grasping 310.16

[billdozier 78]

Had to size a 70 amp load today. Knew the breaker had 75 degree terminals so sized per that table. My question is when does the 90 degree column come into play?

 

[stickboy1375]

Had to size a 70 amp load today. Knew the breaker had 75 degree terminals so sized per that table. My question is when does the 90 degree column come into play?

Derating, OR if you actually find 90 degree termination lugs.

 

[billdozier 78]

Thanks for the reply. Let me take this one step further to make sure I grasp it. Lets say we had a 100 amp load and 7 current carrying conductors in a conduit. A #3 wire in 90 degrees is good for 110 amps. But I have to multiply that 110 amps by 70%. That would be 77 amps. I would run a #1 to carry that load since #1 equals 150 amps less the 45 amps of heat dissipation. How far off am I?

 

[Strathead]

Thanks for the reply. Let me take this one step further to make sure I grasp it. Lets say we had a 100 amp load and 7 current carrying conductors in a conduit. A #3 wire in 90 degrees is good for 110 amps. But I have to multiply that 110 amps by 70%. That would be 77 amps. I would run a #1 to carry that load since #1 equals 150 amps less the 45 amps of heat dissipation. How far off am I?

You are close, but you can also apply 240.4 (B) and run #2's. See also 240.6 for standard breaker sizes. Otherwise, your premise is correct.

 

[Dennis Alwon]

You are close, but you can also apply 240.4 (B) and run #2's. See also 240.6 for standard breaker sizes. Otherwise, your premise is correct.

The OP needs a one hundred amp conductor so I believe he got it correct. #2 at 90C 130 amps but at 70% it is 91 amps and although the breaker can be sized at 100 amps the load cannot be greater than 91.

 

[Strathead]

The OP needs a one hundred amp conductor so I believe he got it correct. #2 at 90C 130 amps but at 70% it is 91 amps and although the breaker can be sized at 100 amps the load cannot be greater than 91.

Yes, I missed the semantics of the 100 amp load in lieu of the 100A breaker. Since we are splitting hairs, it must also be pointed out that, if the load is continuous, then any overcurrent protection must be sized at 125% of the load, which increases the sizes even more.

 

[kwired]

210.19 Conductors ? Minimum Ampacity and Size. (A) Branch Circuits Not More Than 600 Volts.

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

There is similar wording in 215 for feeders and 230 for service conductors.

We generally use 75 degree column for termination temperature as that is what is common. Older devices you may need to use 60 degree column. This gives you minimum size of conductor regardless of what temperature corrections end up allowing. If after calculating temperature derations at 90 degree values you end up with an allowable conductor smaller than the 60 or 75 (whichever applies) degree columns you still must use the 60 or 75 degree size as a minimum.