Motor Branch Circuit Design

[charlie b]

Motors have never been my best friends. One of my engineers asked this question, and I can’t answer it.

You have, in order, a circuit breaker within a branch panel, a cable, a fused disconnect, a cable, a motor controller, a cable, and an elevator motor. The controller is rated at 100 amps. The motor has a starting current of 236 amps and a full load current of 79 amps.

If I sized the breaker or the fuse to allow for the starting current, then neither would protect the controller at its rating. What ratings do I assign for the breaker, the cables, and the fuse?

 

[infinity]

Wouldn't a time delay fuse allow the motor to start?

 

[GoldDigger]

And a typical breaker with both magnetic and thermal trip will also hold the starting surge well enough if sized based on a multiple of the FLA as described/permitted in the code.
The motor overload function will be separate from either fuse or breaker.

 

[ActionDave]

You have, in order, a circuit breaker within a branch panel, a cable, a fused disconnect, a cable, a motor controller, a cable, and an elevator motor. The controller is rated at 100 amps. The motor has a starting current of 236 amps and a full load current of 79 amps.

If I sized the breaker or the fuse to allow for the starting current, then neither would protect the controller at its rating. What ratings do I assign for the breaker, the cables, and the fuse?

Start with the motor and work back. 430.6 takes you to the tables. Table values times 125% sizes your motor conductors and base fuse size. 430.21 and 430.52 lets you oversize you fuses to allow for starting current. That covers the motor up to the fused disconnect.

From the fused disconnect back to the breaker it's a feeder so size the fused disconnect, cable, and breaker accordingly a la 310.

 

[Jraef]

The breaker or fuse only needs to supply the short circuit protection, assuming there is a thermal overload relay associated with the controller (starter, drive etc.). So to that end, the feeder breaker in the panel only needs to be sized to properly protect the conductors, which are sized based on the motor HP and FLA tables in the NEC.

TECHNICALLY, you can use fuses AS the running OL protection as well as the SCPD, but I always advise against that. The NEC makes some allowances for selecting the size of the fuse, but getting a fuse that will still fit the bill for the running OL protection AND not clear on starting current is too much of a hassle if you ask me. I have rarely seen it work.

 

[ActionDave]

The breaker or fuse only needs to supply the short circuit protection, assuming there is a thermal overload relay associated with the controller (starter, drive etc.). So to that end, the feeder breaker in the panel only needs to be sized to properly protect the conductors, which are sized based on the motor HP and FLA tables in the NEC......

So the fact that he has a breaker and a fuse does not change the rules for sizing conductors? The way I read it he has a feeder to the fused disco, not a branch circuit so he can only oversize his overcurrent protection at the fuse down to the starter.

If he had just a breaker or just a fuse I would agree with you without question.

 

[GoldDigger]

So the fact that he has a breaker and a fuse does not change the rules for sizing conductors? The way I read it he has a feeder to the fused disco, not a branch circuit so he can only oversize his overcurrent protection at the fuse down to the starter.

If he had just a breaker or just a fuse I would agree with you without question.

One thing that potentially changes the way the rules apply is if you can categorize the farthest downstream OCPD as "supplemental" protection, in which case the wiring to it is still a branch, not a feeder.

 

[augie47]

So the fact that he has a breaker and a fuse does not change the rules for sizing conductors? The way I read it he has a feeder to the fused disco, not a branch circuit so he can only oversize his overcurrent protection at the fuse down to the starter.

If he had just a breaker or just a fuse I would agree with you without question.

That's a discussion that has gone on for years. Some folks look at it the way you describe, others
like to consider the fuse disconnect as "a disconnect" or as GoldDigger mentions "supplemental" and consider the conductor from the breaker to still be a "branch circuit" (what if the disconnect was non -fusible ) and still a 3rd group look at 430.62 which can be read to allow the feeder to be sized as a branch circuit. It is rare that I see it enforced that the condcutir from the breaker must be sizd larger than the motor branch circuit even though that argument can em made.

