Tap conductors size question

[hhsting]

I have less then ten feet of tap conductors. I have brewker feeding the tap conductors set at 400AT. The breaker is 80% rated. If I take 80% of 400A it would be 320A you see.

I am aware of NEC 2017 Article 240.21(B)(1) but unclear about the part that says rating if overcurrent protection device in my case thus unclear about tap conductor size.

Would the tap conductors be 600kcmil or 500kcmil copper?

 

[don_resqcapt19]

The 80% has ZERO to do with any rules of this type.

What is the load end OCPD? With the 10' rule you could use 8 AWG, termination in a 40 amp OCPD.

 

[hhsting]

The 80% has ZERO to do with any rules of this type.

What is the load end OCPD? With the 10' rule you could use 8 AWG, termination in a 40 amp OCPD.

Load end OCPD is set at 400AT but it is 80% rated.

 

[infinity]

Load end OCPD is set at 400AT but it is 80% rated.

80% is meaningless. The tap conductors need to be a minimum of 400 amps to terminate in a 400 amp OCPD. So 600 kcmils.

 

[don_resqcapt19]

I have brewker feeding the tap conductors set at 400AT

Thats what I am saying load end OCPD is set at 400AT but breaker is 80% rated

Those two statements say the opposite to me.

 

[hhsting]

Those two statements say the opposite to me.

Ah i see as confirmed post #2 the Load end OCPD is set at 400AT but it is 80% rated.

I cant go back and edit post #1

 

[infinity]

Ah i see as confirmed post #2 the Load end OCPD is set at 400AT but it is 80% rated.

I cant go back and edit post #1

So to clarify the tap conductors land on a 400 amp circuit breaker. If so a single set of 600 kcmils is required.

 

[hhsting]

So to clarify the tap conductors land on a 400 amp circuit breaker. If so a single set of 600 kcmils is required.

I guess 80% rated breaker wont change that 600kcmil to 500kcmil?


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[infinity]

I guess 80% rated breaker wont change that 600kcmil to 500kcmil?


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It's been mentioned a few times that the 80% is not relevant to your question about tap conductors. A tap conductor must terminate on the line side of an OCPD that is not larger than the ampacity of the tap conductors itself. For a 400 amp breaker that means 600 kcmil minimum size for a single set of conductors.

What happens on the load side (80% or 100%) of the 400 amp OCPD is irrelevant. The breaker could have a 10 amp load but that wouldn't change the 600 kcmil minimum conductor size on the line side. If you want to use 500 kcmil then the OCPD needs to be changed to 350 amps.

 

[kwired]

I guess 80% rated breaker wont change that 600kcmil to 500kcmil?


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If your continuous load is not over 320 you need a conductor that is 1.25 times 320 in ampacity and can't be protected at more than 1.25 times as well.

If your actual load is 400 amp continuous, then you need 400 x 1.25 = 500 amp conductor ampacity and 500 amp OCPD.

Others are saying the 80% thing has nothing to do with your feeder tap are mostly right. Before selecting that 400 amp breaker you (or somebody) likely already determined that is what is needed and may have used a 1.25 multiplier at some point in determining this for the load being supplied. And the feeder tap rule says you must use a 400 amp conductor here even if the load is less than 400. Now if the load is no more than 380 amps, then the load side of the overcurrent device could actually be 500 kcmil and protected at 400 amps but the feeder tap still needs to be a 400 amp or more conductor.