NEC Code

[dbryan]

An engineer designed the following system and was turned down by the inspector siting a tap rule.

A distribution panel with an 80 amp breaker fed a junction box with #6 copper. At the juction box two taps were made with #10 copper. Each tap conductor was about 6' in lenght and terminated in a fuseible disconnect with 30 amp fuses. The load connected to the disconnect was an A/C load FLA 22.8 with maximum fuse size 30 amp.

The inspector make the electrician replace the 80 amp breaker with a 60 amp breaker. Since there was two such occurances and this was three phase the cost was substancial.

Does anyone know what code was violated?

Thanks for your help.

 

[charlie b]

Re: NEC Code

Table 310.16 gives an ampacity of 65 amps for #6 copper. Section 240.4(B) says that you can go with the next higher rated breaker. Section 240.6(A) gives the standard breaker sizes, and the next higher up for a 65 amp conductor is a 70 amp breaker. So that was one code violation: the 80 amp breaker had too high of a trip point. Now lets see if you needed a 60 amp breaker, or whether a 70 amp breaker would have been acceptable.

Section 240.21(B)(1) gives the applicable tap rule. (1) You are less than 10 feet, (2) The ampacity of the #10 tap conductors was not less than the A/C load, and (3) The ampacity of the #10 tap conductors was not less than the rating of the 30 amp fuse at their load end. There are three other constraints (related to installation) that you would have to review. However, it seems to me most likely that a 70 amp breaker would have been acceptable.

 

[plechtenberg]

Re: NEC Code

Charlie is correct.
What probably happened is someone used the 90 deg C column of table 310.16 which says that if you are using 90deg C wire, it is good for 75A the next standard breaker would have been 80A. I have never seen a breaker with a 90 deg C rated terminals; therefore you are restricted to the 75 deg C column; which as Charlie pointed out is 65A.