 

[Jraef]

So the fact that he has a breaker and a fuse does not change the rules for sizing conductors? The way I read it he has a feeder to the fused disco, not a branch circuit so he can only oversize his overcurrent protection at the fuse down to the starter.

If he had just a breaker or just a fuse I would agree with you without question.

Yeah, I've never been clear on that issue, I always just size it all the way through. You may be technically correct, but it makes no sense to me to have smaller conductors feeding larger ones...

 

[kwired]

430.62(A) permits using higher fuse or breaker setting then conductor ampacity for motor feeders.

If one really want's to get hung up on the fact a feeder is only supplying one motor - by all means add a second tiny motor that has almost no impact and then you are compliant

I think the feeder can supply just a single motor and 430.62(A) still applies because of the wording "A feeder supplying a specific fixed motor load(s)"

 

[ActionDave]

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 the largest rating or setting
of the branch-circuit short-circuit and ground-fault protec-
tive device for any motor supplied by the feeder.....

430.24 Several Motors or a Motor(s) and
Other Load(s)
Conductors supplying several motors, or a motor(s) and
other load(s), shall have an ampacity not less than the sum of
each of the following:........

Staring too long at the code book can make your eyes get itchy and your vision blurry. Gotta look up at the horizon and take a deep breath once in a while. Once I do that I conclude that the code wants feeder conductors sized larger than the branch circuit wires that go to the motor.

 

[kwired]

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 the largest rating or setting
of the branch-circuit short-circuit and ground-fault protec-
tive device for any motor supplied by the feeder.....

430.24 Several Motors or a Motor(s) and
Other Load(s)
Conductors supplying several motors, or a motor(s) and
other load(s), shall have an ampacity not less than the sum of
each of the following:........

Staring too long at the code book can make your eyes get itchy and your vision blurry. Gotta look up at the horizon and take a deep breath once in a while. Once I do that I conclude that the code wants feeder conductors sized larger than the branch circuit wires that go to the motor.

Only if there is more load then just a single motor. The sum of current for all one motors is still the same thing as you have for a branch circuit to that one motor.

 

[iceworm]

Motors have never been my best friends. One of my engineers asked this question, and I can’t answer it.

You have, in order, a circuit breaker within a branch panel, a cable, a fused disconnect, a cable, a motor controller, a cable, and an elevator motor. The controller is rated at 100 amps. The motor has a starting current of 236 amps and a full load current of 79 amps.

If I sized the breaker or the fuse to allow for the starting current, then neither would protect the controller at its rating. What ratings do I assign for the breaker, the cables, and the fuse?

Charlie -
There are already several good posts concerning the code issues - so I won't comment on that. Let's look at this from the point of what are we trying to protect and why. I am going to limit my discussion to normal industry motors/loads installations.

Disclaimer: You used the word "elevator'. I don't do elevators, so there may be elevator codes/Life Safety codes that I am not aware of.

First, the circuit loading is limited by design, not by the circuit breaker or fuse. The motor is selected such that it is operating within spec - normal circuit loading is fine.

Now one must protect the equipment and personnel in the event of a fault/malfunction. For this limited discussion, the controller always has an overload device. This is set to protect the conductors between the feeder CB to the motor - but not necessarily the motor. The overload trip curve is inside of the cable damage curve, but not necessarily inside of the motor damage curve. For example, a typical industrial installation is a combination starter, consisting of a mag-only CB, contactor, overload. I tend to set the overloads (1.15sf motor) to 140%, and mag only to the NEC max (generally 11x to 17x fla, depending on the motor). The overload curve has to be outside of the motor starting curve. The mag trip has to be outside of the inrush (which can be substantially greater than the nominal 6x fla.

Say the motor/mechanical load malfunctions - dragging bearings,stuck relief valve, binding coupling, internal motor electrical fault that does not result in a short circuit. The overloads will trip before the conductors are damaged. If the cause is a motor internal fault, there is no motor to save. It's toast. If it is a mechanical malfunction, again the overloads will trip saving the conductors. And one could get lucky and have a motor still worth rebuilding - but no guarantee on that.

