Motor Feeders, Article 430 Part V (430.63 Weirdness)

[wwhitney]

I'm trying to understand the rules for conductor and OCPD sizing for feeders supplying motors and possibly other loads.

In particular, 430.62 (Part V) on a feeder with just motor loads seems straightforward enough. 430.62(A) says if you size your conductors based on the motor rules in 430.24, then the OCPD for the feeder "shall be provided with a protective device having a rating or setting not greater than" an expected motor-type computation. And 430.62(B) says that if you size your conductors "normally", then you can select your OCPD "normally".

However 430.63 "Rating or Setting — Motor Load and Other Load(s)" uses the language that "the feeder protective device shall have a rating not less" an expected motor-type computation. That would make sense if some other rule provided a link between the feeder conductor size and OCPD size. But 240.4(G) exempts 430.63 from the usual 240.4 rule on OCPD vs conductor size.

Thus it would appear that for a feeder supplying motor loads and other loads, the conductors could be sized by 430.24, and there is no limit on the OCPD size. Which makes no sense, so what I am missing?

Cheers, Wayne

 

[augie47]

I'm unsure what you are quoting. The '17 Code is rather specific that the feeder OCP size is based on the largest motor OCP size + the sum of the FLA of other motors.

 

[wwhitney]

2017 NEC, 430.63 does not seem to place any upper limit on the OCPD size:

430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than . . .

430.63 Rating or Setting — Motor Load and Other Load(s).
Where a feeder supplies a motor load and other load(s), the feeder protective device shall have a rating not less than . . .

Cheers, Wayne

 

[augie47]

Good point.. never noticed

 

[wwhitney]

Any other thoughts? @david luchini ?

There's a 2016 thread with a similar question, but it didn't address the question of what limits OCPD size when applying 430.63:

NEC 430.63 Question

Hello All! I am looking for help interpreting NEC 430.63. I have a panel with a 480VAC, 3 phase feeder that will contain motor starters and 20 amp time delay fuses for three motors that are 5 HP each (7.6 FLA). 20 amp fuses picked according to Table 430.52 (175% times 7.6 FLA = 19; next...

forums.mikeholt.com

Cheers, Wayne

 

[petersonra]

2017 NEC, 430.63 does not seem to place any upper limit on the OCPD size:

430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than . . .

430.63 Rating or Setting — Motor Load and Other Load(s).
Where a feeder supplies a motor load and other load(s), the feeder protective device shall have a rating not less than . . .

Cheers, Wayne

pretty much a feeder can be any size you want. Key with some of these motor feeder things is to understand that they are giving you a way to have a smaller conductor than you would otherwise have if you limit the feeder over current protection device rating. If you go over that rating you have to make the conductors bigger.

 

[wwhitney]

Key with some of these motor feeder things is to understand that they are giving you a way to have a smaller conductor than you would otherwise have if you limit the feeder over current protection device rating. If you go over that rating you have to make the conductors bigger.

This is the part that seems to be missing from 430.63. If I have a feeder carrying motor loads and other loads, what section prevents me from sizing the feeder conductors based on 430.24, and then using an arbitrarily large OCPD under 430.63?

Cheers, Wayne

 

[wyreman]

the ocpd is supposed to open before the wire melts

 

[wwhitney]

the ocpd is supposed to open before the wire melts

Agreed. So which NEC section requires that when an installation is subject to 430.63?

Cheers, Wayne

 

[wyreman]

This is the part that seems to be missing from 430.63. If I have a feeder carrying motor loads and other loads, what section prevents me from sizing the feeder conductors based on 430.24, and then using an arbitrarily large OCPD under 430.63?

Cheers, Wayne

I haven't run the numbers here at all .
I am just going off the words: "what section prevents me from...using an arbitrarily large OCPD"

since the ocpd is supposed to open before the wire melts
why would you use an arbitrarily large ocpd?

I guess you might have found something in the code that needs clarification and I would bet you are probably right.
pretty awesome actually.

beyond the theoretical,
Experience and liability would prevent you from...using an arbitrarily large OCPD

 

[paulengr]

Unrelated but one of the common problems with motor loads is that if you size a breaker to protect a starter it can be up to 300% of FLA and based on trip characteristics it better be. But if you have say a feeder breaker feeding a single starter it is sized to 125%…so nuisance tripping is an issue. The solution is to simply size the feeder and breaker large enough to avoid nuisance tripping although this results in both being oversized, or to treat the breaker as the starter disconnect and the actual disconnect as a secondary disconnect thus sizing the breaker and feeder according to motor FLA. But in the case of one large starter and some small ones again we hit this dilemma unless you can treat it as a multi motor scenario.

