power up refrigeration unit

[david luchini]

Since we're talking a refrigeration unit, I see no problem with the #6 secondary wire OR the manufacturer's required 60amp breaker. The running amps of the unit are well within the ampacity of the #6, and the 60 amp breaker can provide the required local disconnecting means.

I see no reason to over think this.

mxslick, the transformer secondary conductors MUST be protected in accordance with 240.21(C). The #6 secondary has an ampacity of 55 amps. The 60 amp breaker will NOT protect the #6 secondary conductors per 240.21(C)(2). The #6 will be fine from the c/b to the motor, however.

Volta is correct that the primary breaker can be sized up to 250% or 90A. Of course it is not required to be that large. Since the secondary breaker is limiting the secondary current to 83%, I'd use a smaller primary feeder. Maybe a 60A primary breaker (completely arbitrary) with 3#6+#10G feeder the transformer, 3#4+#8 bj to the 60A secondary breaker, and 3#6+#10G from the breaker to the motor.

 

[mxslick]

mxslick, the transformer secondary conductors MUST be protected in accordance with 240.21(C). The #6 secondary has an ampacity of 55 amps. The 60 amp breaker will NOT protect the #6 secondary conductors per 240.21(C)(2). The #6 will be fine from the c/b to the motor, however.

Volta is correct that the primary breaker can be sized up to 250% or 90A. Of course it is not required to be that large. Since the secondary breaker is limiting the secondary current to 83%, I'd use a smaller primary feeder. Maybe a 60A primary breaker (completely arbitrary) with 3#6+#10G feeder the transformer, 3#4+#8 bj to the 60A secondary breaker, and 3#6+#10G from the breaker to the motor.

David, you are right and wrong at the same time. :grin:

The 60 amp breaker is fine for the motor load, which in this case IS the only load on those conductors. So you are right.

Code allows the next highest rated breaker for conductor protection (I don't have my book handy but I'm sure Bob or someone will cite the section) so a 60 amp breaker is considered o.k. for protecting the #6 conductors, especially if the conductor has 90c insulation.

The #4's to the breaker are totally unnecessary in this case.

Now, if we were feeding a subpanel with additional loads from the transformer, I can see a point to using the #4's to the breaker, but I still believe the Code would allow the #6's.

I simply see no need to over-engineer this thing. As long as it's Code-Compliant and won't have voltage drop issues let it go.

Have a good weekend all, off to the Renaissance Festival for the weekend!

 

[david luchini]

David, you are right and wrong at the same time. :grin:

The 60 amp breaker is fine for the motor load, which in this case IS the only load on those conductors. So you are right.

Code allows the next highest rated breaker for conductor protection (I don't have my book handy but I'm sure Bob or someone will cite the section) so a 60 amp breaker is considered o.k. for protecting the #6 conductors, especially if the conductor has 90c insulation.

The #4's to the breaker are totally unnecessary in this case.

Now, if we were feeding a subpanel with additional loads from the transformer, I can see a point to using the #4's to the breaker, but I still believe the Code would allow the #6's.

I simply see no need to over-engineer this thing. As long as it's Code-Compliant and won't have voltage drop issues let it go.

Have a good weekend all, off to the Renaissance Festival for the weekend!

mxslick, I'm afraid it is you who are wrong about the #4's to the breaker. Code allows the next breaker size for devices rated 800Amps or less, per section 240.4(B). However, the code says that the protection of the transformer secondary conductors shall be as specified in 240.21(C)(1) thru (C)(6). It also says that the provisions of 240.4(B) shall NOT be permitted for transformer secondary conductors. (See 240.21(C)).

The 60Amp c/b does NOT provide protection to the #6 (55 ampacity) conductors per the NEC.

Enjoy the Renaissance Festival.

 

[zip1]

David, you are right and wrong at the same time. :grin:

The 60 amp breaker is fine for the motor load, which in this case IS the only load on those conductors. So you are right.

Code allows the next highest rated breaker for conductor protection (I don't have my book handy but I'm sure Bob or someone will cite the section) so a 60 amp breaker is considered o.k. for protecting the #6 conductors, especially if the conductor has 90c insulation.

The #4's to the breaker are totally unnecessary in this case.

Now, if we were feeding a subpanel with additional loads from the transformer, I can see a point to using the #4's to the breaker, but I still believe the Code would allow the #6's.

I simply see no need to over-engineer this thing. As long as it's Code-Compliant and won't have voltage drop issues let it go.

Have a good weekend all, off to the Renaissance Festival for the weekend!

This point confuses me. If you use the 60c rating for terminations rated 100a or less (assuming the terms aren't rated higher,) does the insulation temperature make a difference in the circuit?

 

[david luchini]

This point confuses me. If you use the 60c rating for terminations rated 100a or less (assuming the terms aren't rated higher,) does the insulation temperature make a difference in the circuit?

No, the insulation temperature of the conductor doesn't make a difference. If the terminals are rated for 60 deg, then whether the insulation if 90 deg, 75 deg or 60 deg, you still have to use the 60 deg ampacity column. See NEC 110.14(C)(1).

That is why the #6 secondary conductors from the transformer to the 60A c/b would be too small. The #6 only has an ampacity of 55 from the 60 deg column. Per 240.21(C), the next size circuit breaker is NOT allowed to protect the secondary conductors. You must go to #4 (70A) from the transformer to the 60A c/b in order to be code compliant. It seems kinda silly, since #6 would be fine from the c/b to the motor, but that's what the code requires.

 

[LarryFine]

No, the insulation temperature of the conductor doesn't make a difference. If the terminals are rated for 60 deg, then whether the insulation if 90 deg, 75 deg or 60 deg, you still have to use the 60 deg ampacity column. See NEC 110.14(C)(1).

You can start with the 90 degree column before derating, if the insulation type allows, as long as your final value doesn't exceed the 60 degree rating.