Now if the motor or conductors between the overloads and motor, develops a serious fault, as in short circuit, there is no motor to save, or no conductors to save. So the starter CB/fuses are set to get the fire put out as quickly as possibe. One could say the CB/fuses are selected to protect the structure - and the personnel are protected by not having the structure catch fire.

One might say the conductors between the feeder CB and the overloads are not protected from overload. Well they are - any current going in the feeder CB end has to come out of the overload end. Anything else is a backhoe/forklift attack - again, there are no conductors left to protect, just a fire to put out to protect the structure.

enough with the philosophy

Your specific case:
TM CB in a panel
conductors to fused disconnect (in controller)
controller (with overloads in any of my applications)
Conductors to motor
Motor FLA 79A

This is not something I normally have to deal with - however, here is the method I would use (assuming a design B, code G motor):
Consider the panel TM CB is the 430.52 short circuit protection. Set it at 250% (unless it is a screwie motor). Conductors to controller are sized at 125%. The controller fused disconnect is there to provide the controller disconnect. Slug it if you want - the fuses serve no purpose. Or, use the exception and select the longest time delay fuses available at 225%. Set the overloads to max 140% for sf 1.15, 130% for sf 1.0. Conductors from controller to motor are 125%.

There is no reason to size the conductors from the panel to the controller larger than the motor circuit conductors (other than NEC legal wrangling). I would not expect an AHJ to complain. If they did, I'd suggest the controller fused disconnect was for the required in-sight controller disconnect and did not perform short circuit protection - the panel CB did that. If she drug her feet, I's suggest we could slug the fused disconnect. It the AHJ was still stuck, I'd replace the controller fused disconnect with a non-fused disconnect.

I know this reads somewhat scrambled - but I'm short on time. Susposed to be working a paying job. If someone see flaws in my reasoning - I'm interested. I'm not stuck.

The worm

 

[Dennis Alwon]

I believe you can size the breaker to 250% of the rating in Table 430.2xx

 

[augie47]

Keep in mind, as iceworm alluded to, most of what we have discussed has been about motors generally. Since this is a elevator motor be sure to consult Art 620 and some of the specific numbers may change considerably such as circuit conductor sizes based on 620.13/430.22(E)

 

[kwired]

Charlie -
There are already several good posts concerning the code issues - so I won't comment on that. Let's look at this from the point of what are we trying to protect and why. I am going to limit my discussion to normal industry motors/loads installations.

Disclaimer: You used the word "elevator'. I don't do elevators, so there may be elevator codes/Life Safety codes that I am not aware of.

First, the circuit loading is limited by design, not by the circuit breaker or fuse. The motor is selected such that it is operating within spec - normal circuit loading is fine.

Now one must protect the equipment and personnel in the event of a fault/malfunction. For this limited discussion, the controller always has an overload device. This is set to protect the conductors between the feeder CB to the motor - but not necessarily the motor. The overload trip curve is inside of the cable damage curve, but not necessarily inside of the motor damage curve. For example, a typical industrial installation is a combination starter, consisting of a mag-only CB, contactor, overload. I tend to set the overloads (1.15sf motor) to 140%, and mag only to the NEC max (generally 11x to 17x fla, depending on the motor). The overload curve has to be outside of the motor starting curve. The mag trip has to be outside of the inrush (which can be substantially greater than the nominal 6x fla.

Say the motor/mechanical load malfunctions - dragging bearings,stuck relief valve, binding coupling, internal motor electrical fault that does not result in a short circuit. The overloads will trip before the conductors are damaged. If the cause is a motor internal fault, there is no motor to save. It's toast. If it is a mechanical malfunction, again the overloads will trip saving the conductors. And one could get lucky and have a motor still worth rebuilding - but no guarantee on that.

