Multiple motors on a single branch circuit

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cppoly

Senior Member
Location
New York
I’m trying to connect three motors (under 1 HP, 3 amps each) on a single branch circuit but need help with the wording of 430.53 “Several Motors or Loads on One Branch Circuit”.

My first concern is that this sections says you can follow 430.53(A), (B), or (C). So if all of the motors are under 1 HP, it seems like I can simply follow (A) “Not Over 1 Horsepower” and not be concerned with (B) or (C). But following (A) alone gives no information on branch circuit sizing requirements. Devils advocate: can I connect 10, 20, 30, or 50, motors one branch circuit? Also, how do I size the branch circuit OCPD for multiple motors (in my case 3 motors)?

Section (B) “If Smallest Rated Motor Protected” is a little bit more descriptive and gives more insight into branch circuit sizing. But is this section only followed if one motor or more motors are over 1 HP?

Section (C) sounds like it’s only used for factory assembled installations, is this correct? Section (D) sounds like it only applies to section (C).
 

jeremy.zinkofsky

Senior Member
Location
nj
I’m trying to connect three motors (under 1 HP, 3 amps each) on a single branch circuit but need help with the wording of 430.53 “Several Motors or Loads on One Branch Circuit”.

My first concern is that this sections says you can follow 430.53(A), (B), or (C). So if all of the motors are under 1 HP, it seems like I can simply follow (A) “Not Over 1 Horsepower” and not be concerned with (B) or (C). But following (A) alone gives no information on branch circuit sizing requirements. Devils advocate: can I connect 10, 20, 30, or 50, motors one branch circuit? Also, how do I size the branch circuit OCPD for multiple motors (in my case 3 motors)?

Section (B) “If Smallest Rated Motor Protected” is a little bit more descriptive and gives more insight into branch circuit sizing. But is this section only followed if one motor or more motors are over 1 HP?

Section (C) sounds like it’s only used for factory assembled installations, is this correct? Section (D) sounds like it only applies to section (C).


Simple. Take 125% the nameplate FLA of the largest HP motor, 100% of the nameplate FLA of the rest of the motors on the circuit, and add it all together for minimum circuit ampacity.

Example:

Given (1) 20HP, (1) 10HP, (1) 5HP, & (1) 2HP motors at 460V, 3 Phase respectively.

Looking at NEC table 430.250 to get the FLA's gives us (27 FLA x 1.25) + (14 FLA x 1) + (7.6 FLA x 1) + (3.4 FLA x 1) = 58.75A. So a 60A breaker to feed this multi-motor circuit.
 

cppoly

Senior Member
Location
New York
Thanks but I'm asking about sizing an OCPD for a single branch circuit feeding multiple motors.

Also, I think you sized the OCPD based on minimum circuit ampacity, but OCPD isn't sized this way.
 

jeremy.zinkofsky

Senior Member
Location
nj
Thanks but I'm asking about sizing an OCPD for a single branch circuit feeding multiple motors.

Also, I think you sized the OCPD based on minimum circuit ampacity, but OCPD isn't sized this way.


Sorry, I was thinking conductor ampacity for some reason.

From your explanation, it looks like you meet 430.53(A) which tells you that the OCPD cannot be greater than 20A at 120V. You also need to comply with 430.53(D) which should be easy in your case if you use #12AWG for everything.
 

jrohe

Senior Member
Location
Omaha, NE
Occupation
Professional Engineer
Simple. Take 125% the nameplate FLA of the largest HP motor, 100% of the nameplate FLA of the rest of the motors on the circuit, and add it all together for minimum circuit ampacity.

Example:

Given (1) 20HP, (1) 10HP, (1) 5HP, & (1) 2HP motors at 460V, 3 Phase respectively.

Looking at NEC table 430.250 to get the FLA's gives us (27 FLA x 1.25) + (14 FLA x 1) + (7.6 FLA x 1) + (3.4 FLA x 1) = 58.75A. So a 60A breaker to feed this multi-motor circuit.

