Conductor Sizing

[mskaltz]

I've recently been asked to explain the 125% rule for conductor sizing and where it applies. I know about the branch circuit rule using 125% of FLA for the largest load in a set of branch circuits, but I think there are other places this is used. Please consider the following when responding.

Transformer Secondary conductors

Motor Control Centers, Switchgear, and other load centers.

Thank You

 

[ryan_618]

It applies anytime the load is continuos. The basis is that circuit breakers aren't listed to operate at 100% of their rating for three hours. Due to this, we must increase the size of the breaker. Once we increase the size of the breaker, the wire is no longer protected, so we increase it in size as well.

Notice in the 2008 NEC the changes to 210.19(A) and 215.2 that allow the nuetral to be sized at 100% of the continuous load if it isn't connected to an overcurrent device.

 

[iwire]

Ryan, what a great concise answer.

I read his question and could not see any way that could be simply answered.

Interesting about the change for the neutral.

 

[don_resqcapt19]

Ryan,

Notice in the 2008 NEC the changes to 210.19(A) and 215.2 that allow the nuetral to be sized at 100% of the continuous load if it isn't connected to an overcurrent device.

Can we really do that? What code section says you don't have to protect the grounded conductor at its ampacity?
Don

 

[ryan_618]

Ryan,

Can we really do that? What code section says you don't have to protect the grounded conductor at its ampacity?
Don

I would turn that question on its head and ask "what section requires that the nuetral have overcurrent protection"? We size the conductors to carry the load, as specified in 210.19, 215.2 and 230.42, and we size breakers to carry the load in accordance with 210.20, 215.3 and 230.90. All of these requirements are a function of the load, including the sizing of the neutral conductor.

I think the code takes care of the neutral conductors in two ways:
1) Inflated calculations in Article 220, and
2) The performance requirements of 310.10 that prohibits conductors from operating above the temperature rating of the insulation.

 

[ryan_618]

Ryan, what a great concise answer.

I read his question and could not see any way that could be simply answered.

Interesting about the change for the neutral.

Thanks Bob! I appreciate the compliment.