Now if the motor or conductors between the overloads and motor, develops a serious fault, as in short circuit, there is no motor to save, or no conductors to save. So the starter CB/fuses are set to get the fire put out as quickly as possibe. One could say the CB/fuses are selected to protect the structure - and the personnel are protected by not having the structure catch fire.

One might say the conductors between the feeder CB and the overloads are not protected from overload. Well they are - any current going in the feeder CB end has to come out of the overload end. Anything else is a backhoe/forklift attack - again, there are no conductors left to protect, just a fire to put out to protect the structure.

enough with the philosophy

Your specific case:
TM CB in a panel
conductors to fused disconnect (in controller)
controller (with overloads in any of my applications)
Conductors to motor
Motor FLA 79A

This is not something I normally have to deal with - however, here is the method I would use (assuming a design B, code G motor):
Consider the panel TM CB is the 430.52 short circuit protection. Set it at 250% (unless it is a screwie motor). Conductors to controller are sized at 125%. The controller fused disconnect is there to provide the controller disconnect. Slug it if you want - the fuses serve no purpose. Or, use the exception and select the longest time delay fuses available at 225%. Set the overloads to max 140% for sf 1.15, 130% for sf 1.0. Conductors from controller to motor are 125%.

There is no reason to size the conductors from the panel to the controller larger than the motor circuit conductors (other than NEC legal wrangling). I would not expect an AHJ to complain. If they did, I'd suggest the controller fused disconnect was for the required in-sight controller disconnect and did not perform short circuit protection - the panel CB did that. If she drug her feet, I's suggest we could slug the fused disconnect. It the AHJ was still stuck, I'd replace the controller fused disconnect with a non-fused disconnect.

I know this reads somewhat scrambled - but I'm short on time. Susposed to be working a paying job. If someone see flaws in my reasoning - I'm interested. I'm not stuck.

The worm

I don't entirely agree with some of what you said.

Motor overload protection does protect the motor - is sized to the motor nameplate rating, must be re-evaluated if you change to a different motor. Conductors must be sized to carry rated motor current (and usually 25% more) so if you protect the motor you also protect the conductors. Motors want to maintain shaft speed, if you add more load, they draw more current to try to keep the shaft speed maintained. This is why we need motor overload protetion. If motors drew fixed current things would be different, but current varies according to mechanical driven load conditions.

 

[Jraef]

I don't entirely agree with some of what you said.

Motor overload protection does protect the motor - is sized to the motor nameplate rating, must be re-evaluated if you change to a different motor. Conductors must be sized to carry rated motor current (and usually 25% more) so if you protect the motor you also protect the conductors. Motors want to maintain shaft speed, if you add more load, they draw more current to try to keep the shaft speed maintained. This is why we need motor overload protetion. If motors drew fixed current things would be different, but current varies according to mechanical driven load conditions.

I agree with kwired here. Motor thermal OL curves are SPECIFICALLY designed with the MOTOR thermal damage curve in mind, that is part and parcel to the use of an I²t curve based on a "NEMA Class", i.e. 10, 20. 30 etc. That NEMA Class is based on being just under the maximum point in the motor damage curve at Locked Rotor Current from NEMA design criteria (averaged out at 600%) and the number, 10, 20, 30, is the number of seconds the motor WINDINGS can safely handle that. So Class 20 dictates 600% FLC for 20 seconds, Class 10 is 600% FLC for 10 seconds etc.. The fact that this will ALSO protect the conductors in consequential, but is taken advantage of in the case of a factory combination starter with a mag-only breaker.

I tend to set the overloads (1.15sf motor) to 140%

Motor shops probably love you...
Many, if not most, OL relays are ALREADY factoring in a pick-up point of between 115 -125% of the FLC selection point, which follows the 1.0SF rule as a "worst case" scenario. By THEN basing your OL setting / selection on 140% of FLC, your actual pickup point becomes 140% of 125% (175%), which can allow motor winding damage on a sustained OL condition. But not all OLs are like this, you have to RTFM... that said, ALL of the IEC dial adjustable OL relays ARE like this already, that's a hard and fast rule for IEC because they don't have "Service Factor" motors (they are all 1.0SF in other words), so they will warn you to NOT set it above the FLC of the motor. now if you are using a 1.15SF NEMA design motor that was likely made for a Class 20 OL, and you use a Class 10 IEC OL relay, you can probably get away with a slightly higher setting. But there is no direct linear relationship to this, so you really don't know what you are setting it at if you do that.

