copper chopper
Senior Member
- Location
- wisconsin
On the new hospital where doing, we have a 150 kva xmer that has 412 amps on the secondary, how can the enginneers have us hooking this up to a panel with a 800 amp main. what gives?????
how is this possible ???
- Thread starter copper chopper
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Your 800 Amp main is probably total connected load, or calculated load. The XF is diversity factor rated. The demand factor is probably around 38-42%.
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IF the transformer protected on the primary at 125% and the secondary conductors are installed per 240.21(C) (sized 800 amps or more, etc), it is likely not a violation. (assuming your post means the secondary breaker is 800 amps)
copper chopper
Senior Member
- Location
- wisconsin
augie you are correct after finding the other half of this ripped print I now see the primary at 225 amps... thanks..
Primary protection is only part of what is required...
What is the secondary configuration (e.g. 208/120V 3? 4W)?
Only "Conductors supplied by the secondary side of a singlephase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, ..." [240.4(F)]
beanland
Senior Member
- Location
- Vancouver, WA
125% Secondary, 250% Primary
125% Secondary, 250% Primary
I understand that a 480VD-208Y/120V transformer is to be protected up to 125% on secondary and up to 250% primary. The secondary protection is required because it is a delta-wye 3-phase transformer. A 150kVA transformer has a full load rating of 417A. 125% means 520A rounded up to nearest standard size. You can have an 800A bus with a 600A main but not an 800A main. The 480V OCPD can be 451A, rounded up to 500A. However, a better choice is to have the 480V OCPD slightly larger than the 208V OCPD once reflected through the transformer. 600A * 208V / 480V = 260A so a 300A 480V OCPD is a good choice.
125% Secondary, 250% Primary
I understand that a 480VD-208Y/120V transformer is to be protected up to 125% on secondary and up to 250% primary. The secondary protection is required because it is a delta-wye 3-phase transformer. A 150kVA transformer has a full load rating of 417A. 125% means 520A rounded up to nearest standard size. You can have an 800A bus with a 600A main but not an 800A main. The 480V OCPD can be 451A, rounded up to 500A. However, a better choice is to have the 480V OCPD slightly larger than the 208V OCPD once reflected through the transformer. 600A * 208V / 480V = 260A so a 300A 480V OCPD is a good choice.
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