transformer OCPD

Status
Not open for further replies.
S

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
Looking at installing a European-sourced machine in the shop. It nameplate states 40KW, 64A, 50/60Hz, 3-ph. We're debating the relative merits of a 45KVA vs 75KVA transformer. I'm leaning toward the 75KVA unit, since we may add another 400V machine in the future, and the larger transformer will run cooler.

We have a 240V corner-grounded service in the building, and a spare 225A breaker in the switchgear.

Assuming I go with the 75KVA trafo, do I need additional OCPD beyond the 225A breaker on the primary side of the trafo (240V) if I include secondary OCPD (400V)? I'm looking at an 80A breaker for the secondary (64A x 120% = 76.8A --> 80A). Probably a small 480V main breaker panel, so I have room for additional 400V loads in the future. Do I size the 400V panel's main breaker to the output amperage of the trafo?

I don't do feeder and OCPD sizing for transformers very often, and I'd rather get this right the first time.


Thanks,

SceneryDriver
 
SceneryDriver said:
I'm looking at an 80A breaker for the secondary (64A x 120% = 76.8A --> 80A). Probably a small 480V main breaker panel, so I have room for additional 400V loads in the future. Do I size the 400V panel's main breaker to the output amperage of the trafo?

I don't do feeder and OCPD sizing for transformers very often, and I'd rather get this right the first time.


Thanks,

SceneryDriver
Click to expand...

Where did the 64 amps come from
 
Last edited:
2014 NEC Table 450.3(B) Max Rating or Setting of Overcurrent Protection for Transformers 1000 volts or less (as a percentage of Transformer-Rated Current)

............ Primary .................................................... Secondary

Primary only protection, 125% ................................ Not required

Primary and secondary protection, 250%....................... 125%



Will the piece of equipment be, ever be, started under load?
If yes, 125% may not pass the primary high inrush current and trip the breaker open.
 
SceneryDriver said:
Looking at installing a European-sourced machine in the shop. It nameplate states 40KW, 64A, 50/60Hz, 3-ph. We're debating the relative merits of a 45KVA vs 75KVA transformer. I'm leaning toward the 75KVA unit, since we may add another 400V machine in the future, and the larger transformer will run cooler.
We have a 240V corner-grounded service in the building, and a spare 225A breaker in the switchgear.
Assuming I go with the 75KVA trafo, do I need additional OCPD beyond the 225A breaker on the primary side of the trafo (240V) if I include secondary OCPD (400V)? I'm looking at an 80A breaker for the secondary (64A x 120% = 76.8A --> 80A). Probably a small 480V main breaker panel, so I have room for additional 400V loads in the future. Do I size the 400V panel's main breaker to the output amperage of the trafo?
I don't do feeder and OCPD sizing for transformers very often, and I'd rather get this right the first time.
Thanks,
SceneryDriver
Click to expand...

Primary side 240 volts
Secondary side 400 volts

transformer over load protection:

240 x 1.73 (412.5)
75000 / 412.5 = 181.81 x 1.25=227 amps

or

181.81 x 2.50 = 454.52 Amps

400 x 1.73 = 692
75000 / 692= 108.38
108.38 X 1.25 = 135.37 amps

in addition to the above you need to protect the conductors
 
Jamesco said:
2014 NEC Table 450.3(B) Max Rating or Setting of Overcurrent Protection for Transformers 1000 volts or less (as a percentage of Transformer-Rated Current)

............ Primary .................................................... Secondary

Primary only protection, 125% ................................ Not required

Primary and secondary protection, 250%....................... 125%



Will the piece of equipment be, ever be, started under load?
If yes, 125% may not pass the primary high inrush current and trip the breaker open.
Click to expand...

It's a thermoforming machine for plastic. It shouldn't be started under any appreciable load (no large motors, so far as I've been told). 125% of 180A @ 240V works out nicely to 225A, so I wouldn't need to supply OCPD for the secondary, which would save some $$. How likely is magnetizing inrush on a 75KVA transformer to trip a 225A breaker?

SceneryDriver
 
SceneryDriver said:
It's a thermoforming machine for plastic. It shouldn't be started under any appreciable load (no large motors, so far as I've been told). 125% of 180A @ 240V works out nicely to 225A, so I wouldn't need to supply OCPD for the secondary, which would save some $$. How likely is magnetizing inrush on a 75KVA transformer to trip a 225A breaker?

SceneryDriver
Click to expand...

You should be fine.
 
Status
Not open for further replies.
Top