, 480VAC, 65KAIC in the main panel to the transformer. ...

As noted the 175A is a fine size.

Following other comments, as for using a 100A:

37.5kva, 480V, 1 ph, FLA = 78A
78A x 1.25 = 98A

Unless the transformer is a known screwball - I'd consider a 100A feed.

ice

 

[kwired]

Has the transformer's inrush current when the transformer is first energized even being considered here? Basing the pri OCPD on load or cable size isn't going to change what that transformer magnetizing current is.
Being limited to a 100a breaker frame in a panel can be a bit depressing though. A 100at breaker will have a inst trip of 1000aa, maybe ip to 1200a. The question is will the breaker nuisance trip or not. Expect the inrush for a transformer of this size should be about 10x the pri FLA.

I tried to address this some. I once used a 25 kVA (480 x 120/240) unit as temp power source for a construction project - because it was what was readlily available at the time though about a 7.5 would have been enough to meet the needs. Hooked it to 10 AWG primary conductors with 30 amp RK5 fuses and never had any problems. Connect it closer the the source and maybe less impedance in supply conductors has more impact on inrush? This one was probably at least 200 feet from the source with a 200 amp feed (probably 250 MCM aluminum) from that source. Primary rated full load would have been about 52 amps.

 

[templdl]

I tried to address this some. I once used a 25 kVA (480 x 120/240) unit as temp power source for a construction project - because it was what was readlily available at the time though about a 7.5 would have been enough to meet the needs. Hooked it to 10 AWG primary conductors with 30 amp RK5 fuses and never had any problems. Connect it closer the the source and maybe less impedance in supply conductors has more impact on inrush? This one was probably at least 200 feet from the source with a 200 amp feed (probably 250 MCM aluminum) from that source. Primary rated full load would have been about 52 amps.

That is what is so intriguing about this stuff is that there are variables that sometimes don't make any sense the causes it to work or not. When you are dealing with a spike of current the affect of the cable and what the breaker actually ends up seeing sometime one has to get lucky.

 

[templdl]

I tried to address this some. I once used a 25 kVA (480 x 120/240) unit as temp power source for a construction project - because it was what was readlily available at the time though about a 7.5 would have been enough to meet the needs. Hooked it to 10 AWG primary conductors with 30 amp RK5 fuses and never had any problems. Connect it closer the the source and maybe less impedance in supply conductors has more impact on inrush? This one was probably at least 200 feet from the source with a 200 amp feed (probably 250 MCM aluminum) from that source. Primary rated full load would have been about 52 amps.

One more thing to consider though.
I was commissioned by a drytype transformer manufacture marketing manager to do a study of the inrushes of transformers and pri OCPDs because of nuisance trips being blamed on a defective transformer. This study resulted in a big spread sheet. At a given KVA inrushes will very greatly from a standard efficiency transformer to a high efficiency, 80, 115, 150deg temp rise, 'K' factors etc. There were instances where you get to the raged edge when using the max rating pri OCPD as allowed by the NEC. But must of the time if you use the max as allowed there should be no nuisance tripping. Some times it may be dependent of how lucky you are if you get an OCPD that happens to be calibrated of the high side as the mag calibration which is often based upon +-20 of 10x the rating of the breaker.