 

[iceworm]

I don't entirely agree with some of what you said.

Motor overload protection does protect the motor - is sized to the motor nameplate rating, must be re-evaluated if you change to a different motor. Conductors must be sized to carry rated motor current (and usually 25% more) so if you protect the motor you also protect the conductors. Motors want to maintain shaft speed, if you add more load, they draw more current to try to keep the shaft speed maintained. This is why we need motor overload protetion. If motors drew fixed current things would be different, but current varies according to mechanical driven load conditions.

i'm okay with you not agreeing, but give this some thought: Of course the overload protection must be evaluated if one changes the motor, inertia, geaing, pump size .... The first two I bolded are a change in design. As I said, the loading is protected through design. What is going to change that will increase loading? Someone in the dead of the night changing gearing? Yes, I have seen a few that can change loading through operation procedure. Still, ignorance I can fix. I can not fix stupid. And I can't make fool proof. The universe is in competition with engineers. The engineers design better, more foolproof equipment. The universe breeds better fools. So far the universe is winning.

Overload protection is not designed to prevent some moron from sneaking out and "fixing" the equipment.

My favorite one of these was the guy that went out and put in smaller overloads to "protect the motor". It was a pulp mill debarker, 100hp tin-can Lincoln. The overloads were set at 130% - class 30, high inertia, long start time. The crew could shove the logs through fast enough to overload - they had to work at it , but they could. Occasionally they would trip the overloads - three or four times a week. So one night after a trip, he changes out the overloads (110% as I recall). Within a couple of hours, the motor trips - reset. Trips agin - reset. Trips again - reset. On the fourth start within an hour the smoke comes out of the motor.

There is a point. You can't fix overload resulting from poor design by changing the overloads. One fixes a poor design by ................... drum roll ............
Fix the design

You are correct that overload trip curves are also picked to be inside of the motor starting current current profile and the starting current current damage curve. Both of those usually come up up for larger motors, high inertia loads. Truly, I don't believe I have ever seen a mechanic put in the worng 2000Hp motor. However, both of those are outside of my discussion. This discussion is the basic protection philosophy - industrial grade.

And yes, there are electronic overloads that can be tailored to a motor thermal model. AB E3+ is an excellent example, and yes they work well down in the integral horsepower sizes. That will still not fix someone putting in the wrong motor.

The worm

 

[kwired]

i'm okay with you not agreeing, but give this some thought: Of course the overload protection must be evaluated if one changes the motor, inertia, geaing, pump size .... The first two I bolded are a change in design. As I said, the loading is protected through design. What is going to change that will increase loading? Someone in the dead of the night changing gearing? Yes, I have seen a few that can change loading through operation procedure. Still, ignorance I can fix. I can not fix stupid. And I can't make fool proof. The universe is in competition with engineers. The engineers design better, more foolproof equipment. The universe breeds better fools. So far the universe is winning.

Overload protection is not designed to prevent some moron from sneaking out and "fixing" the equipment.

My favorite one of these was the guy that went out and put in smaller overloads to "protect the motor". It was a pulp mill debarker, 100hp tin-can Lincoln. The overloads were set at 130% - class 30, high inertia, long start time. The crew could shove the logs through fast enough to overload - they had to work at it , but they could. Occasionally they would trip the overloads - three or four times a week. So one night after a trip, he changes out the overloads (110% as I recall). Within a couple of hours, the motor trips - reset. Trips agin - reset. Trips again - reset. On the fourth start within an hour the smoke comes out of the motor.

