Derating

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shaw0486

Senior Member
Location
baltimore
When derating conductor per 310.15Ba should the actual load be used or the breaker protecting the conductors? For example i have a bunch or motors a total of 42 conductors all being 2 hp. 4.25 amp each I should be able to install all of this in one conduit using #14?
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
The derated ampacity of the conductors for motors must be equal to or greater than 125% of the motor full load current as found in the tables at the end of Article 430. The conductors must be protected per 430.52. Note that for other than motors or a couple of other special cases, the derated ampacity would have to equal or exceed the rating of the OCPD.
 

resistance

Senior Member
Location
WA
I agree with Don. We also need more information to help you further. Example: Saying a bunch of motors doesn?t tell us anything.
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
The design process should use the following sequence:
  1. Calculate the load.
  2. Pick a wire size that has sufficient ampacity to handle that load.
  3. Pick a breaker that will protect that size of wire.
Please note that these steps are independent of each other. The derating that is required due to the number of conductors in the conduit is a part of step 2. You don't use the results of step 1 in determining the ampacity of the conductors in step 2. Instead, you take a first guess at the wire size you would like to use, then you calculate its ampacity, and compare that value to the calculated load. If the ampacity is not high enough, you try again with a larger wire.

 

charlie b

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Lockport, IL
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More information would be helpful, but you did give us enough information for me to strongly suspect that you would not be able to do the installation as suggested. Absent any reason to believe otherwise, I infer that all 42 conductors in the conduit will count as being "current carrying conductors." That gives us a derating factor of 35%. When you apply that to the 90C rating of a #14 wire (presuming you are using 90C rated wire), the ampacity calculates to be 8.75 amps. Although that is higher than the value you are giving us for the load of each motor, you will not be able to protect that wire with a 15 amp breaker. I know that for motors, the relationship between wire ampacity and breaker rating is different than it is for other loads, and that motor overload protection comes into the discussion, but it just seems to me that this won't do what you want.

By the way, when we calculate motor loads, we do not get to use any nameplate values for load current. We have to use the tables - 430.248 for single phase and 430.250 for three phase. I don't see the value of 4.25 amps in the 2 HP row of either table. So I am curious where that number came from.
 

kwired

Electron manager
Location
NE Nebraska
I agree with Don. We also need more information to help you further. Example: Saying a bunch of motors doesn?t tell us anything.
If he said "all" motors it tells us what we need to know. Full load values (usually from tables in 430) is what ampacity should be based or adjusted from. Non motor loads, as as Don said most cases ampacity must match overcurrent devices.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
....... When you apply that to the 90C rating of a #14 wire (presuming you are using 90C rated wire), the ampacity calculates to be 8.75 amps. Although that is higher than the value you are giving us for the load of each motor, you will not be able to protect that wire with a 15 amp breaker. I know that for motors, the relationship between wire ampacity and breaker rating is different than it is for other loads, and that motor overload protection comes into the discussion, but it just seems to me that this won't do what you want.

Why can't the conductors be protected by 15Amp breaker? I know there are smaller breakers but isn't 15Amp the first available for most of the panels.
 

kwired

Electron manager
Location
NE Nebraska
More information would be helpful, but you did give us enough information for me to strongly suspect that you would not be able to do the installation as suggested. Absent any reason to believe otherwise, I infer that all 42 conductors in the conduit will count as being "current carrying conductors." That gives us a derating factor of 35%. When you apply that to the 90C rating of a #14 wire (presuming you are using 90C rated wire), the ampacity calculates to be 8.75 amps. Although that is higher than the value you are giving us for the load of each motor, you will not be able to protect that wire with a 15 amp breaker. I know that for motors, the relationship between wire ampacity and breaker rating is different than it is for other loads, and that motor overload protection comes into the discussion, but it just seems to me that this won't do what you want.

By the way, when we calculate motor loads, we do not get to use any nameplate values for load current. We have to use the tables - 430.248 for single phase and 430.250 for three phase. I don't see the value of 4.25 amps in the 2 HP row of either table. So I am curious where that number came from.

So you have derated the ampacity of a 14 AWG to 8.75 amps. Assuming 4.25 amps is the motor rating for whatever reason that is still under the derated ampacity of the conductors. The motor overload protection still limits the current the conductors will see for any extended amount of time. You do not have this kind of protection with most other circuits and therefore the fuse or breaker is closely related to conductor sizing.
 

don_resqcapt19

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Location
Illinois
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retired electrician
The code in 430.52 permits the motor branch circuit short circuit and ground fault protective device to be rated at 250% of the motor current if the OCPD is a inverse time breaker. The exception also permits rounding up to the next standard size OCPD. The smallest standard size (240.6) OCPD is 15 amps. As long as the derated ampacity of the conductors is at least 125% of the motor full load current as shown in the Article 430 tables, you would be permitted to protect the conductor with a 15 amp thermal magnetic breaker. Is this case even though the ampacity of the conductor under the conditions of use is only 8.75 amps, it is code compliant to connect that conductor to a 15 amp breaker.
 