So the loophole may just be a solution to the oversized feeder issue.

 

[xptpcrewx]

This is the part that seems to be missing from 430.63. If I have a feeder carrying motor loads and other loads, what section prevents me from sizing the feeder conductors based on 430.24, and then using an arbitrarily large OCPD under 430.63?

Cheers, Wayne

Since 430.63 is under Part V (Feeders), I would think the OCPD limit doesn't matter so long as the conductors were protected. The only thing the OCPD needs to do is protect the conductors and allow the largest motor to start under base load (everything else running). The assumption I am making is based on motors and other loads already having their own overcurrent protection in accordance with the rules in Part III (Branch Overload) and Part IV (Branch Short-Circuit/Ground-Fault).

 

[wwhitney]

Since 430.63 is under Part V (Feeders), I would think the OCPD limit doesn't matter so long as the conductors were protected.

Yes, but the question is what is the maximum OCPD allowed to protect the feeder conductors for 430.63? That section doesn't specify, so as far as I can see, it is unlimited.

I should have titled this thread "#14 conductors protected at 100A" to get more attention. As far as I can see, that is allowed for this situation:

- 240V single phase 1 HP motor, Table 430.248 FLC is 8A.
- Fixed continuous non-motor load of 240V, 4A.
- 4 position panelboard with (2) 15A two pole breakers supply two #14 circuits for the two loads
- Minimum feeder conductor ampacity is 15A per 430.24, so #14 is OK even at 60C.
- A #14 feeder protected at 100A complies with 430.63, as 100 > 250% * 8A + 125% * 4A = 25A.
- 240.4 first paragraph does not apply, as 240.4(G) includes 430 Part V, which includes 430.63.

So if the above is a violation, what section is violated?

Cheers, Wayne

 

[xptpcrewx]

Yes, but the question is what is the maximum OCPD allowed to protect the feeder conductors for 430.63? That section doesn't specify, so as far as I can see, it is unlimited.

I should have titled this thread "#14 conductors protected at 100A" to get more attention. As far as I can see, that is allowed for this situation:

- 240V single phase 1 HP motor, Table 430.248 FLC is 8A.
- Fixed continuous non-motor load of 240V, 4A.
- 4 position panelboard with (2) 15A two pole breakers supply two #14 circuits for the two loads
- Minimum feeder conductor ampacity is 15A per 430.24, so #14 is OK even at 60C.
- A #14 feeder protected at 100A complies with 430.63, as 100 > 250% * 8A + 125% * 4A = 25A.
- 240.4 first paragraph does not apply, as 240.4(G) includes 430 Part V, which includes 430.63.

So if the above is a violation, what section is violated?

Cheers, Wayne

240.4 always applies, unless unless otherwise permitted or required in 240.4(A) through (G). It seems to me 430.63 might be one of those vague or poorly written permissions, but 430.61 clearly explains the intent of Part V as intending to protect the conductors.

 

[wwhitney]

240.4 always applies, unless unless otherwise permitted or required in 240.4(A) through (G).

And as mentioned, 240.4(G) covers 430 Part V, so the basic rule in 240.4 does not apply to the feeder in my example.

It seems to me 430.63 might be one of those vague or poorly written permissions

Certainly agreed.

but 430.61 clearly explains the intent of Part V as intending to protect the conductors.

Yes, it says "Part V specifies protective devices," and the specification in 430.63 is only for a minimum size, not a maximum. So no violation of 430 Part V.

Cheers, Wayne

 

[xptpcrewx]

And as mentioned, 240.4(G) covers 430 Part V, so the basic rule in 240.4 does not apply to the feeder in my example.


Certainly agreed.


Yes, it says "Part V specifies protective devices," and the specification in 430.63 is only for a minimum size, not a maximum. So no violation of 430 Part V.

Cheers, Wayne

IMO the intent of sections 110.10 and 430.61 imply the maximum OCPD rating/setting would need to be below and to the left of the feeder conductor short-circuit damage curve. As mentioned, the only reason for allowing an OCPD rating/setting higher than the feeder ampacity would be to avoid nuisance tripping during simultaneous motors starts. Since 430.61 mentions protection against short-circuits or ground faults, this would seem to suggest any overcurrents beyond the feeder continuous conductor rating must be limited to the intermediate damage curve.

The OCPD rating/setting rules basically account for the largest motor to start plus the FLC of any other motors and/or FLA of the other loads. What about several large motors starting simultaneously? How would the NEC implement a maximum OCPD rating/setting limit for a situation like this without requiring a motor starting and coordination study to be performed?