There is a point. You can't fix overload resulting from poor design by changing the overloads. One fixes a poor design by ................... drum roll ............
Fix the design

You are correct that overload trip curves are also picked to be inside of the motor starting current current profile and the starting current current damage curve. Both of those usually come up up for larger motors, high inertia loads. Truly, I don't believe I have ever seen a mechanic put in the worng 2000Hp motor. However, both of those are outside of my discussion. This discussion is the basic protection philosophy - industrial grade.

And yes, there are electronic overloads that can be tailored to a motor thermal model. AB E3+ is an excellent example, and yes they work well down in the integral horsepower sizes. That will still not fix someone putting in the wrong motor.

The worm

I deal with a lot of grain handling equipment. Equipment is presumably sized to handle a specific volume per minute - within certain conditions. Change those conditions for whatever reason whether intentional or not and motor overload may be called on to respond.

Real common situation is storage bin unloading augers moving corn. Get some high moisture corn in that bin and the auger may be loaded more then originally intended. Motor overload trips - operator learns that he needs to close the trap so it doesn't feed as much into the auger and can continue operation.

Another common situation elevator leg - especially one that handles multiple types of grain, feed or other ingredients. Depending on what material is being moved operators need to adjust traps, gates, etc to keep flow at a rate the leg can handle. Most of these we do put an ammeter at the operator station so they can monitor load instead of just having it trip unexpectedly. Once had a owner of a new system complain - we need to be able to unload trucks faster then that. To which my reply was something like - you needed your equipment guys to sell you larger equipment then because this motor is doing about all it is rated for and if we set things so it can go more - it will fail sooner instead of later.

Even pumping applications - a change in media density, a break in a line, a partially plugged line, open or closed valve that shouldn't be... will effect load on the pump motor

So yes motors are sized for certain conditions, but conditions can vary at times and that is where overload protection protects the motor.

There is probably more to the story on your pulp mill. High inertia - maybe too many starts in too short of time period was a bigger problem then a little overloading.

 

[iceworm]

...Many, if not most, OL relays are ALREADY factoring in a pick-up point of between 115 -125% of the FLC selection point, which follows the 1.0SF rule as a "worst case" scenario. By THEN basing your OL setting / selection on 140% of FLC, your actual pickup point becomes 140% of 125% (175%), which can allow motor winding damage on a sustained OL condition. But not all OLs are like this, you have to RTFM... that said, ALL of the IEC dial adjustable OL relays ARE like this already, that's a hard and fast rule for IEC because they don't have "Service Factor" motors (they are all 1.0SF in other words), so they will warn you to NOT set it above the FLC of the motor. now if you are using a 1.15SF NEMA design motor that was likely made for a Class 20 OL, and you use a Class 10 IEC OL relay, you can probably get away with a slightly higher setting. But there is no direct linear relationship to this, so you really don't know what you are setting it at if you do that.

Ahhhh ... Yes one must be capable and willing to read.

I know this will be suprising, but when I said I tend to set the overloads at 140% fla (for a 1.15sf) - that's what I meant - not some oddball, anecdotal calculation. As for figuring out the manuals to determine the actual overload setting, the AB E3+ holds the prize for the most convoluted. But it is decipherable.

As for IEC tin-cans - that is not part of my discussion. So surprisingly, none of my comments address setting overloads for IEC motors.

As for "probably get away with a slightly higher setting". That is riduculous. One does not "get away" with anything. One does the modeling, does the calculations, installs the appropriate equipment. Commissions and measures the results. Adjusts for process variances. This is industry. It has to make money and to do that it has to stay running - generally flatout, 24/7, probably +10%

And yes the motor rewinders do like me. I tend to spec balance rotors and fans, high temp wire and varnish, vacuumed and baked, high temp sleeving on the leads, ngli2 arctic grease in the bearings. Yeah, they make money.

If you have a valid dissent (that is spelled wrong), I'm listening. But you will need to stick in the limits of the discussion. Telling me that what I do doesn't fit IEC motors doesn't help much. Of course it doesn't fit.

The worm