Multiple motors on same branch circuit

[cppoly]

Need a little help with 430.53 Several Motors or Loads on One Branch Circuit

Can someone break this down for 430.53(A), (B), and (C) and give quick examples of what these are applied to.

430.53(A) Not Over 1 HP
430.53(B) If Smallest Rated Motor Protected
(By the way this section doesn't make sense to me since it takes about protecting the SMALLEST motor with ground fault short circuit protection, but 430.53(C)(4) does the complete OPPOSITE and talks about protecting the LARGEST motor with ground fault short circuit protection??)
430.53(C) Other Group Installations
(Examples of this?)

 

[cppoly]

Any input at all would be helpful

 

[kwired]

Generally not happening with 1 HP 120 volt single phase motors, maybe with 1/2 hp or less it can happen.

480 volt three phase 1 HP motor only draws ~2 amps at full load rating, plenty of capacity on a 15 amp circuit to get 4-6 motors on that circuit. Each one still needs individual overload protection somehow. 15 amp is smallest standard overcurrent device if using circuit breakers, if using fuses 1,3,6 and 10 are also standard sizes and you won't be doing the multiple motor thing as easily since you would need at least a 6 amp fuse for the 1 HP 480 volt motor I mentioned.

 

[cppoly]

Thanks. Do you know what Other Group Installation refers to? There's more requirements in this section.

 

[kwired]

First thing that is key in (C) is :

"where the motor controller(s) and overload device(s) are (1) installed as a listed factory assembly and the motor branch-circuit short-circuit and ground-fault protective device either is provided as part of the assembly or is specified by a marking on the assembly, or (2) the motor branch-circuit short-circuit and ground-fault protective device, the motor controller(s), and overload device(s) are field-installed as separate assemblies listed for such use and provided with manufacturers’ instructions for use with each other"

Basically you can't design/build this yourself, it must be listed assembly or listed to to be assembled that way.

 

[cppoly]

Thanks again!

So if you have motors over 1 HP, you would need to follow (B).

But honestly (B) makes no sense. It allows you to have several motors on the same branch circuit (in this case over 1 HP) but requires you not to exceed the smallest motor OCPD while ensuring the OCPD of this branch circuit will not open under the most severe normal conditions. To me that means both motors starting at the same time. How you could protect the smallest motor if the largest motor starts at the same time. This OCPD would most likely trip.

 

[petersonra]

Thanks again!

So if you have motors over 1 HP, you would need to follow (B).

But honestly (B) makes no sense. It allows you to have several motors on the same branch circuit (in this case over 1 HP) but requires you not to exceed the smallest motor OCPD while ensuring the OCPD of this branch circuit will not open under the most severe normal conditions. To me that means both motors starting at the same time. How you could protect the smallest motor if the largest motor starts at the same time. This OCPD would most likely trip.

The code allows for a 15 amp CB to be used as the OCPD. At 480 V, you could conceivably have several 1 HP motors start simultaneously and not have it trip a 15 A CB.

 

[kwired]

Thanks again!

So if you have motors over 1 HP, you would need to follow (B).

But honestly (B) makes no sense. It allows you to have several motors on the same branch circuit (in this case over 1 HP) but requires you not to exceed the smallest motor OCPD while ensuring the OCPD of this branch circuit will not open under the most severe normal conditions. To me that means both motors starting at the same time. How you could protect the smallest motor if the largest motor starts at the same time. This OCPD would most likely trip.

As perersonra mentions with 15 amp breaker being smallest standard overcurrent device, your three phase motors, and especially 480 volt systems is where you would find this used the most. 208 or 240 volts you probably only have about 2 Hp motors max that you would be able to do this with, and in those cases probably only two motors maybe three but might have unexpected trip occasionally. With 480 volt you could have two 5 Hp motors but maybe 3 or even 4 2 HP motors - if the breaker will hold during starting Code kind of says 4 is too much minimum wise, experiences tell me unless they are fully loaded and run long periods of time, they likely will work, especially if they don't start simultaneously.

The code allows for a 15 amp CB to be used as the OCPD. At 480 V, you could conceivably have several 1 HP motors start simultaneously and not have it trip a 15 A CB.

 

[Biogas Sparky]

Hey guys.

First time writing in, but used a lot. Going over this thread plus NEC I am wondering if I am understanding this. This is what I typically work with.

