I currently have a machine that is in place and running on a 40A breaker. The nameplate was just discovered and has 25HP,2HP, and 1/2HP motors on it (480V, 3ph). When taking 125% of largest plus the other two motors I get 47A. This requires a 50A breaker. We only ran #6 to the load because of 70% derating based on 9 current carrying conductors. I need to know is there something in the code that allows me either to use a smaller breaker or not have to double derate the load by taking 125% and then 70% of conductor ampacity. Right now I am faced with pulling out the #6 and installing #4 which may cause some pull by problems based on the age of the other conductors. The conduit run is under floor and therefore a wet run.
Cable derating
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Re: Cable derating
#6 thhn ampacity is 75amps if you derate by 70% ampacity is 52.2 amps, and I realize that conductor ampacity selection has to match the equipment temp. terminations, which most are rated for 60 degrees C or 75 degrees c, it looks to me like you will have to go to #4 thhn.
[ May 19, 2003, 10:54 PM: Message edited by: jro ]
#6 thhn ampacity is 75amps if you derate by 70% ampacity is 52.2 amps, and I realize that conductor ampacity selection has to match the equipment temp. terminations, which most are rated for 60 degrees C or 75 degrees c, it looks to me like you will have to go to #4 thhn.
[ May 19, 2003, 10:54 PM: Message edited by: jro ]
Re: Cable derating
From Table 430.150 a 25 hp 480 = 34 amps
2 hp 480 v = 3.4 amps and a 1/2 hp 480 = 1.1 amp.
Wire capacity = 1.25 x 34 + 3.4 + 1.1 = 47 amps.
If this machine is a unit and the motors are part
of it I think you only need to run one feeder to the machine. Wire size for 47 amps with 3 phase conductors, a neutral if required and a EGC
would be #8 thhn. Breaker size would be a max
of 250% x 47 amps = 110 amp. I do not understand why you think you need to install 9 conductors.
If you need 3 ckts, then you need 3 breakers.
From Table 430.150 a 25 hp 480 = 34 amps
2 hp 480 v = 3.4 amps and a 1/2 hp 480 = 1.1 amp.
Wire capacity = 1.25 x 34 + 3.4 + 1.1 = 47 amps.
If this machine is a unit and the motors are part
of it I think you only need to run one feeder to the machine. Wire size for 47 amps with 3 phase conductors, a neutral if required and a EGC
would be #8 thhn. Breaker size would be a max
of 250% x 47 amps = 110 amp. I do not understand why you think you need to install 9 conductors.
If you need 3 ckts, then you need 3 breakers.
Re: Cable derating
We didn't use the 75A because it is underground and we classified it as THWN which only allows 65A. We are only running three conductors to this one machine, the other 6 conductors are for other machines but hit us on the derating. A couple of questions: First I read in the code that you may chose the next size circuit interruption does this mean that I can use a 45A or 50A breaker for this 47A load? Second since I am derating the conductor 70% because of # of current carrying do I also have to derate the load by adding an additional 25% to the largest motor?
We didn't use the 75A because it is underground and we classified it as THWN which only allows 65A. We are only running three conductors to this one machine, the other 6 conductors are for other machines but hit us on the derating. A couple of questions: First I read in the code that you may chose the next size circuit interruption does this mean that I can use a 45A or 50A breaker for this 47A load? Second since I am derating the conductor 70% because of # of current carrying do I also have to derate the load by adding an additional 25% to the largest motor?
Re: Cable derating
The section that allows you to use the next size breaker does not apply to motors. The breaker is sized from table 430.52 which allows you to use
a breaker up to 250% of the full load amps. You must use a larger breaker because of the large starting amps of the motor. Starting amps are estimated a 6 x full load amps.
If you are installing the conductors as in your post then you must derate for the number of conductors. The NEC requires that the full load amps x 1.25 = the amp rating of the conductor.
The FLA is equal to 47 amps x 1.25 = 59 amps
required conductor capacity. If you choose #6
the rating is 75 amps x .70 = 52.5 amps. This is
not adequate. Use #4 at 95 amps x .7 = 66 amps.
When you use the 1.25 multipler, it is not a derating factor. You are increasing the size of the conductor to act as a heat sink to cool the breaker.
[ May 20, 2003, 12:52 PM: Message edited by: bob ]
The section that allows you to use the next size breaker does not apply to motors. The breaker is sized from table 430.52 which allows you to use
a breaker up to 250% of the full load amps. You must use a larger breaker because of the large starting amps of the motor. Starting amps are estimated a 6 x full load amps.
If you are installing the conductors as in your post then you must derate for the number of conductors. The NEC requires that the full load amps x 1.25 = the amp rating of the conductor.
The FLA is equal to 47 amps x 1.25 = 59 amps
required conductor capacity. If you choose #6
the rating is 75 amps x .70 = 52.5 amps. This is
not adequate. Use #4 at 95 amps x .7 = 66 amps.
When you use the 1.25 multipler, it is not a derating factor. You are increasing the size of the conductor to act as a heat sink to cool the breaker.
[ May 20, 2003, 12:52 PM: Message edited by: bob ]
Re: Cable derating
The 47A total already accounted for 125% of the largest motor so we are only sizing it for 47A but because it is THWN we can only derate at 65A. #6@75C will handle 65A(.70)=45.5A. Because of this 1.5A I'm moving to #4. This machine has currently been up and running on a 40A breaker for approximately 20 years. We are just moving it within the same room and were planning on reconnecting it to the same breaker because at the time no nameplate could be found. Since we know it ran on 40A without tripping can I again run it on the 40A ?
The 47A total already accounted for 125% of the largest motor so we are only sizing it for 47A but because it is THWN we can only derate at 65A. #6@75C will handle 65A(.70)=45.5A. Because of this 1.5A I'm moving to #4. This machine has currently been up and running on a 40A breaker for approximately 20 years. We are just moving it within the same room and were planning on reconnecting it to the same breaker because at the time no nameplate could be found. Since we know it ran on 40A without tripping can I again run it on the 40A ?
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Re: Cable derating
Bob,
Unless the insulation type has a "-2" after the type letters, you can only use the 75?C for derating purposes if the conductor is in a wet location. The original post says it is in a wet location.
Don
Bob,
Unless the insulation type has a "-2" after the type letters, you can only use the 75?C for derating purposes if the conductor is in a wet location. The original post says it is in a wet location.
Don
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