kwired

Electron manager
Location
NE Nebraska
The code in 430.52 permits the motor branch circuit short circuit and ground fault protective device to be rated at 250% of the motor current if the OCPD is a inverse time breaker. The exception also permits rounding up to the next standard size OCPD. The smallest standard size (240.6) OCPD is 15 amps. As long as the derated ampacity of the conductors is at least 125% of the motor full load current as shown in the Article 430 tables, you would be permitted to protect the conductor with a 15 amp thermal magnetic breaker. Is this case even though the ampacity of the conductor under the conditions of use is only 8.75 amps, it is code compliant to connect that conductor to a 15 amp breaker.

Correct - if we have a conductor that has an ampacity the next standard size circuit breaker is 15 amps. If fuses are used the next size higher is 10 amps according to 240.6. That one is a little easier to overlook for some people.
 

don_resqcapt19

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Location
Illinois
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retired electrician
240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.
Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.
 

resistance

Senior Member
Location
WA
If he said "all" motors it tells us what we need to know. Full load values (usually from tables in 430) is what ampacity should be based or adjusted from. Non motor loads, as as Don said most cases ampacity must match overcurrent devices.
Do you like to start conversations with no purpose!! Give it a rest!!! If you have an answer then pitch. The fact is, we do need more information!! I’m not saying we can’t gather a base. Goodness!!
 
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kwired

Electron manager
Location
NE Nebraska
Do you like to start conversations with no purpose!! Give it a rest!!! If you have an answer then pitch. The fact is, we do need more information!! I?m not saying we can?t gather a base. Goodness!!

If supplied loads are all motors I don't see what the problem is. In motor circuits the conductor ampacity need not be more than 125% of motor current, but overcurrent device may well be higher, in most other circuits overcurrent device can not be any higher than next standard size higher than conductor ampacity.

Why didn't you ask OP if it was all motors instead of just ask for more information? Other informatin really does not matter too much as the motor circuits are still acceptable on 14 AWG, but other loads may have to be increased conductor sizes.
 

resistance

Senior Member
Location
WA
If supplied loads are all motors I don't see what the problem is. In motor circuits the conductor ampacity need not be more than 125% of motor current, but overcurrent device may well be higher, in most other circuits overcurrent device can not be any higher than next standard size higher than conductor ampacity.

Why didn't you ask OP if it was all motors instead of just ask for more information? Other informatin really does not matter too much as the motor circuits are still acceptable on 14 AWG, but other loads may have to be increased conductor sizes.
Seems charlie b agreed we needed more info.. This isn’t to say we don’t have a base. Not sure why what I said is wrong. Charlie b said the same thing. I just didn’t use the OP information (base) to do what Charlie did—which was give some input based on the information he has. Why couldn’t you have just did what Charlie b. did: Give your input based on the information the OP provided. Why focus on what I said. Kwired, really!! Lets not waste our time talking about nothing.

Back on topic,
I agree with Charlie b’s response—since I don’t have a “base” to continue the topic now.
 

kwired

Electron manager
Location
NE Nebraska
Seems charlie b agreed we needed more info.. This isn?t to say we don?t have a base. Not sure why what I said is wrong. Charlie b said the same thing. I just didn?t use the OP information (base) to do what Charlie did?which was give some input based on the information he has. Why couldn?t you have just did what Charlie b. did: Give your input based on the information the OP provided. Why focus on what I said. Kwired, really!! Lets not waste our time talking about nothing.

Back on topic,
I agree with Charlie b?s response?since I don?t have a ?base? to continue the topic now.

I was not trying to belittle anyone in any way. Sorry if it was taken the wrong way.

My point is motors and non motor loads will be treated differently. Charlie did point that out also, but I feel he was wrong when he said you can not protect that conductor with a 15 amp breaker - 15 is the next standard size breaker and we already discussed this.
 
. Note that for other than motors or a couple of other special cases, the derated ampacity would have to equal or exceed the rating of the OCPD.

So if I understand this statement correctly. If I am pulling 15 current carring conductors in a 1 inch EMT and I want those to be landed on a 20 amp breaker. I must use # 10 THHn derated gives me 20 amp at 50% percent correct? even tho my "calculated" load is 900 watts on a 120 volt circuit. I could not use # 12 THHn and land still land that on a 20 amp breaker since 50% is 15 amps. correct?

I was told I was reading the charts incorrectly and that since the "calculated" load is under 15 amps that I would be able to pull 15 CCC's in #12 THHn and still land them on 20 amp breakers. Please help I am so confused.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The calculated load does not enter into the sizing of the overcurrent protective device. Conductors, not used for motors or a couple of other special cases, must be protected at or below there ampacity. The adjustment and correction factors change the ampacity of the conductor.
 
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