Overcurrent Protection

[davidray4999]

A 150KVA transformer set on a pad to supply power to a new restaurant. From the transformer the wire is run parallel with 3/0 to a 600AMP main disconnect with 600AMP fuses mounted on outside of building. From the disconnect it runs to a single 350KCMIL 400AMP disconnect in panel box mounted inside electrical room. Would the 600AMP or 400AMP disconnect be the overcurrent protection? What is the sufficiant wire sizes for this service and the overcurrent protection requirement? This service is also 3phase.

My interpretation is that:
75C (degrees) at 600AMP you would use parallel 350KCMIL or 1500KCMIL and for 400AMP you would use parallel 3/0 or 600KCMIL.
90C (degrees) at 600AMP you would use parallel 300KCMIL or 500KCMIL and for 400AMP you would use parallel 3/0 or 500KCMIL.

This is probably very confusing and easier to explain, but if you understand any help would be very appreciative.

 

[augie47]

A 150KVA transformer set on a pad to supply power to a new restaurant. From the transformer the wire is run parallel with 3/0 to a 600AMP main disconnect with 600AMP fuses mounted on outside of building. .

How many 3/0. Only two per pahse ? That would be a problem.

From the disconnect it runs to a single 350KCMIL 400AMP disconnect in panel box mounted inside electrical room.

again, undersized

Would the 600AMP or 400AMP disconnect be the overcurrent protection? What is the sufficiant wire sizes for this service and the overcurrent protection requirement? This service is also 3phase.

My interpretation is that:
75C (degrees) at 600AMP you would use parallel 350KCMIL or 1500KCMIL and for 400AMP you would use parallel 3/0 or 600KCMIL.
90C (degrees) at 600AMP you would use parallel 300KCMIL or 500KCMIL and for 400AMP you would use parallel 3/0 or 500KCMIL.

This is probably very confusing and easier to explain, but if you understand any help would be very appreciative.

For practical purposes forget the 90? column, unless your equipment is 90? rated (doubtful). The numbers you give will work, but you could possibly use smaller conductors if you met the load and used the "next higher size" rule, but, if I correctly understand the set-up, the present conductors are insufficient.

 

[petersonra]

Does the transformer belong to the utility?

I don't think you can use the 90 Deg column except for derating purposes unless the terminations of everything is rated for 90 Deg.

I would not be in favor of using 1500 KCMil wire at all. It is just too unwieldy.

I might be inclined to put 400A fuses in the 600A disconnect.

 

[iwire]

I would not be in favor of using 1500 KCMil wire at all. It is just too unwieldy.

I agree, it is also often less expensive to run multiples conduits and conductors then to run very large copper

 

[augie47]

The lamp above my head flashes..
Thanks Bob
As Bob mentioned, if the wire from the transformer to the disconnect is utility supplied, then they size per their rules, not NEC.

 

[davidray4999]

The wires from the transfromer is not utility that is the customers side. He has paralleled the wires from transformer to disconnect but undersized for 600AMP. He could use the 400AMP fuses and then the wires would work, but not from the disconnect to panel box which should be 500KCMIL. This wouldn't even be a problem if he didn't oversize his service. He has the 600AMP service but will never use it. Most equipment being used is gas.