I have heard two diffrent versions of how to calculate for the correct sizing of conductors. Some say to calculate for the load, others say to calculate for the OCPD. The NEC under article 240.4 stipulates that you need to protect conductors against overcurrent, therefore the sizing of the OCPD is relevnet to the ampacity of the conductor that it is protecting and not the load. i will give you an expample. Let's say that you have an exsiting misc. continuous load of 286 ampers and the OCPD is rated 400 amperes. The conductors that feed this circuit are 600MCM. According to the NEC article 240.4 the OCPD is sized to protect the conductors since the 600MCM is rated at 475 at 90c. Even at 75c the conductors are rated at 420. The distance to the load is approx. 320 feet. There is really only a small voltage drop at this point. Am I correct in saying that the ampacty of the conductor must meet the the sizing of OCPD even after any derating or correction factors are done? Of course the conducots will need to be sized a minimum of 125% of the load. Why do some people stipulate that you must size to the load instead of the OCPD?
Sizing Conductors
- Thread starter marasmit
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- Lockport, IL
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- Retired Electrical Engineer
The load is everything. The entire purpose of our work is to serve the load. The proper design sequence begins with defining the load, and calculating the amount of current it will draw. Sometimes we need to add 25% to the current, for example if the load will be on continuously (i.e., for more than 3 hours at a time). The next step is to make sure the conductor that supplies the load has enough ampacity to handle the calculated amount of current. The final step is to protect that conductor against overcurrent.
Sizing conductors
Sizing conductors
Charles, so if i had a load of 853 (3 ph motor, Compressor @ 480v) S.F. of 1.25. shouldn't the size of the conductor be designed with the 75c colum for ampacity? Secondly, there are (3) 500MCM feeding this motor and the OCPD is a 1200 amp breaker. I would think even though the conductors ampacity is over the 125% of the motor load, but since you have a S.F of 1.25, don't you need to add this to your calculation? i know that if the OCPD is 800 amperes or larger, then the conducotrs must meet of exceed this ampacity. Does this make sense?
Mark
Sizing conductors
Charles, so if i had a load of 853 (3 ph motor, Compressor @ 480v) S.F. of 1.25. shouldn't the size of the conductor be designed with the 75c colum for ampacity? Secondly, there are (3) 500MCM feeding this motor and the OCPD is a 1200 amp breaker. I would think even though the conductors ampacity is over the 125% of the motor load, but since you have a S.F of 1.25, don't you need to add this to your calculation? i know that if the OCPD is 800 amperes or larger, then the conducotrs must meet of exceed this ampacity. Does this make sense?
Mark
For example, if the load is 2 amps and you want to feed it from a 15 amp breaker in a panel then you would use a #14 conductor (240.4(D)(3). You could use a smaller conductor, because the load is small, but not with a 15 amp breaker.
You have to make sure your system provides enough power for the load and also provides proper protection for the electrical equipment in the system.
You have to make sure your system provides enough power for the load and also provides proper protection for the electrical equipment in the system.
- Location
- Illinois
- Occupation
- retired electrician
Mark,
For the purposes of sizing the conductors for a motor circuit, in most cases, you size them based on a minimum of 125% of the full load current as found in Tables 430.247 through 430.250. there is no need to look at the service factor. Yes, you would size based on the 75?C ampacity in most cases.
For the purposes of sizing the conductors for a motor circuit, in most cases, you size them based on a minimum of 125% of the full load current as found in Tables 430.247 through 430.250. there is no need to look at the service factor. Yes, you would size based on the 75?C ampacity in most cases.
Derating
Derating
for purposes of code compliancy, is it correct in saying that your conductor ampacity needs to meet the load ampacity + 125% before any correction factors? So, if i have a load of 125 amps and i run a #1 thwn conductor good for 130 amps @75c even though after the correction factor is applied the #1 is only good for 113 amps. Correction factor is .87 for ambient temp.
