Motor Feeder Overload

Status
Not open for further replies.
erickench

erickench

Senior Member
Location
Brooklyn, NY
As I look through the NEC I notice that there is no rule for the overload protection of motor feeders. There is however rules for Short Circuit-Ground Fault protection. Does anyone know of a rule that covers the overload protection for motor feeders?
 
It is assumed that the motor overload protection will also protect a correctly sized motor feeder from overloads.
 
Volta said:
Wouldn't it? If one or more motors become overloaded, and the current stops flowing to those motors, wouldn't that protect the feeder?
Click to expand...

This is exactly right. (IMO) The feed doesn't care if you're powering motors, or lighting circuits. As long as no fault conditions exist within the feeder circuits, the supply will remain intact. The overload protection for the motor is just that. FOR THE MOTOR. if a motor burns up, shorts, etc., your overcurrent protection device will seperate the motor from the source (feeder). The feeder will not be harmed as there is no longer a connection to the fault.

erickench said:
As I look through the NEC I notice that there is no rule for the overload protection of motor feeders. There is however rules for Short Circuit-Ground Fault protection. Does anyone know of a rule that covers the overload protection for motor feeders?
Click to expand...

The feeder is required to have it's OWN overcurrent and ground fault protection, (breaker, fuses, etc). They are kinda like 2 seperate entities serving a common purpose.
 
CONTROL FREQ said:
The feeder is required to have it's OWN overcurrent and ground fault protection, (breaker, fuses, etc). They are kinda like 2 seperate entities serving a common purpose.
Click to expand...

If the feeder is required to have both overload and groundfault-short circuit protection then wouldn't the rules of NEC 215.3 cover this requirement for motor feeders?
 
erickench said:
Then why doesn't the same rule apply to feeders that feed loads other than motors? Those other feeders are protected against overload.
Click to expand...

It might be that these feeders already have 25% of the largest added per 430.62(A) > 430.24, less exceptions, which might mitigate the short-term effects of motor overloads, while non-motor and non-continuous feeders don't have that 25%, and need tighter OL control.
 
erickench said:
If the feeder is required to have both overload and groundfault-short circuit protection then wouldn't the rules of NEC 215.3 cover this requirement for motor feeders?
Click to expand...

There are motors in my plant that are fed straight from a breaker box, through a 3 pole paddle switch (motor switch), and that's IT... they've been that way for 30 years. As far as I know, if the breaker and wires are sized properly for that motor, it's fine. The down side is... by the time that breaker trips, the motor... well... I don't know if you've ever tried to get that smoke back into a motor... but I've never seen anyone who could do it.:happyno:

PS, I really would not be surprised to find out I'm wrong about it being "fine" in this day and age, but it HAS worked for a long time. I'm always ready to learn something though.
 
The problem is that article 430 does not reference NEC 215.3. However, both overload and short circuit-ground fault protection can be achieved with a single inverse time circuit breaker that has both a long time and short time trip setting.
 
erickench said:
The problem is that article 430 does not reference NEC 215.3. However, both overload and short circuit-ground fault protection can be achieved with a single inverse time circuit breaker that has both a long time and short time trip setting.
Click to expand...
I don't have my NEC copy here with me, but I belive that although 430 doesn't mention 215, 215 (or somewhere above that) does mention that MOTOR circuits fall under 430.

430 requires an overload protection device in all ungrounded conductors. If that device is an Overload Relay, that is considered the overload protection for the entire circuit, i.e. the conductors INSIDE the box as well as the motor leads. That does not however apply to the conductors that are bringing power TO the starter box, which must have their own separate feeder or branch OCPD.
 
Jraef said:
I don't have my NEC copy here with me, but I belive that although 430 doesn't mention 215, 215 (or somewhere above that) does mention that MOTOR circuits fall under 430.

430 requires an overload protection device in all ungrounded conductors. If that device is an Overload Relay, that is considered the overload protection for the entire circuit, i.e. the conductors INSIDE the box as well as the motor leads. That does not however apply to the conductors that are bringing power TO the starter box, which must have their own separate feeder or branch OCPD.
Click to expand...


The overload protection rules outlined in article 430 apply to the branch circuits only not the feeders.
 
CONTROL FREQ said:
There are motors in my plant that are fed straight from a breaker box, through a 3 pole paddle switch (motor switch), and that's IT... they've been that way for 30 years. As far as I know, if the breaker and wires are sized properly for that motor, it's fine. ...<snip>

PS, I really would not be surprised to find out I'm wrong about it being "fine" in this day and age, but it HAS worked for a long time. I'm always ready to learn something though.
Click to expand...

