Sizing Motor Branch Circuit Conductors and OCPD's

13842 Views 5 Replies 4 Participants Last post by micromind, Jun 23, 2011

electures

Discussion Starter · Jun 22, 2011

I am going to explain how to size branch circuit conductors for a motor. For this example I am going to use a motor with the following specs;

5HP
230 Volt AC
FLA = 15A
Single phase
Continuous duty class
Continuous duty cycle
Service factor 1.15
Tempature rise not over 40 degrees C.

First look at NEC 430.6A which states;

(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

What this means is that for sizing the branch circuit conductors, motor disconnect and OCPD, the amperage listed on the nameplate is not used. Instead the tables in NEC 430 are required to be used.

So even though the nameplate states 15 amps the values in Table 430.248 (Single phase AC motor shall be used).

430.22(A) General states;

(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).

A 5HP single phase 240V motor draws 28 amps. Multiply the 28 by 125%

28A x 125% = 35A

The conductors have to be sized for 35A.

Table 310.16 indicates #8AWG is rate for 40A.

Next we look at sizing the ground fault, short circuit, overcurrent protective device (OCPD).

430.52 states;

430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.

(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Table 430.52 states that for an inverse time circuit breaker protecting a single phase motor it is to be sized at 250% of the FLA as stated in Table 430.248.

28A x 250% = 70A

Based on the above calculation a 70A ITCB is correct.

If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;

(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.

28A x 400% = 112A

Since we can’t exceed 400% of the motor FLA we round down to a 110A ITCB on a piece of #8 AWG conductor.

There is also an online calculator that will do all the work for you. Please follow the link.

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electures

Discussion Starter · #2 · Jun 22, 2011 (Edited)

1HP, Single phase motor

Below is the procedure for a 1HP 115V single phase motor.

430.6(A) 1 states;
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Table 430.248 indicates 16 amps for a 115V 1HP single phase motor.

430.22(A) General states;

(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of
not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).

16A x 125% = 20A

Table 310.16 indicates #14AWG is rate for 20A.

Now before you get your panties in a bunch the asterisk refers us to the bottom of Table 310.16 where it states: See 240.4(D).

240.4(D) states;

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.

240.4(E) refers to tap conductors which is not applicable here. However, 240.4(G) is a table and tells us that Article 430 applies for motor branch circuit conductors. This means that #14 AWG is specifically permitted to be used for conductors supplying this 20A motor load.

Next we look at sizing the ground fault, short circuit, overcurrent protective device.

430.52 states;

430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.

(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Table 430.52 states that for an inverse time circuit breaker protecting a single phase motor it is to be sized at 250% of the FLA as stated in Table 430.248.

16A x 250% = 40A

Based on the above calculation a 40A ITCB is correct.

If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;

(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.

16A x 400% = 64A

Since we can’t exceed 400% of the motor FLA we round down to a 60A ITCB on a piece of #14 AWG conductor.
The real kicker is that if this was a pool filter motor it is code compliant.

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a7ecorsair

· #3 · Jun 22, 2011 (Edited)

I have a question:yes:
Let's say all the paint has been rubbed off the compressor tank (It used to say 5 HP) and now we don't know the HP rating of the motor. All all we have to work with is the motor name plate which is very easy to read. It says 15 amps and SF 1.0
How would this be handle as compared to a compressor that is supposedly 5 HP?
I'm not arguing with your posting, I'm just trying to get a better understanding of how it is done using different information.
As I mentioned in the other thread, my 3 HP motor says 14 amps and SF 1 where the 5 HP motor had 15 amps and a SF 1. How can we see a 1 amp difference on the motor plate and a 2 HP change painted on the tank?
Added:
The motors do not have a HP rating on the plate. The only mention of HP is what is painted on the tank.

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WDR

· #4 · Jun 22, 2011

Is the three HP amp draw at 120 and the five HP at 240? That could explain the small 1 Amp difference. Or the way the horse power is rated has been inflated.

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electures

Discussion Starter · #5 · Jun 22, 2011

a7ecorsair said:
I have a question:yes:
Let's say all the paint has been rubbed off the compressor tank (It used to say 5 HP) and now we don't know the HP rating of the motor. All all we have to work with is the motor name plate which is very easy to read. It says 15 amps and SF 1.0
How would this be handle as compared to a compressor that is supposedly 5 HP?
I'm not arguing with your posting, I'm just trying to get a better understanding of how it is done using different information.
As I mentioned in the other thread, my 3 HP motor says 14 amps and SF 1 where the 5 HP motor had 15 amps and a SF 1. How can we see a 1 amp difference on the motor plate and a 2 HP change painted on the tank?
Added:
The motors do not have a HP rating on the plate. The only mention of HP is what is painted on the tank.

Tough question. Without the needed information it impossible to size everything correctly. Now there is a danger of undersizing the conductors which could lead to them burning up. Motors are one of the most misunderstood articles in the NEC. SInce people don't understand them they try to apply various other articles and are wrong. I hope this thread will help people understand motors a little better.

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micromind

· #6 · Jun 23, 2011

a7ecorsair said:
I have a question:yes:
Let's say all the paint has been rubbed off the compressor tank (It used to say 5 HP) and now we don't know the HP rating of the motor. All all we have to work with is the motor name plate which is very easy to read. It says 15 amps and SF 1.0
How would this be handle as compared to a compressor that is supposedly 5 HP?
I'm not arguing with your posting, I'm just trying to get a better understanding of how it is done using different information.
As I mentioned in the other thread, my 3 HP motor says 14 amps and SF 1 where the 5 HP motor had 15 amps and a SF 1. How can we see a 1 amp difference on the motor plate and a 2 HP change painted on the tank?
Added:
The motors do not have a HP rating on the plate. The only mention of HP is what is painted on the tank.

Read the part in post #1 right after the red writing.

Yes, motors are indeed the most misunderstood part of the code. You need to look at 4 separate articles to correctly size conductors and breakers/fuses.

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