Derating
for purposes of code compliancy, is it correct in saying that your conductor ampacity needs to meet the load ampacity + 125% before any correction factors? So, if i have a load of 125 amps and i run a #1 thwn conductor good for 130 amps @75c even though after the correction factor is applied the #1 is only good for 113 amps. Correction factor is .87 for ambient temp.
Exisitng Condition
Exisitng Condition
Here is an exisitng condition. Can you see if what i am saying is correct?
2) The Centac 2, 800 H.P. compressor out by the cooling towers will need to have the main feeders reworked. After doing the calculations on the motor F.L.A., (853 amperes X 125% = 1,066.25 amperes) ambient temperature correction factors and using the N.E.C. requirements for motor loads, the existing (3) 500 MCM conductors are compliant per the sizing of the load, but because you need to have the OCPD (overcurrent protection device) rated at a minimum of 125 to 150% of the F.L.A., you need the 1200 OCPD. This present a problem, even though you are compliant with the motor load conductor ampacity, once you protect this motor with a 1200 OCPD you now will need to increase the conductor size accordingly. This is all due to article 240.4 (c). There are already three 3? conduits ran to this compressor unit, you would need to run an additional 3? conduit from the main switchgear over to the control cabinet and finally run one more additional 3? over to the motor. Multiply by temperature correction factor of .76 (direct sunlight correction factor) the 500MCM is now only good for 326.80 amperes. To simplify this, you could use the (3) 500 MCM conductors and add another set of 500MCM to get the required conductor ampacity. You would need to run an additional conduit for these conductors. The ground conductor size needs to be increased to 3/0.
Exisitng Condition
Here is an exisitng condition. Can you see if what i am saying is correct?
2) The Centac 2, 800 H.P. compressor out by the cooling towers will need to have the main feeders reworked. After doing the calculations on the motor F.L.A., (853 amperes X 125% = 1,066.25 amperes) ambient temperature correction factors and using the N.E.C. requirements for motor loads, the existing (3) 500 MCM conductors are compliant per the sizing of the load, but because you need to have the OCPD (overcurrent protection device) rated at a minimum of 125 to 150% of the F.L.A., you need the 1200 OCPD. This present a problem, even though you are compliant with the motor load conductor ampacity, once you protect this motor with a 1200 OCPD you now will need to increase the conductor size accordingly. This is all due to article 240.4 (c). There are already three 3? conduits ran to this compressor unit, you would need to run an additional 3? conduit from the main switchgear over to the control cabinet and finally run one more additional 3? over to the motor. Multiply by temperature correction factor of .76 (direct sunlight correction factor) the 500MCM is now only good for 326.80 amperes. To simplify this, you could use the (3) 500 MCM conductors and add another set of 500MCM to get the required conductor ampacity. You would need to run an additional conduit for these conductors. The ground conductor size needs to be increased to 3/0.
- Location
- Illinois
- Occupation
- retired electrician
The correction and adjustment factors change the ampacity of the conductors and that becomes the new ampacity. If the code rule in question requires a conductor with an ampacity of 100 amps, your would have to use a conductor that has an ampacity of 100 amps after you have applied the required correction and adjustment factors. Note that if your conductor is rated for 90?C, you apply the correction and adjustmetn factors to the 90?C ampacity.
A far as your compressor circuit, motor feeders are a special case and their overcurrent protection is covered by Article 430 and not by Article 240. The conductor is sized at 125% of the motor full load and the feeder short circuit and ground fault protection can be up to 250% of the motor full load without changing the conductor size. The motor overload device also provides the overload protection for the conductors.
A far as your compressor circuit, motor feeders are a special case and their overcurrent protection is covered by Article 430 and not by Article 240. The conductor is sized at 125% of the motor full load and the feeder short circuit and ground fault protection can be up to 250% of the motor full load without changing the conductor size. The motor overload device also provides the overload protection for the conductors.
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