It may indeed have been acceptable 30+ years ago, it's theoretically still possible if all the stars align in terms of breaker ratings and motor sizes, but I have never seen it work. The rules have definitely tightened up over the years. You may be grandfathered in now but if you change anything, you may find yourself in a heap of hurt.

The down side is... by the time that breaker trips, the motor... well... I don't know if you've ever tried to get that smoke back into a motor... but I've never seen anyone who could do it.:happyno:
Click to expand...
Aaaaand there you go. I guess the cost of down time is irrelevant to your company. Either that or they carry a stock of spare motors with quick disconnect plugs on them for swapping them out quickly. It might be worth investing in some proper motor protection... :p

Just FYI, this is a chart of survey results of the average daily cost of downtime by different industries. Divide that by your daily work hours to see how much they are "saving" by being cheap on the OL relays.
cost_survey.gif


The colors have to do with the number of employees in the companies, i.e. 0-30, 31-50, 200-300 etc. You can download the complete paper for free from here. or view it as an image file here.
 
erickench said:
Then why doesn't the same rule apply to feeders that feed loads other than motors? Those other feeders are protected against overload.
Click to expand...

Because motors are the only loads that are subject to overload. The 'thermal' overloads that are mimicking the motor thermal profile, protecting against overheating by repeated starts and mechanical overload. Transformers are capable of more severe and sustained overloads, thus fuses and circuit breakers, sized per the NEC rules, will protect against immediate failure,however sustained and repeated overloads will shorten the life of those.
 
erickench said:
The overload protection rules outlined in article 430 apply to the branch circuits only not the feeders.
Click to expand...

Maybe you can elaborate what do you mean by feeders. If you mean that a feeder has multiple branches off of it and only the branch will have full 430 protection, then the answer would be that it is unlikely that ALL motors(branches) would overload at the same time.
 
Weressel, other feeders have their ampacities protected using the 125% continuous plus 100% noncontinuous rule. By motor feeder I mean conductors feeding a panel that has motor loads and maybe some other types of loads connected through branch circuits. Article 430 does provide rules for overload protection of motors but those rules would also protect the branch circuit conductors at their ampacities as well. There is nothing in article 430 that provides protection at the ampacity of motor feeders.
 
erickench said:
Weressel, other feeders have their ampacities protected using the 125% continuous plus 100% noncontinuous rule. By motor feeder I mean conductors feeding a panel that has motor loads and maybe some other types of loads connected through branch circuits. Article 430 does provide rules for overload protection of motors but those rules would also protect the branch circuit conductors at their ampacities as well. There is nothing in article 430 that provides protection at the ampacity of motor feeders.
Click to expand...

Because 430 would have nothing to do with your definition of 'motor feeders' above. They are NOT motor feeders, even by your definition, they are panel feeders.
 
erickench said:
...Article 430 does provide rules for overload protection of motors but those rules would also protect the branch circuit conductors at their ampacities as well. ...
Click to expand...
Given the NEC sizing rules for multimotor feeder conductors, the multiple motor overload devices will also protect the feeder conductors from overload.
 
weressl said:
Because 430 would have nothing to do with your definition of 'motor feeders' above. They are NOT motor feeders, even by your definition, they are panel feeders.
Click to expand...

Okay, part V of article 430 gives rules for motor feeder short-circuit ground-fault protection but not for overload. Maybe I meant to say motor control center although I would think that there are panels that supply motors as well. A feeder would feed two or more motor loads through branch circuits. The NEC does not provide rules for the overload protection of the feeders. NEC 430.24 does give rules for sizing the ampacity of the feeder but not the OCPD at that ampacity. The rules for most other feeders are similar for both ampacity and OCPD in that they are sized according to the 125% continuous plus 100% noncontinuous rule.
 
Last edited:
The NEC does not require the OCPD for a motor feeder to provide overload protection for that feeder. It only requires that the feeder OCPD provide ground fault and short circuit protection for the feeder. The code recognizes that motor feeders, just like motor branch circuit conductors may require the use of an OCPD that is has a rating larger than the ampacity of the conductor to avoiding triping from the motor starting current. If any one of the motors that are fed from the motor feeder have an overload condition, the motor overload device will cause the motor to be disconnected from its power source. This will indirectly provide overload protection for the motor feeder conductors.
Yes motor feeders are not treated the same as other feeders just like motor branch circuits are not treated the same as other branch circuits.
 
Status
Not open for further replies.
Top