Motor Feeder Conductor Sizing/Protection

[Jraef]

You cannot consider the wires between a CB acting as SC/GF protection and an OL as also being protected by the OL any more than the wires of a feeder (or feeder tap) that terminate on a main CB of a MDP as being protected by that main CB that you termiante on at the end of the run.

Why not? It seems to me that the overload device is protecting the entire circuit from overload.

Correct. The motor circuit in your scenario #1 is a BRANCH circuit, not a feeder, because the OCPD for that circuit is the last device before the load (look up the definitions of Branch and Feeder). The thing that's different in a motor circuit is just that the long time Over Current Protective Device, in this case the OL relay, is (usually) separate from the Short Circuit Protective Device, in this case the circuit breaker or fuse. Whether it's 25", 25' or 250' is irrelevant. Current in a circuit is the same in the entire circuit, regardless of where you measure it. So if there is an over current situation, meaning in this case an overloaded motor, then the OL relay sees it the same as if the trip element were in the circuit breaker.

Where you are messing yourself up is, as iceworm had said earlier, that you CAN'T have an "overload" in the middle of the circuit. If anything goes awry in that 250' of conductor between the CB and the OL relay, it will likely be a SHORT CIRCUIT OR GROUND FAULT, not an "overload", and the SC trip unit in the CB would take it off line. While it's technically true that you could have a high resistance ground fault in those cables that MIGHT not be enough to trip the SC protection, that situation is rare enough that it's overlooked because if that's the case, it's likely not enough to have tripped the lower level of the thermal trips either, so it would continue on until it does turn into a Short Circuit or Ground Fault and the breaker clears it.

 

[bwat]

I'm aware of the definitions.

..you CAN'T have an "overload" in the middle of the circuit. If anything goes awry in that 250' of conductor between the CB and the OL relay, it will likely be a SHORT CIRCUIT OR GROUND FAULT, not an "overload", and the SC trip unit in the CB would take it off line. While it's technically true that you could have a high resistance ground fault in those cables that MIGHT not be enough to trip the SC protection, that situation is rare enough that it's overlooked because if that's the case, it's likely not enough to have tripped the lower level of the thermal trips either, so it would continue on until it does turn into a Short Circuit or Ground Fault and the breaker clears it.

Agreed, agreed, and agreed. The point that I've been trying to make this whole time (see my first post, #15) is that you can make the same statements you just made about a feeder (or feeder tap) that terminates on a single overcurrent device, but yet the code has a significantly different set of sizing and length requirements. As I said, a different "philosophy" in what is actually a risk. Because a circuit is categorized as a branch circuit and not a feeder circuit, that does not mean it is less likely to experience a high resistance fault like you described. If I'm wrong here, please describe how.

Why not? It seems to me that the overload device is protecting the entire circuit from overload.

I'm either very intrigued to hear how terminating on a OL is more protective for the 250' run than terminating on a CB, or I think you missed the words "any more" in my post that you quoted.


Maybe it's worth stating this: my saying that this is "strange" is a lot more focused on the significant difference in the requirements per code.. not necessarily as much that this one is right and that one wrong kind of thing. And I get that the circuits are defined differently, but the resulting requirements are REALLY different. Same conductors.. protected by same upstream OCPD.. with same risk (or lack thereof) of a fault before the next overcurrent device.. but largely different rules.

 

[iceworm]

Actually the motor feeder

... I'm either very intrigued to hear how terminating on a OL is more protective for the 250' run than terminating on a CB, or I think you missed the words "any more" in my post that you quoted. ...

I'm missing your point.
Here's my take:

Industrial Motor feeder usually have the Disconnect/OCP right with the contactor and overload. Then the conductors run the 250' to the motor.
So, the conductors are clearly protected by the OCP and the overload.

For your case, the Disconnect/OCP is in a remote panel, conductors run 250' to the controller (contactor/overload) and then to the motor.

Aside: As you know, the controller has to have an additional disconnect right next to it. This doesn't change anything, but it does have to have the extra disconnect.​

For the motor case, the OCP is set for 2.5X fla (310.15. tables), the conductors are picked for 1.25 fla (31.15 tables), the overloads are set for 1.15X to 1.4X fla (nameplate) The only thermal protection for the conductors are the overloads at the end of the 250' run

240.21 tap conductors
For this case the conductors can be quite small compared to the feeder end OCP
As I mentioned earlier,
240.21.B.5 Outside taps of unlimited length
240.21.C.4 Transformer secondary conductors, Outside Secondary Conductors
240 Part VIII, Supervised Industrial Installations.

