Motor Feeder Conductor Sizing/Protection

[CharlesK]

Journeyman Exam Question:

1.) If a single-phase motor draws 50 Amperes and its branch circuit is protected by a 150-ampere nontime-delay fuse, the feeder overcurrent protective device may not be larger than:
A.) 87.5 amperes C.) 300 amperes
B.) 100 amperes D.) 150 amperes

Answer: D.) 150 amperes ART. 430.62(A.)

Follow-up Question:
If you size the feeder to 125% of the motor FLA, this gives you ,at 75Celsius, a #6 AWG conductor. A #6 AWG is rated for 65 Amperes.
For feeders with a long length, what is protecting the conductors in between the 150A feeder OCPD and the fusible disconnect(150A nontime-delay fuse)?

 

[augie47]

150 amp protection is for short-circuit-fault-to- ground protection. The conductors are protected against overload my the motor overload protection device. In the event of a short or fault the 150 amp fuse will protect the conductor. If the length is so long that there is question about the ability of the fuse to provide the protection the conductor is probably undersized for voltage drop,

 

[Smart $]

Journeyman Exam Question:

1.) If a single-phase motor draws 50 Amperes and its branch circuit is protected by a 150-ampere nontime-delay fuse, the feeder overcurrent protective device may not be larger than:
A.) 87.5 amperes C.) 300 amperes
B.) 100 amperes D.) 150 amperes

Answer: D.) 150 amperes ART. 430.62(A.)

...

FWIW, that may be the desired test answer, but it is inaccurate.

If the feeder conductor has an ampacity of 800A, would the feeder OCPD still be limited to 150A? See 430.62(B).

 

[CharlesK]

Motor Feeder Conductor Sizing/Protection

The branch circuit conductors are protected against overload by the motor overload protection device.

What is protecting the "feeder conductor" in between the feeder OCPD and the Fusible disconnect?
If the feeder conductor length in between the feeder OCPD and the fusible disconnect is substantial what is protecting the conductor, if there is a short circuit or overload? The feeder conductor is sized to 125%*(Motor FLA). This yields a #6AWG THWN conductor, rated for 65 Amperes. If there is a short circuit or overload in feeder conductor, the 150A feeder OCPD will not protect the #6AWG conductor.

I have attached an image, that may help with any confusion.
My apologies if my explanation is not concise.


~Charles

150 amp protection is for short-circuit-fault-to- ground protection. The conductors are protected against overload my the motor overload protection device. In the event of a short or fault the 150 amp fuse will protect the conductor. If the length is so long that there is question about the ability of the fuse to provide the protection the conductor is probably undersized for voltage drop,

 

[Smart $]

The branch circuit conductors are protected against overload by the motor overload protection device.

What is protecting the "feeder conductor" in between the feeder OCPD and the Fusible disconnect?
If the feeder conductor length in between the feeder OCPD and the fusible disconnect is substantial what is protecting the conductor, if there is a short circuit or overload? The feeder conductor is sized to 125%*(Motor FLA). This yields a #6AWG THWN conductor, rated for 65 Amperes. If there is a short circuit or overload in feeder conductor, the 150A feeder OCPD will not protect the #6AWG conductor.

I have attached an image, that may help with any confusion.
My apologies if my explanation is not concise.


~Charles

The confusion you have is in part how protection for non-motor circuits are required to be sized. The difference between these and motor circuits is that the OCPD

 

[Smart $]

The confusion you have is in part how protection for non-motor circuits are required to be sized. The difference between these and motor circuits is that the OCPD

...rating is much closer to the ampacity of the conductors AND they do not have separate overload protection.

For motor circuits, overload protection is what provides conductor protection. The OCPD's provide short-circuit and ground-fault protection only.

(Sorry about the previous incomplete post... it got submitted by accident)

 

[ActionDave]

....This yields a #6AWG THWN conductor, rated for 65 Amperes. If there is a short circuit or overload in feeder conductor, the 150A feeder OCPD will not protect the #6AWG conductor.

The part in red is key. There is a difference between a short circuit and an overload. The #6 wire is protected from overload by the thermal protection that is required for motors, us old guys call them heaters. Short circuit and ground faults are more than 65 amps but they are also more than 150 amps, more like 10000 amps for a split second, so either way the fuse is going to blow.

 

[Smart $]

The part in red is key. There is a difference between a short circuit and an overload. The #6 wire is protected from overload by the thermal protection that is required for motors, us old guys call them heaters. Short circuit and ground faults are more than 65 amps but they are also more than 150 amps, more like 10000 amps for a split second, so either way the fuse is going to blow.

Short-circuit and ground faults only last for a split second because a fuse blows or a circuit breaker trips.

If either were to fail to function or were not part of the circuit, we'd have to concern ourselves with more than the insulation on the wire being compromised. As a last best-case scenario, we hope the wiring functionally acts as a fuse or the source fails in some non-catastrophic manner.

 

[ActionDave]

Short-circuit and ground faults only last for a split second because a fuse blows or a circuit breaker trips.

Yes, you are more correct.

If either were to fail to function or were not part of the circuit, we'd have to concern ourselves with more than the insulation on the wire being compromised. As a last best-case scenario, we hope the wiring functionally acts as a fuse or the source fails in some non-catastrophic manner.

Odds are in our favour.

 

[jumper]

Odds are in our favour.

Blimey mate!!!

You going British on me?

 

[ActionDave]

Blimey mate!!!

You going British on me?

Aye. All sophisticated and whatnot.

