Wire sizing

[Redeagle91]

I was on a jobsite and I came across this junction box with a terminal block in it. There are 3-480v 15 amp Blower coil units that are being fed from a 480v 50 amp breaker with #8 wire . It comes to this box and changes wire size to 3 sets of #12. Leaves there and goes to 3 separate 30 amp fused disconnects with 15 amp fuses in them. Should the wire from the terminal block to the disconnects be rated for 50 amps

 

[Dennis Alwon]

I think they may be using the tap rules in art. 240 so it looks okay

 

[infinity]

How long are the conductors between the terminal box and the 15 amp OCPD?

 

[Redeagle91]

How long are the conductors between the terminal box and the 15 amp OCPD?

Maybe 25ft

 

[Redeagle91]

I think they may be using the tap rules in art. 240 so it looks okay

Okay I will look at it

 

[infinity]

Maybe 25ft

25' would be the maximum length if applying the 25' tap rule.

 

[wwhitney]

25' would be the maximum length if applying the 25' tap rule.

The 25' tap rule requires an ampacity of at least 1/3 of the upstream OCPD, and for 50A, that would be 16.7A. So 75C rated #14 would comply, and #12 is only required if 60C rated or 60C limited (like NM per 334.80)?

Cheers, Wayne

 

[infinity]

The 25' tap rule requires an ampacity of at least 1/3 of the upstream OCPD, and for 50A, that would be 16.7A. So 75C rated #14 would comply, and #12 is only required if 60C rated or 60C limited (like NM per 334.80)?

Cheers, Wayne

Correct. The existing conductors are already #12 so they would be adequate for the 1/3 minimum ampacity of the 50 amp OCPD.

 

[david luchini]

The 25' tap rule requires an ampacity of at least 1/3 of the upstream OCPD,

At least 1/3 of the ampacity of the feeder conductors. (Still 50A in this example).

 

[wwhitney]

At least 1/3 of the ampacity of the feeder conductors. (Still 50A in this example).

Huh?

2023 NEC 240.21(B)(2)(1) says "The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors."

Cheers, Wayne

 

[david luchini]

Huh?

2023 NEC 240.21(B)(2)(1) says "The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors."

Cheers, Wayne

See 430.28(2)

 

[wwhitney]

See 430.28(2)

Good point, the OP is about motor circuits, so 430.28(2) applies rather than the more general 240.21(B)(2). (*)

Still, I think the phrase "25' tap rule" is commonly understood to mean 240.21(B)(2), so my statement was correct, if off point relative to the OP. But the motor/non-motor aspect of the OP was not relevant for the point I was asking about.

Cheers, Wayne

(*) Does 430.28(2) mean that if I have grossly upsized my feeder conductors due to voltage drop, I am required to use a larger tap conductor than I would under 240.21(B)(2)? Or is it merely an additional allowance to what 240.21(B)(2) allows? Seems rather pointless to require a short section of "normal" sized feeder conductors between the upsized feeder conductors and the tap.

 

[infinity]

I'm thinking that the single feeder OCPD for a bunch of motors could be larger than the feeder conductor size so the actual size of the feeder conductor is used for the 1/3 rule.