CB----Contactor----100to800ft--------CB inside enclosure (could have 1-2 enclosures per group)------motor OL(sometimes up to 4 motors, each has OL)--------Motor

So what I understand is, because I will run multiple motors on one starter, the first CB is for the group and is considered "branch"and is sized 125% the largest plus the sum of the others (all the same size motors), and the second inside the enclosure is protecting the motor leads sized 125% of the largest and the sum of the others (all continuous)

This has always been a hard one for me to fully understand and make sure I am sizing it correctly. I can give a more in depth description if needed.

Thanks in advance.

 

[kwired]

Hey guys.

First time writing in, but used a lot. Going over this thread plus NEC I am wondering if I am understanding this. This is what I typically work with.

CB----Contactor----100to800ft--------CB inside enclosure (could have 1-2 enclosures per group)------motor OL(sometimes up to 4 motors, each has OL)--------Motor

So what I understand is, because I will run multiple motors on one starter, the first CB is for the group and is considered "branch"and is sized 125% the largest plus the sum of the others (all the same size motors), and the second inside the enclosure is protecting the motor leads sized 125% of the largest and the sum of the others (all continuous)

This has always been a hard one for me to fully understand and make sure I am sizing it correctly. I can give a more in depth description if needed.

Thanks in advance.

Does each motor have no more than max allowed short circuit/ground fault protection?

When there is multiple motors on same branch circuit this generally means they are going to be motors that draw less than maybe 6-8 amps or they maybe don't run simultaneously.

 

[Biogas Sparky]

Technically yes because we use the Fuji Duo series OL and it has an instantaneous trip ratting set to protect the conductors. Each motor has its own OL.

What I have come to realize the last few days of digging in is:
-The first CB in line (being for feeder) shall be permitted to be based on the ampacity of feeder conductors, 430.62(B). I size feeder conductors for 125% of the FLC of all the motors, although typically a little larger because I have to take into account voltage drop. 430.24 says I can size to 125% the largest motor+ sum of all others. (They are continuous duty)
- The contactor that controls all the motors has to be rated for combination use. UL 508
- The CB inside the enclosure that holds the OLs is sized to protect the conductors of all the motors in that box. Lets say 2 motors so it is 20a. 430.52
- There is an OL for each motor and is based on 125% of the motor FLA and is also set to protect each individual conductor because of the dual function of the specific OL be purchase. 430.87. (this also acts as an individual motor controller because it is able to LOTO if needed)
- They are a "4" Hp motor with an FLA of 5.8. This is also a bit of a job because no 4hp motor listen in the NEC.

I am hoping this makes sense

 

[paulengr]

Technically yes because we use the Fuji Duo series OL and it has an instantaneous trip ratting set to protect the conductors. Each motor has its own OL.

What I have come to realize the last few days of digging in is:
-The first CB in line (being for feeder) shall be permitted to be based on the ampacity of feeder conductors, 430.62(B). I size feeder conductors for 125% of the FLC of all the motors, although typically a little larger because I have to take into account voltage drop. 430.24 says I can size to 125% the largest motor+ sum of all others. (They are continuous duty)
- The contactor that controls all the motors has to be rated for combination use. UL 508
- The CB inside the enclosure that holds the OLs is sized to protect the conductors of all the motors in that box. Lets say 2 motors so it is 20a. 430.52
- There is an OL for each motor and is based on 125% of the motor FLA and is also set to protect each individual conductor because of the dual function of the specific OL be purchase. 430.87. (this also acts as an individual motor controller because it is able to LOTO if needed)
- They are a "4" Hp motor with an FLA of 5.8. This is also a bit of a job because no 4hp motor listen in the NEC.

I am hoping this makes sense

The difference between “group” motors and just regular distribution is whether or not they are intended to be operated together. That does not necessarily mean started and stopped from one contactor...stages starting is allowed, but that generally it’s all one multi motor machine. Part of this is philosophical. For instance rock quarries might have several conveyors in series with a rock crusher and screens. They routinely run different numbers of conveyors depending on where they are feeding the product so those are generally not treated as group motors but if the rock crusher has two motors load sharing, it is. It is also partly practical. The main disconnect is just that. It is the point where we do LOTO and is generally responsible for tripping on short circuits. So we lose “everything” at that point. That is really not what you have. Because you are feeding multiple motors from a single starter protected individually by an MMS if I understand you correctly we could treat the starter as a simple switch in the feeder and each motor+MMS is it’s own branch starting at the feed terminals of the MMS. The issue here being the very low SCCR (5 kA) of the contactor. This is generally a problem for most of the MMS’s on the market and no contactor is rated for circuit interruption duty...they are switches, not breakers. At higher voltages and power we tend to blur the lines.

I would start at the C.B. feeding the contactor(s) as the branch unless the it is not set up for LOTO in which case it would be the main breaker.