NEC Issue

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Enile

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Hello all.

I have an issue with article 430, I read and read... and read and read, but I cannot find the answer to my question. I think the NEC is very clear when it comes to branch circuit conductor rating and protection against SC and GF rating, even feeder protection rating its quite clear.

But, how do you rate the conductor ampacity for the feeder?

Do you use 430.24?

Let me se if I get things right...

Say you have a feeder supplying 2 motors and lighting load.

Motor 1 = 32 FLA
Motor 2 = 5 FLA
Lload = 5 Amp

So the breaker rating would be:

80+5+5 = 90 Amp

And the feeder conductor ampacity would be:

32*1.25+5+5 = 50 Amp

That would give me a combination of a 90 Amp breaker with a 6 AWG wire, which would not be protected by the breaker.

So...
What do you do?
Am I getting this wrong?
Does the NEC requires an overload protection for the feeder? (although I can't find anything related to this matter)

Any help would be appreciated, thanks in advance :).
 
Does the feeder need protection? Well ... look at the 'tap rules.' It's possible - though not likely - that you don't. At least, not at the source of the feeder; you'll still need it at/ before the motor starter.
 
240.4 Protection of Conductors.
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
 
Enile said:
Hello all.

I have an issue with article 430, I read and read... and read and read, but I cannot find the answer to my question. I think the NEC is very clear when it comes to branch circuit conductor rating and protection against SC and GF rating, even feeder protection rating its quite clear.

But, how do you rate the conductor ampacity for the feeder?

Do you use 430.24?

.
Click to expand...
430.24, .25 and .26
Take a look at fiugure 430.1, it will help guide you.
Enile said:
Let me se if I get things right...

Say you have a feeder supplying 2 motors and lighting load.

Motor 1 = 32 FLA
Motor 2 = 5 FLA
Lload = 5 Amp

So the breaker rating would be:

80+5+5 = 90 Amp

And the feeder conductor ampacity would be:

32*1.25+5+5 = 50 Amp
.
Click to expand...
Looks right
Enile said:
That would give me a combination of a 90 Amp breaker with a 6 AWG wire, which would not be protected by the breaker.

So...
What do you do?
Am I getting this wrong?
Does the NEC requires an overload protection for the feeder? (although I can't find anything related to this matter)

Any help would be appreciated, thanks in advance :).
Click to expand...

In practice I normally see the feeder ampacity equal to the breaker feeding it, but, as your calculations show, that does not need to be the case with motor loads (and some other loads as david points out above)
Keeps electricians on their toes.:)
 
430.24 requires the min a ampacity the conductors shall have. You calculated that to be 50 amps. 240.4 provides for the overcurrent protection of the feeder conductors. If you choose a feeder conductors with a 50 amp rating you would provide overcurrent protection at 50 amps. However if you design a feeder with a greater than min ampacity ( greater than fifty amps) you would protect the feeder in accordance with its ampacity rating.
 
Thank you all for your answers :).

I agree with everyone here.

I've always rated my SC and GF protection according to 430.62, and made my conductors meet that rating. In my example above, I would've used a 2 AWG wire (keeping in mind 240.4).

My problem is:

Then why 430.24?

If you chose to size your conductors with 430.24 and your breaker by 430.62, there would be no overcurrent protection at the feeder, therefore you would have to use a separate overcurrent device or a special type of breaker, but NEC does not say anything about it.

430.24 can't be use to size branch circuit conductors (because of the limitations of 430.53), and, at least for me, there?s no logic behind finding the minimum ampacity if you're not going to use it anyway, so I don't know what's the real deal..

Any thoughts?
 
