Feeder Example for Motors Plus Non-Motors

[wwhitney]

I've never properly understood all of Article 430 Part V. So maybe someone could help me understand the following example:

A feeder supplies a non-continuous, non-motor load of 40A along with 2 motors with a Table FLC of 40A each. No derating is required and all terminations are 75C. What is the minimum allowable Cu 90C insulated conductor size, and what is the allowable range of standard size inverse time breaker(s) that may be used for those conductors?

Note I haven't specified voltage or number of phases. I don't think those matter, but if I'm mistaken and they do, let's say 240V, single phase (so the motors are 7.5 HP per FLC in Table 430.248).

Thanks, Wayne

 

[wwhitney]

If we delete the non-motor load, then 430.62 applies, and I believe I understand that.

We may size our conductors per 430.24, which gives us a minimum ampacity of 90A for the two motors (125% * 40A + 40A). The load is also 90A per (2026 NEC) 120.50(A). So the minimum Cu conductor size is #3, based on the 75C column. The minimum OCPD size is 90A per 215.3, and the maximum OCPD size if we don't want to use 430.52(C)(1) Exception 2 is 250% * 40A + 40A = 140A; not a standard size, so 430.52(C) Exception 1 allows going up to the next standard size, 150A.

Or if 430.52(C)(1) Exception 2 does apply, the maximum OCPD size for one motor is 400% * 40A = 160A, so the maximum size OCPD is 160A + 40A = 200A.

In either case, if we want to use an OCPD larger than the maximums above, then we may per 430.62(B), but the minimum ampacity will increase and needs to comply with 240.4 (including 240.4(B) if not over 800A OCPD).

But 430.63 is what I find confusing.

Cheers, Wayne

 

[wwhitney]

and the maximum OCPD size if we don't want to use 430.52(C)(1) Exception 2 is 250% * 40A + 40A = 140A; not a standard size, so 430.52(C) Exception 1 allows going up to the next standard size, 150A.

Correction: 430.62(A) only refers to 430.52(C) for the largest motor, and that contribution is 100A, a standard size. 40A additional makes 140A, but 430.62(A) doesn't have the equivalent of 430.52(C) Exception 1, so we have to round down to a standard size. Making the maximum standard size 125A.

Cheers, Wayne

 

[Elect117]

We are assuming the motor's have their own overload protection protection? This is just for short circuit + ground fault protection right?

The feeder's ground fault protection would be the largest branch circuit's plus the FLA of the other motors.

The size is based on 430.52(C) and the table so the exceptions would still apply.

You don't need to take into account more than the GF+SC of the largest motor in most circumstances. If you had 200 motors on a feeder and the largest one was 100hp and the rest were various numbers below, than you would, in order to protect the feeder, only need to consider the largest SC/GF + FLAs so it trips appropriately. Then you can size up based on the table. You wouldn't want the normal operation to trip the SF/SC device, so they add the FLAs to it.

what is the allowable range of standard size inverse time breaker(s) that may be used for those conductors?

Sizing the conductors to 430.24/430.26. I would assume that the branch circuits with overload at the motor can have a large difference. A feeder's overload protection is sort of ignored. As so long as it meets one of the 430.31-430.44 sections. They call circuits with multiple types of loads motor branch circuit conductors in 430 so I don't know if there is a real distinction there.

But if anything other than a motor is on the same circuit, that might change things. A lot of 430 is permissive. So you run into issues when you get to the restrictive parts of code. Like motor control centers 430.94. In situations like that you need to increase the conductor size to catch up to the oversized OCPD for the GF/SC / inrush tripping issues.

 

[wwhitney]

We are assuming the motor's have their own overload protection protection?

Yes, this is just about the feeder and feeder OCPD, branch circuit details are beyond the scope.

But if anything other than a motor is on the same circuit, that might change things.

Yes, that is the whole point of the thread, per the thread title and per the OP which states "a non-continuous, non-motor load of 40A along with 2 motors with a Table FLC of 40A each." So any comments on that situation?

Cheers, Wayne

 

[Elect117]

Yes, this is just about the feeder and feeder OCPD, branch circuit details are beyond the scope.


Yes, that is the whole point of the thread, per the thread title and per the OP which states "a non-continuous, non-motor load of 40A along with 2 motors with a Table FLC of 40A each." So any comments on that situation?

Cheers, Wayne

Motor 1 + Motor 2 + Other non-motor loads -- Assuming this is from a panelboard.---
40A FLA + 2.5 *40 + 40A is the largest OCPD you could have (without assuming it wasn't enough) = 180A and rounding up, 200A

The Feeder conductor would need to meet, at a minimum, 430.24 and the panel board would need to be protected at it's rating. 408.36
So, if you picked a 200A panelboard with a 200A OCPD that is okay. But, the conductor size needs to be 3/0. Because 240.4 does not allow for anything else. It isn't in 240.4(G) for example.

If it is taps,

430.24 for the feeder conductor size.

(1.25 * 40) + 40 + 40 = 130A -> #1 or 1/0A

430.53 for the feeder protection

2.5*40+40+40=180A --> 200A

240.21 + 430.21 for the loads branch circuits conductors and then their respective codes for OCPD size.

 

[Elect117]

To keep going, for the loads, the smallest conductor for a 25' tap would be #6 cu.

Your OCPD would be 40A for the non motor load. You can reduce the wire size after the OCPD to #8 or whatever.

Your SC/GF device for motor 1 would be 2.5* 40 = 100A with #6.

Your SC/GF device for motor 2 would be 2.5 * 40 = 100A with #6