Feeder wire and breaker sizing 600A

[Ron Poehlmann]

Guys,
I need some help. I have one of those bad projects that keeps coming back to me. The contractor purchased gear and wire and delivered it to the job site. The gear is not based on my design but on a previous job that they did. The job is a hotel and the circuit is a 208V branch feed for each floor. There is one circuit with two problems.

Here is the situation I need help with. I have calculated the load for a particular floor at 440 amps. The breaker in the MDP is a 600 A, and the wire the contractor supplied to use is 2 parallel 350 kcmil Al.

I would have selected 2 runs of 500 kcmil Al.

If the 2 runs of 350 MCM are run in seperate conduits the rating of the feeder wire would be 500A Table 310.16.

I went to the code to prove that the wire size was to small for the breaker size, but I have been unable to come up with a "chapter & verse" for my claim.

Articles 210.19(A)(1) and 215.2 only requires the conductor size to have an allowable ampacity size not less than 100% noncontinuous load + 125% of the continuous load. The 500A of the 2 runs of 350 MCM meet this requirement because the load is calculated at 440A.

If this were a service feeder, I would be able to say 230.90(A) requires the breaker to be sized at or below the allowable ampacity of the conductors.

Article 215.3 says the 100% noncont. & 125% cont. about protection, but adds Article 240. Now 240.4 says the conductors must be protected against overcurrent according to 310.15. I think I just found it!?

Since 500A is a standard breaker size, then I can't go up to the 600A breaker with this wire.

Does this all sound correct, or did I miss anything?


The other issue with this circuit is that the contractor wants to run the parallel conductors from the 600A breaker to the floor and then tap the conductors to feed 5 200A MCB panels. My initial reaction was that it could not be done without installing a 600A disconnect at each floor and coming out of the disconnect with 3/0 CU to each of the panels with 200A MCB's.

After looking at 240.21(B)(2) again it looks like I may be wrong on this point. The 3/0 would be rated at 1/3 of the 600A.

My original understanding was the the tap had to be 25' from the protective device, but this does not seem to be the case, and as long as the wire being tapped is properly protected, I can see that the 25' would start from the tap point.

Do I have this one right or wrong?

 

[charlie b]

Since 500A is a standard breaker size, then I can't go up to the 600A breaker with this wire.

That is correct. Either set the breaker to trip at 500 amps (if it is adjustable), or replace it with a 500 amp breaker, or bring some more aluminum into the party.

. . .I can see that the 25' would start from the tap point.

That is correct. The wire from the panel to the tap point is protected by the breaker at the panel. That breaker would not protect a 3/0 copper. But we are allowed to take the risk of protecting the smaller wire at the panel it serves, rather than at the point from which it gets its power. We reduce the risk by limiting the distance and by physically protecting the conductors from damage.

 

[raider1]

Since 500A is a standard breaker size, then I can't go up to the 600A breaker with this wire.

Correct, take a look at 240.4 and 240.4(B)

My original understanding was the the tap had to be 25' from the protective device, but this does not seem to be the case, and as long as the wire being tapped is properly protected, I can see that the 25' would start from the tap point.

The length mentioned in 240.21(B)(2) is that of the tap conductors themselves not the combine lenght of the tap and the feeder it is connected to.

Chris

 

[Ron Poehlmann]

Thanks to all for the help.