Secondary Conductors Utility Transformer

[aelec84]

Hello Forum,

I understand that (3) sets of 500kcmil conductors are not protected by a 1200A overcurrent protection device as per NEC 240.4(B) but if they are used as incoming service entrance conductors and terminating on a 1200A, 3P main circuit breaker inside switchboard, will that be allowed? Please advise. Thank you.

 

[wwhitney]

See NEC 230.90(A). Your arrangement does not comply with the test of that section, so do any of the exceptions apply?

Edit: this answer assumes the service point is on the utility side of the 1200A OCPD, so that the 1200A OCPD is the service disconnect.

Cheers, Wayne

 

[Elect117]

If the utility did that, then it doesn't need to met the NEC. They can terminate however many runs they want from their transformer to the utility pull section.

If it is not the utility, then you need to meet the normal articles for services. Typically, with 1200A OCPD, you can not round. So 3x 380A is 1140A and is less than 1200A and therefore insufficient.

 

[infinity]

The only way that would be code compliant is if the POCO has determined that the service point is on the load side of the 1200 amp OCPD.

 

[wwhitney]

The only way that would be code compliant is if the POCO has determined that the service point is on the load side of the 1200 amp OCPD.

Seems like it wouldn't take much motor load for Exception 1 to 230.90(A) to make it allowable.

Cheers, Wayne

 

[aelec84]

See NEC 230.90(A). Your arrangement does not comply with the test of that section, so do any of the exceptions apply?

Edit: this answer assumes the service point is on the utility side of the 1200A OCPD, so that the 1200A OCPD is the service disconnect.

Cheers, Wayne

Yes, the service point is on the line side of the 1200A OCPD, making it a service disconnect. No exceptions apply, but isn't Exception #2 irrelevant here since the conductors aren’t actually protected by the 1200A OCPD? Instead, they just land there, which requires the use of #600 conductors per 230.90 (A).

 

[infinity]

Yes, the service point is on the line side of the 1200A OCPD, making it a service disconnect. No exceptions apply, but isn't Exception #2 irrelevant here since the conductors aren’t actually protected by the 1200A OCPD? Instead, they just land there, which requires the use of #600 conductors per 230.90 (A).

So you're correct you'll need 3 sets of 600's or larger conductors.

 

[wwhitney]

No exceptions apply, but isn't Exception #2 irrelevant here since the conductors aren’t actually protected by the 1200A OCPD?

The service conductors are protected against overload by the service OCPD. Exception #2 could apply, but it doesn't help you as 240.4(B) stops at 800A.

Cheers, Wayne

 

[wwhitney]

So you're correct you'll need 3 sets of 600's or larger conductors.

So how does 230.92(A) Exception #1 work?

In the configuration of the OP, we would have 1140A ampacity conductors protected at 1200A. It's not uncommon with large motor circuits that the difference in the MCA and MOCP would be more than 60A.

For example, suppose the load on the service consisted of 1000A of non-continuous non-motor load, plus a 100A motor load. The MCA would be 1125A. The MOCP per 430.63 would be 1250A. So it seems like under 230.92(A) Exception #1 the OP's install would comply.

It's odd that this conclusion depends on the magnitude of the non-motor load. Change the example to 900A of non-continuous motor load plus a 100A motor load, and now the MCA is 1025A, and the MOCP is 1150A. In this case it seems 230.92(A) Exception #1 no longer helps.

Cheers, Wayne

P.S. It's also odd that the text of 430.63 says that the OCPD "shall have a rating not less than" some number, rather that following the formulation of 430.62 that "if the conductors are sized per 430.24, then the OCPD shall be not more than" some number. I can only conclude that this is a long-standing mistake in 430.63.

 

[Elect117]

So how does 230.92(A) Exception #1 work?

In the configuration of the OP, we would have 1140A ampacity conductors protected at 1200A. It's not uncommon with large motor circuits that the difference in the MCA and MOCP would be more than 60A.

For example, suppose the load on the service consisted of 1000A of non-continuous non-motor load, plus a 100A motor load. The MCA would be 1125A. The MOCP per 430.63 would be 1250A. So it seems like under 230.92(A) Exception #1 the OP's install would comply.

It's odd that this conclusion depends on the magnitude of the non-motor load. Change the example to 900A of non-continuous motor load plus a 100A motor load, and now the MCA is 1025A, and the MOCP is 1150A. In this case it seems 230.92(A) Exception #1 no longer helps.

Cheers, Wayne

P.S. It's also odd that the text of 430.63 says that the OCPD "shall have a rating not less than" some number, rather that following the formulation of 430.62 that "if the conductors are sized per 430.24, then the OCPD shall be not more than" some number. I can only conclude that this is a long-standing mistake in 430.63.

This is where I get tossed up between overload and overcurrent. 230.90 uses both terms.

That exception sounds more like a LSIG breaker where you modify the short time and instantaneous time for a large motor. But the long time should still protect the conductors.

 

[wwhitney]

That exception sounds more like a LSIG breaker where you modify the short time and instantaneous time for a large motor. But the long time should still protect the conductors.

I am confident that the exception allows the following installation:

A service supplies only one load, a 100A motor load. The feeder to the motor requires 125A of conductors. The SC/GF protection for the motor is selected at 200A. Separate motor overload protection is provided. The service conductors consist of 125A ampacity conductors protected by a 200A service OCPD supplying 125A ampacity conductors to the motor et al.

In this configuration, the motor overloads are providing the "overload protection" required by the first sentence of 230.90.

Cheers, Wayne