Derating calculation problem

[jonnytsa]

In the mike holts study master contractor practice exam they derate 10awg @ 70% with 8 current carrying conductors thhn for 21A loads on 30A breaker.

90° thhn 10awg is 40A x 70% = 28A. they say 10awg can be on 30A breaker derated at 28A since the load is 21A? I thought the derated wire had to be calculated not less than the breaker itself?? Have i been wrong all this time? Or did Mike holt make a mistake?

Since the wire is derated to 28A doesn't that mean its only good for a 25A breaker?

 

[Dennis Alwon]

In general you are correct but look at 240.4(B) for your answer

240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The
next higher standard overcurrent device rating (above the
ampacity of the conductors being protected) shall be permitted
to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cord-andplug-
connected portable loads.
(2) The ampacity of the conductors does not correspond with
the standard ampere rating of a fuse or a circuit breaker
without overload trip adjustments above its rating (but
that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes.

 

[Dennis Alwon]

Let me be clear that the wire is rated 28 amps but you can use a 30 amp overcurrent protective device however, the calculated load must not be greater than 28 amps.

 

[kwired]

In the mike holts study master contractor practice exam they derate 10awg @ 70% with 8 current carrying conductors thhn for 21A loads on 30A breaker.

90° thhn 10awg is 40A x 70% = 28A. they say 10awg can be on 30A breaker derated at 28A since the load is 21A? I thought the derated wire had to be calculated not less than the breaker itself?? Have i been wrong all this time? Or did Mike holt make a mistake?

Since the wire is derated to 28A doesn't that mean its only good for a 25A breaker?

One other piece of the puzzle is whether or not the load is continuous or not. If it is a continuous 21 amp load then your minimum conductor ampacity is 125% of 21 = 26 amps. But that is still below your adjusted ampacity of 28 amps for the 10 AWG conductor.

If it is not a continuous load there is some possibility you must use a 25 amp breaker instead of a 30, kind of depends on what the load is and what code sections may apply to it. If it is a motor or refrigeration compressor, it likely still needs 10 AWG but breaker can be even more than 30.

 

[david luchini]

One other piece of the puzzle is whether or not the load is continuous or not. If it is a continuous 21 amp load then your minimum conductor ampacity is 125% of 21 = 26 amps. But that is still below your adjusted ampacity of 28 amps for the 10 AWG conductor.

The 125% applies before the application of any adjustment or correction factors. After the application of any adjustment or correction factors, the conductor would require an ampacity of at least 21 amps to serve the load, but it would have to be greater than 25A to be protected by the 30A c/b.

 

[kwired]

The 125% applies before the application of any adjustment or correction factors. After the application of any adjustment or correction factors, the conductor would require an ampacity of at least 21 amps to serve the load, but it would have to be greater than 25A to be protected by the 30A c/b.

Yes, had a brain cramp there, 125% applies to selecting conductor based on termination rating.

Ampacity adjustments and temp correction factors apply for insulation rating of conductor - which only needs to be at 100% of load in all cases.

OP can still have some situations where the OCPD needs to be 25 amps for his 21 amp load, some where it maybe can be 30 amps, and some where it could be more than 30.

 

[mohsan514]

Could you please provide any code reference..

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[mohsan514]

The 125% applies before the application of any adjustment or correction factors. After the application of any adjustment or correction factors, the conductor would require an ampacity of at least 21 amps to serve the load, but it would have to be greater than 25A to be protected by the 30A c/b.

Could you please provide any code reference that says 125% applies before the application of any adjustment or correction factor..

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[kwired]

Could you please provide any code reference..

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There are several that can apply, any particular part of the process you have a question about?

First you must determine if you are applying to a branch circuit, feeder or a service conductor, to know whether to start in 210, 215 or 230 though they all have mostly same general rules. Then you go to 310.15 and select a conductor based on minimum required ampacity, but may need to make adjustments/corrections to ampacity in accordance with number of conductors in a raceway or cable or because of ambient temperature.

Then it is on to art 240 to determine what overcurrent protection is permitted, but conditions of the application may send you to different parts of 240 or even to other parts of code for some specific applications.

 

[mohsan514]

There are several that can apply, any particular part of the process you have a question about?

First you must determine if you are applying to a branch circuit, feeder or a service conductor, to know whether to start in 210, 215 or 230 though they all have mostly same general rules. Then you go to 310.15 and select a conductor based on minimum required ampacity, but may need to make adjustments/corrections to ampacity in accordance with number of conductors in a raceway or cable or because of ambient temperature.

Then it is on to art 240 to determine what overcurrent protection is permitted, but conditions of the application may send you to different parts of 240 or even to other parts of code for some specific applications.

I have chiller units and each unit is 600 amps and required OCPD is 800amp. I used the 2 runs of cable and total ampacity is 820 amp and these are laid in perforated cable tray . After applying grouping factor / correction factor , the cable ampacity is reduced to 720 amps. But i am worried that how 800 amp CB will protect the conductor in case of any abnormal condition.
Since nec 240.4 allows to use the higher rated OCPD. Even the conductor is less rated..
Please advise

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[kwired]

I have chiller units and each unit is 600 amps and required OCPD is 800amp. I used the 2 runs of cable and total ampacity is 820 amp and these are laid in perforated cable tray . After applying grouping factor / correction factor , the cable ampacity is reduced to 720 amps. But i am worried that how 800 amp CB will protect the conductor in case of any abnormal condition.
Since nec 240.4 allows to use the higher rated OCPD. Even the conductor is less rated..
Please advise

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Motors (and refrigeration units) have higher branch circuit devices than conductor ampacity quite often, this to allow for overcurrent device to hold during starting. There is still motor overload protection that will limit current to conductor ampacity or only allow limited overloading before it interrupts the circuit.

Take a simple 10 Hp 480 volt motor - FLA may be about 12 amps, so overload protection is set according to 12 amps (probably going to be 115% to 125% of that most cases but based on that 12 amps) A branch circuit breaker could possibly be as high as 35 amps. You are still allowed 14 AWG conductor because the motor overload protection will never allow 14 AWG to be overloaded for long enough to cause any damage to the conductor.

 

[mohsan514]

I am looking for correction factor or de-rating factor of touching single core cables installed in trefoil arrangement at cable tray . 3 ckt at each cable tray. Please advise with code reference.
Thanks

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[Dennis Alwon]

I am looking for correction factor or de-rating factor of touching single core cables installed in trefoil arrangement at cable tray . 3 ckt at each cable tray. Please advise with code reference.
Thanks

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Is this what you are looking for [MENTION=160389]mohsan514[/MENTION]

Table 392.22(A)(5) Allowable Cable Fill Area for
Multiconductor Cables in Ventilated Channel Cable Trays for
Cables Rated 2000 Volts or Less

 

[mohsan514]

Thank you.. same i was looking for..

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