 

[jumper]

Aye. All sophisticated and whatnot.

Eh, a bit of a nob now, are you? Rubbish I say.

 

[CharlesK]

Revisiting this example after a few years.....

In our example the motor controller and overload protection is installed next to the motor, while the short circuit protection is installed at the switchboard let's say 250' feet away.

If this motor feeder is installed underground and the conduit is filled with water an overload condition can occur to damage the conductor before the upstream OCPD will trip.

Therefore, the motor overload will not be able to protect the wire from an overload condition in this case.

 

[iceworm]

(Circuit Breaker) .........................................250" UG and water filled .....................................................(Controller - Overload) ......... (Motor)

.... an overload condition can occur to damage the conductor before the upstream OCPD will trip ....

Any current entering the CB end has to exit the overload end. So, if the circuit is overloaded, the overload will trip.

Now, if by

underground and the conduit is filled with water

, you are implying the fault occurs in this portion,
The only way an overload occurs between the CB and the overloads is if the conductors are damaged - like a backhoe attack.

(Circuit Breaker) .............................250" UG and water filled .............................(FAULT).................................................(Controller - Overload) ......... (Motor)

Well, you already have the answer, the conductors are damaged - there is nothing to protect or save. So, the overloads won't trip because the fault current is not going through the overloads.

Eventually the fault gets bad enough the CB trips.

One might say the structure is protected by the CB. To protect the conductors, consider - more concrete and steel.

 

[bwat]

CharlesK, if you are saying you find this strange, you are not alone. I have also thought of this same scenario and wondered if there was somewhere in the code that would prohibit you from doing exactly what you describe. I haven't found one, so I was excited to see that you asked this. I find it surprising that there isn't somewhere in the code, in article 430 for example, that puts a length limit to the distance between the CB that serves as the SC/GF and the motor overload.

With the situation you describe, you could have a 250' run with a CB at about 2x the ampacity of the conductors, and that's legal. But now if we were to switch this from a motor circuit to a regular feeder (so only the device we terminate on changes), even though you end up terminating on a device rated at the ampacity of the conductors, you probably end up being in the tap rule territory and could be in violation because you are over 25 feet (assuming indoors).

iceworm, I get what you're saying, but I hope I showed above that we don't apply the same philosophy to feeders.

 

[iceworm]

... worm, I get what you're saying, but I hope I showed above that we don't apply the same philosophy to feeders.

(emphasis by the worm)
Not exactly. Look at:
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

240.92.C​

240.92.D​

Yep, these are all part of my work - as in normal.

An overload in the middle of a feeder doesn't happen - unless the event murdered them. "In the pump room, by Mr. Operator, with the backhoe" Feeders overloaded in the middle are dead. An OCPD can't stop that from happening. The best it can do is put out the fire.

Or in the case of a transformer secondary, the primary OCPD is hopefully set to trip inside of the transformer damage curve. They are not trying to save the secondary conductors, they are already dead. But it would be good if the primary protection tripped before the transformer caught fire.

 

[ActionDave]

... but I hope I showed above that we don't apply the same philosophy to feeders.

No, you didn't show anything. A motor circuit and a feeder are different and different rules apply.

 

[Jraef]

But now if we were to switch this from a motor circuit to a regular feeder (so only the device we terminate on changes), even though you end up terminating on a device rated at the ampacity of the conductors, you probably end up being in the tap rule territory and could be in violation because you are over 25 feet (assuming indoors).

The highlighted area is where you are mistaken. The termination is not the difference. These special OCPD rules for motor circuits are BECAUSE there is a separate requirement for running overload protection in a motor circuit. If you don’t have running overload protection, those same rules allowing fuses or breakers that large don’t apply. The only time I’ve seen that happen is when people attempt to use fuses AS the motor overload protection. It’s possible under the right set of circumstances, but very difficult to attain because of standard fuse sizes.

 

[bwat]

To be clear, we're talking about the wires of a long length before the OL. I understand that in a motor circuit the duties of the OCPD is (can be) split between multiple devices. I apply this code all this time and it makes sense.

What is strange is that there isn't a "length limit", such as there is with indoor feeder taps, between the two devices that combine to accomplish the overcurrent protection; CB and OL. 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. Agree? And I mean in theory here... not what the code says.

Let's say I have a unicorn motor with FLA = FLC = 100A. CB can be 250A, conductors to be at least 125A, and let's say the OL is 100A. I could then have this:

Scenario 1, motor circuit:
(250A CB)................(250' of 125A wire)..........(100A OL)

But if it's not a motor circuit, but rather a feeder, I could have this following scenario. Note that with respect to the 250' run, the only thing that differs is the device at the end of the 250' run... and they're both overcurrent devices.

Scenario 2, feeder circuit:
(250A CB)................(250' of 125A wire)..........(100A CB)


Scenario 1 above is completely to code. Scenario 2 is not even close. I recognize that code sections can be very application specific. What is ok for this circuit, doesn't have the same leniency for this other circuit and vice versa. No problem there and a lot of times it's a small adjustment or exception that can be made for certain circuit types. But for scenario 2 to be ok, either the 25' indoor feeder tap rule would have to not actually have a length limit, or the feeder conductor next size up rule would have to be a 2x conductor ampacity rule... which is of course a lot more than a next size up/round up.

I get that this is the way the code is. But how is this not strange?


I really like to understand the why or the principle behind code. It's easy for me to forget certain parts of the what in the code. But if I understand the why, it will tend to stick better and be easier to apply.

 

[david luchini]

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.