For motors except for low speed, high torque, or multispeed motors, NEC section 430.6(A)(1) requires the use of the full-load current (FLC) values from T430.248, T430.249, or T430.250 (not the nameplate FLA values as you stated) for sizing the conductors and short-circuit and ground-fault protection. The nameplate FLA values are used for sizing the overloads per NEC section 430.6(A)(2).

The 27, 14, 7.6, and 3.4 amps noted in the example above are full-load currents (FLC) per T430.250, not nameplate full-load amps (FLA). There is a distinct difference.

This example does accurately convey the correct method for determining the minimum required ampacity for a multi-motor feeder circuit per NEC section 430.24. Therefore, the feeder is required to have a minimum ampacity of 58.75 amps.

However, that does not also equate to the breaker (the feeder's short-circuit and ground-fault protection device). You could be asking for trouble by way of breakers tripping if you size the breaker that tight. Per NEC section 430.62, the first thing you need to figure out is the largest breaker that would be permitted for the largest motor. For an inverse-time circuit breaker serving the 20 horsepower motor, this would be 27 amps x 2.5 = 67.5 amps, rounded up to 70 amps (see NEC section 430.52(C)(1)). Then per NEC section 430.62, you take that value and add the FLC of the remaining motors on the feeder to determine the largest breaker that would be permitted. Therefore, 70 + 14 + 7.6 + 3.4 = 95 amps, rounded down to 90 amps.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
My first concern is that this sections says you can follow 430.53(A), (B), or (C). So if all of the motors are under 1 HP, it seems like I can simply follow (A) “Not Over 1 Horsepower” and not be concerned with (B) or (C).


Yes, that is correct.

But following (A) alone gives no information on branch circuit sizing requirements. Devils advocate: can I connect 10, 20, 30, or 50, motors one branch circuit?


430.58 directs you to 430.110 which says that the OCPD must have an ampacity not less than 115% of the sum of the full load currents.

Also, how do I size the branch circuit OCPD for multiple motors (in my case 3 motors)?

430.53(A) gives you two options for OCPD size....20A for 120v branch circuits and 15A branch circuits 1000v nominal or less.

Section (B) “If Smallest Rated Motor Protected” is a little bit more descriptive and gives more insight into branch circuit sizing. But is this section only followed if one motor or more motors are over 1 HP?


No, section (B) can also apply to motors not over 1HP.

Section (C) sounds like it’s only used for factory assembled installations, is this correct? Section (D) sounds like it only applies to section (C).


Section (C) applies to factory assembled installations and to field installed separate assemblies listed for such use.

Section (D) applies to (A), (B) and (C).
 
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rhamblin

Senior Member
Along these same lines...

430.53(C)(1) "Each motor overload device...listed for group installation with a specified maximum rating of fuse" Where would someone find this information? I don't recall ever reading in manufacturer specifications about an overload device being listed for group installation.

What I am trying to figure out:

Today I was asked to wire up 2 motors utilizing the same branch circuit. The equipment has a 3 HP motor, and a 5 HP motor, one is for a chopper, the other is for a blower. This equipment is custom made for our application on our presses. This cart that has the chopper/blower unit, will be wired with a 10' SO cord and a male 480V 3 phase 30 amp plug. In the past, we wired a 30 amp fusible disconnect, then ran to a starter, with overloads, then to the motor. I have been asked if we need the fusible disconnect as we can simply unplug the unit if we need power disconnected.

So essentially I have 30 amp branch circuit that will have a 5HP and 3HP motor on it. I'm trying to figure out if this is legal or not, as the 30 amp breaker will be the short-circuit and ground-fault protective device.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
The equipment has a 3 HP motor, and a 5 HP motor, one is for a chopper, the other is for a blower. This equipment is custom made for our application on our presses.
Then it's one piece of equipment with two motors like an air conditioner, no?

So essentially I have 30 amp branch circuit that will have a 5HP and 3HP motor on it. I'm trying to figure out if this is legal or not, as the 30 amp breaker will be the short-circuit and ground-fault protective device.
Where is your overload protection?
 
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