Enile,
Sorry but you lost me.
430.24 sets the rules for sizing a conductor supplying several motors.
430.53 is for selecting a SC-GF protection device or several motors (single device-multiple motors) .
430.62 is for selecting the SC-GF protection for a motor feeder with individual SC-GF at for each motor.
For your original example, you stated you would use a #2 conductor.
I assume this bases on your 90 amp breaker (unless one is using NM or SE, #3 or #4 might be more common), however, IMO. based on 240.4(G) and 430.62 you would be Code compliant using the #6 (or even #8) and your 90 amp breaker as long as each motor has it's own OL and SCGF protection.
 
I'd think the minimum required ampacity of the feeder would be 51.25A per 430.24.

That would be 125% of the FLC of the largest motor, 100% of the FLC of the other motors, and the "ampacity required" for the other loads.

I'd say the "ampacity required" for the 5A lighting load would be 6.25A, for continuous load.

So, 40+5+6.25=51.25.

And while I'd agree that 90A is the proper breaker, I wouldn't arrive at it by taking 32*2.5 + 5 + 5.

Per 430.63 (for motor loads plus power and lighting loads) I'd first look at the maximum rating for the two motors per 430.62. This would be 32*2.5 + 5 = 85, so 80A is the maximum rating for the two motors. Then per 430.63, I'd look at a device with a rating sufficient to carry the lighting load plus the maximum rating from 430.62.

6.25 + 80 = 86.25. So a 90A c/b would be permitted.
 
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I know I would be code compliant, but you see, my main concern is that you cannot control what kind of load the customer will connect after everything is finished (additional load to that projected), and if by some chance there could be the case where the line current would be higher than the ampacity of the conductor but lower than the breaker, it would not create and entirely safe condition, that's why I wanted to know if being code compliant would be enough.

Thank you everyone, you've been of much help :).

PS: Sorry for my english, I'm Mexican :p.
 
Enile said:
I know I would be code compliant, but you see, my main concern is that you cannot control what kind of load the customer will connect after everything is finished (additional load to that projected), and if by some chance there could be the case where the line current would be higher than the ampacity of the conductor but lower than the breaker, it would not create and entirely safe condition, that's why I wanted to know if being code compliant would be enough.

Thank you everyone, you've been of much help :).

PS: Sorry for my english, I'm Mexican :p.
Click to expand...

Welcome to the forum, Enile :)

...and your written English is better than some that it is their first language.

I've read through this post (not skimmed, but rather fast) and cannot discern a definitive answer to your question from the replies. Here's the answer...

You have to separate the concept of a feeder to branch-circuit equipment from a "motor" feeder to equipment having one or more motors and other non-motor loads. From your question, I believe you are asking about the former of the two...

As such, we first go to 220.50 which says motor loads shall be calculated according to 430.24, 430.25, and 430.26, and with 440.6 when applicable. These are all regarding conductor ampacity rather than load, but I interpret them to mean that you have to consider their determined ampacity as the load for Article 220 caluclations.

The gist of these sections say essentially the ampacity must be 125% of the largest motor's FLA, plus the FLA of the other motors, plus the ampacity required for the other loads. Note that these sections say nothing about sizing overcurrent protection (OCP; OCPD for the device). For OCP you must go to 215.3, which sends us to Part I of Article 240 and adds the common 125% for continuous loads requirement.

In Part I of Article 240, there is no leniency(?) for protecting a conductor which serves branch-circuit equipment having motor loads at higher than the feeder conductor's ampacity.

Getting back to your example, the minimum feeder conductor ampacity would be 50A, if the lighting load is non-continuous, 51.25A if the lighting load is continuous. Note these are minimums, and the conductor ampacity/OCP rating would have to be 50A/50A and 51.25/60A minimum respectively. However, you may want to upsize these to allow for starting current of the motors not tripping the OCPD, but you have to upsize the conductors and the OCP respective of each other. If you wanted to use a 90A OCPD, your feeder conductors would have to have an ampacity of 81 or greater (#4 copper or #2 aluminum 75-90?C-rated; #3 copper or #1 aluminum 60?C-rated), not accounting for correction or adjustment.
 
