Transformer Calculations

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Dexie123

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Have a 112.5 kva 480/240 volt 3 phase transformer on the job I working on. The secondary has 500's terminating into a 400amp switch. Is this permissable? I determine a 240v sec as this:

112,500 kva / 240 * 1.73 as 271 amps.

What should the ampacity be?

Thanks
 
The conductor can be sized based on the load. 271 is the full load current at the transformer secondary. The 500 kcmil conductors cannot be protected by a 400 amp OCPD in this application. They must be protected at or below there ampacity as shown in Table 310.16. 240.4(B) does not apply.
Don
 
don_resqcapt19 said:
The conductor can be sized based on the load. 271 is the full load current at the transformer secondary. The 500 kcmil conductors cannot be protected by a 400 amp OCPD in this application. They must be protected at or below there ampacity as shown in Table 310.16. 240.4(B) does not apply.
Don
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Ummm... he said, "a 400amp switch." Doesn't necessarily mean a 400 amp OCPD.
 
Smart $ said:
Ummm... he said, "a 400amp switch." Doesn't necessarily mean a 400 amp OCPD.
Click to expand...


It could mean that they have no overcurrent protection at all.
 
Hmmm...given the voltage it could be a delta to delta transformer and not even require protection on the secondary side.
 
It's actually a step down wye to wye transformer. There is a fused disconnect right next to it and the fuses are 400 amps. From the switch it goes to a MDP right next to it with 6 switches. 4 rated at 100amps 2 at 125amps feeding panels on the floor (I don't know what the load calcuations on the panel are).... Not sure what the primary feed consists of .....

So Don if 240.4(B) does not apply are you in the opinion that the largest wire size and fuses that should be used be 250amps?
 
Dexie123 said:
It's actually a step down wye to wye transformer. There is a fused disconnect right next to it and the fuses are 400 amps. From the switch it goes to a MDP right next to it with 6 switches. 4 rated at 100amps 2 at 125amps feeding panels on the floor (I don't know what the load calcuations on the panel are).... Not sure what the primary feed consists of .....

So Don if 240.4(B) does not apply are you in the opinion that the largest wire size and fuses that should be used be 250amps?
Click to expand...

Dexie, If this is a 4-wire secondary xfrmr @ 208 volt, then secondary protection is 112500/360 = 312.5 x 125% =390 amps max. A higher rating not exceeding the next higher rating is 350 amps. The fuses should be set @ 350 .
Rick
 
?*!whatthe? said:
As long as you have Primary protection at 125% of xfmr rating you do not need secondary protection.
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That is true as far as the transformer, the secondary conductors and the panel will need protection.
 
RUWired said:
Dexie, If this is a 4-wire secondary xfrmr @ 208 volt, then secondary protection is 112500/360 = 312.5 x 125% =390 amps max. A higher rating not exceeding the next higher rating is 350 amps. The fuses should be set @ 350 .
Rick
Click to expand...

Thanks RUWired.....

This brings up another question that I'm not sure I understand about transformers. If the computed ampacity of a transfomer is 312.5amps but you fuse it for 390 and furthmore set the fuses to 350 what if the loads are at full throttle say pulling 350. How can the transformer handle a load that it's not designed to do? I understand that your fuses and OCPD will and that the wire will be protected at it's ampacity but how does the transformer put out more that it should or is designed to do?
 
Dexie123 said:
If the computed ampacity of a transfomer is 312.5amps but you fuse it for 390 and furthmore set the fuses to 350 what if the loads are at full throttle say pulling 350. How can the transformer handle a load that it's not designed to do?
Click to expand...

Dexie, Transformers are rated for 100% continuous (112500 kw ).Breakers are rated for 80% continuous.( 350 x 80% = 280 amps x 360 = 100800 kw.)If your panel is running more than 80% continuously it will need to be upgraded eventually.
Rick
 
Also, This is why the 400 amp fuses are to big for the xfrmf. If you kept the 400 amp fuses in and ran it up to 80% continuously, your kw load would be 115200kw. That would be 2700 watts over what the xfrmr is rated for, and that would slowly cook the xfrmr to failure.
Rick
 
Dexie,
So Don if 240.4(B) does not apply are you in the opinion that the largest wire size and fuses that should be used be 250amps?
Click to expand...
In the case of this installation the maximum overcurrent protective device rating permitted for the 500 kcmil conductors is 350 amps. The maximum permitted OCPD for the secondary of the transformer is 400 amps assuming that the primary OCPD is between 125% and 250% of the rated primary current. You have to install smaller fuses (350A) or increase the wire size to 600 kcmil.
Don
 
don_resqcapt19 said:
Dexie,

The maximum permitted OCPD for the secondary of the transformer is 400 amps assuming that the primary OCPD is between 125% and 250% of the rated primary current. Don
Click to expand...

Don, What is allowing you to go to 400 amps.Note 1 table 450.3(B) limits you to 125%,no higher.A higher setting is anywhere between 312.5 and 390.Or is this industry standard?
Rick
 
Rick,
See note 1 after table 450.3(B).
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
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The 125% value is 390.8 amps. The note clearly permits you to use the next standard rated OCPD above 390.8 amps and that would be a 400 amp OCPD.
The 125% value only applies to the transformer protection. The secondary conductors must be protected at or below their ampacity.
Don
 
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Ok I understand what you guys are saying here. Let me ask the same question a bit of a different way. If you don't mind.....


My transformer is rated at 112.5kva. Being it's a 208 3ph transformer the load ampacity works out to 312 amps. (112500/208*1.73).

We can rate the switch or fused disconnect in accordance to 125% for continuous duty. That brings us up to 390 amps. Being 390 is more than the 380 allowed for 500's MCM according to 310.16 you can either up the wire to 600 or the breaker down to 350 and leave the 500's.

Think I'm right so far...... NOW.....

What am I allowed to load the transformer's load to exactly? In other words lets say I have this transformer that is being feed off the load side to a MDP with a set of 500's fused at a disconnect at 350amps. The disconnect is feeding 6 motors NON continous and each motor draws 57.5amps which is a total of 345amps. Our transformer is only going to give us 312 amps. However, we've still installed this transformer according to code (I think). Breaker is within the 125% and wire is protected at it's ampacity. Is this acceptable or as RUWired states the transformer would be sure to "slowly cook"?

Thoughts?
 
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