Motor conductors and 240.4(G) HELP!

[Steve McKinney]

Having a dilema fellas. Last night in class we were going over motor branch circuits. I was telling the class that the 240.4 (D) did not apply to motor circuits. As you know, if you look at T310.16, the asterik will tell you to go to 240.4D. This is the #10 on 30 amp, # 12 on a 20 amp ocp and #14 on a 15 amp ocp. It also tells you to look at 240.4 (E) and 240.4 (G), which is the table for "Specific Conductor Applications".

My dilema is that Motor and motor-control circuit conductors is listed with "430, Parts III, IV, V, VI, VII"

Motor circuit conductors is Part II!

How do I read this? MY problem is that I am looking to justify the branch circuit conductor to be utilized at their rated values per T 310.16 and not being obligated to tie a #10 to a maximum 30 amp breaker.

How can I best explain this Gentlemen?

Thnx in advance,

Steve McKinney

 

[George Stolz]

I'm going to go out on a limb and guess that it's because the OCPD is determined in Part IV, not Part II.

I do find the distinction kinda odd, though - why don't they just say "Article 430" and be done with it?

 

[infinity]

How do I read this? MY problem is that I am looking to justify the branch circuit conductor to be utilized at their rated values per T 310.16 and not being obligated to tie a #10 to a maximum 30 amp breaker.



Steve McKinney

For one thing the OCPD is not protecting the conductors from overcurrent. That is the job of the thermal overload protection built into the motor or within the motor starter. The OCPD protecting the circuit is only providing short circuit and ground fault protection therefore the 30 amp limit for #10 wire doesn't need to apply because the conductor is protected by another device within the circuit.


I agree with George, since you would use part IV to determine the protection device.

 

[masterelect1]

Ocpd

Ocpd

the OCPD must be capable of carrying the starting load imposed upon it up to ,I believe, 800 % of motor FLA. Therefore the conductors to the motor would not have to match the OCPD rating as the motor conds are protected by the OL device.

 

[infinity]

the OCPD must be capable of carrying the starting load imposed upon it up to ,I believe, 800 % of motor FLA. Therefore the conductors to the motor would not have to match the OCPD rating as the motor conds are protected by the OL device.

800% would be for an instantaneous trip CB, 250% would be for an inverse time CB and 175% would be for a time delay fuse.

 

[Wes G]

Electrical Technology Instructor

Electrical Technology Instructor

Steve, as I see the issue, the 430 part II section of the code deals with sizing the conductors for feders and has nothing to do with protection of the conductors. T 310.16 covers conductor ampacty, not conductor protection. 240.4(D) and 430 parts III, IV, V, VI & VII cover protection and not ampacity. As long as you keep your sizing and protection separate I think you'll solve your problem

 

[Steve McKinney]

Thank you so much for the insights gentlemen. I wasn't thinking of it as much as overcurrent protection, which is Part III for overloads and Part IV which is short circuit and ground fault protection. The different view really helped me. As I was wrestling with this again last night, I ran over and read the Parts III and IV and in 430.51 I believe I ran across what also addresses the original question.

430.51 General. Part IV specifies devises intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or grounds. THESE RULES ADD TO OR AMEND THE PROVISIONS OF ARTICLE 240.

Thanks again fellas,

Steve