Your concern is valid but not much different than having a 1200 amp ML panel with (6) 400 amp breakers, etc.
Thats why in the earlier post I mentioned keeping on your toes.
Hopefully in most situations the electrician takes all the variables into account.
 
Smart $ said:
Welcome to the forum, Enile :)
Getting back to your example, the minimum feeder conductor ampacity would be 50A, if the lighting load is non-continuous, 51.25A if the lighting load is continuous. Note these are minimums, and the conductor ampacity/OCP rating would have to be 50A/50A and 51.25/60A minimum respectively. However, you may want to upsize these to allow for starting current of the motors not tripping the OCPD, but you have to upsize the conductors and the OCP respective of each other. If you wanted to use a 90A OCPD, your feeder conductors would have to have an ampacity of 81 or greater (#4 copper or #2 aluminum 75-90?C-rated; #3 copper or #1 aluminum 60?C-rated), not accounting for correction or adjustment.
Click to expand...

430.24 Several Motors or a Motor(s) and Other Load(s).

[/QUOTE]but you have to upsize the conductors and the OCP respective of each other. If you wanted to use a 90A OCPD, your feeder conductors would have to have an ampacity of 81 or greater (#4 copper or #2 aluminum 75-90?C-rated; #3 copper or #1 aluminum 60?C-rated), not accounting for correction or adjustment..[/QUOTE]

You lost me here.

It was my belief in his example he sized his conductor ampacity in accordance with the rules in 430.24 then went to table 430.52 using an inverse time breaker used a multiplier of 250% (32amps X 2.5 = 80 amps)
80 + 5 (6.25) + 5 = 91.25 amps = 100 amp breaker

Feeder min. ampacity
430.24 Several Motors or a Motor(s) and Other Load(s).
32amps X 1.25 = 40 amps
40 + 6.25 + 5 = 51.25 = 6awg thwn
 
david said:
430.24 Several Motors or a Motor(s) and Other Load(s).

but you have to upsize the conductors and the OCP respective of each other. If you wanted to use a 90A OCPD, your feeder conductors would have to have an ampacity of 81 or greater (#4 copper or #2 aluminum 75-90?C-rated; #3 copper or #1 aluminum 60?C-rated), not accounting for correction or adjustment..
Click to expand...

You lost me here.

It was my belief in his example he sized his conductor ampacity in accordance with the rules in 430.24 then went to table 430.52 using an inverse time breaker used a multiplier of 250% (32amps X 2.5 = 80 amps)
80 + 5 (6.25) + 5 = 91.25 amps = 100 amp breaker

Feeder min. ampacity
430.24 Several Motors or a Motor(s) and Other Load(s).
32amps X 1.25 = 40 amps
40 + 6.25 + 5 = 51.25 = 6awg thwn
Click to expand...
Those sections (aka rules) are for motor feeders (aka branch-circuits to motor controllers, multi-motor or combination motor and non-motor equipment, etc.), not panelboard feeders, where the panelboard's branch-circuits supply motor and non-motor loads through more than one OCPD's. I am of the belief the OP/OP'er is asking about the latter.
 
Smart $ said:
Those sections (aka rules) are for motor feeders (aka branch-circuits to motor controllers, multi-motor or combination motor and non-motor equipment, etc.), not panelboard feeders, where the panelboard's branch-circuits supply motor and non-motor loads through more than one OCPD's. I am of the belief the OP/OP'er is asking about the latter.
Click to expand...

Agreed, I might add that the rules also apply to MC centers as long as 430.94 is applied to overcurrent protection for the MCC
430.62 Rating or Setting ? Motor Load.
(A) Specific Load
Exception No. 2: Where the feeder overcurrent protective device also provides overcurrent protection for a motor control center, the provisions of 430.94 shall apply.
 
Thank you all for your answers, it seems this issue has been solved!

Gonna stick around in this forum, there's a lot of good questions and really smart people here!
 
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