We could have #14, 1000' from the secondary of a 1000kva xfm terminating at a 15A CB. The only protection is the 1500A equivalent on the transformer primary. Yes, this is ridiculous, no, it isn't very likely.

Is your concern that with motors the motor circuit conductor can be run inside the building with the overload protection at the end.
And for taps, except for supervised industrial, long taps with the OCP at the end can only be done outside.

Well, with motor feeders, the OCP is 2X (could be a bit higher) the motor circuit conductor ampacity.
With taps, the OCP could be anything.

In either case, the conductor protection is concrete and steel - that is about the only way to beat a backhoe or a forklift.

One might say, that in either case, the purpose of the feed end OCP is to put out the fire, not save the conductors.

 

[bwat]

Worm:

I appreciate you continuing with this.

The keyword in the other sections you mentioned is “outside”. It’s not surprising that things are more lax for outside feeders and secondary conductors. If a short happens, and the wires burn, the chance of a larger fire is a lot less likely. The “why” on those can be justified. And yes, I get that if something happens on the run, the wires won’t be saved, regardless of the ocpd size. Nothing to save... except for a fire.


So yes, this isn’t far off from what I’m trying to point out:

Is your concern that with motors the motor circuit conductor can be run inside the building with the overload protection at the end.
And for taps, except for supervised industrial, long taps with the OCP at the end can only be done outside.

With taps, the OCP could be anything.

Anything for indoor? 1/10 gets you 10’, and 1/3 gets you 25’ IIRC. Plus the physical routing requirements. For a motor circuit, I could put in CC fuses at 300% of FLC so my ratio there could be 1.25/3 which of course isn’t very far from 1/3 and I could run that unlimited length indoor and without any other special physical requirements like taps have.

A “general” rule that I see throughout the code is that conductors usually fall into at least 1 of these 3 options:
1. They are protected at or around their ampacity at the source. Around being due to round ups and other slight adjustments.
2. They are outside
3. If not protected at or around ampacity, they better be short and then also be physically protected. Tap rule requirements.

It’s a little rare to find code permissible cases that don’t fit into one of those three options, but even when you do find those, and I’m sure you and others can list several, the “why” can typically be discovered.

The scenario that CharlesK posted doesn’t fall into one of those options but it is to code. You (and then Jraef) then posted that the likely hood of anything happening before the OL is small. And I agree. But I then pointed out that this same exact thought process could be applied to a feeder, but the code does not do this. So since the “why” isn’t evident for the difference in the code, it seems reasonable to call this strange.

 

[david luchini]

A “general” rule that I see throughout the code is that conductors usually fall into at least 1 of these 3 options:
1. They are protected at or around their ampacity at the source. Around being due to round ups and other slight adjustments.
2. They are outside
3. If not protected at or around ampacity, they better be short and then also be physically protected. Tap rule requirements.

4. They have a sc/gf protective device at the source and an ol protective device near the end of the run (Art. 430 requirements.)

The length of the circuit conductors is irrelevant as each device provides its protection to the entire circuit.

 

[bwat]

4. They have a sc/gf protective device at the source and an ol protective device near the end of the run (Art. 430 requirements.)

Can you provide a reason for why your #4 exists (i.e. the code we're discussing) and why the reasoning that you would use is invalid for feeders? That's been the question... Because 1-3 can be explained, and so far I've only heard reasoning for #4 that doesn't make sense for it to only apply to a motor circuit and if it were valid, then #3 would be very different or eliminated.


You also stated:

The length of the circuit conductors is irrelevant as each device provides its protection to the entire circuit.

But doesn't this also happen for a 1/3 feeder tap, for example? Quite different results (length and physical protection) and I haven't heard a reason for why they are treated so differently.

There very well may be a reason out there, but I've yet to hear it. I really think this is one of those "yes it's to code... yes, many would say it's kinda weird in the context of the rest of the code... no we can't really explain the difference, but it is what it is."

This discussion has turned me into one of my kids just continuing to yell "But why?!?"