Why can you use the 90 degree Column when you are doing adjustment factors for more than three current caring conductors when the conductors are number eight and larger but 18 gaug to 10 gauge wire you are not allowed to use the 90° column amperage When you use do your adjustment factors for more than three current carting conductors unless Specifically permitted in 240.4 (E) or (G). What’s the difference ?
Conduit fill
- Thread starter Hitschamp
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You use the temperature value for the insulation on the conductors. The conductor size is not relevant.
@infinity ; I agree. So this is my scenario I was at work and my foreman was doing conduit fill we have ac units being fed by a 3 pole breaker. I told him we could run a three-quarter EMT conduit as a home run for three units that would be 10 # tens witch based on table 310.15 (b) (3)(a) is a 50% adjustment factor. These AC units are being fed from a 20 amp breaker. Based on table 310.15(b) (16) #10 THHN I use the 90 degree column that puts me at 40amps 40x .5 = 20 so we’re good to land on 20 amp breaker. My foreman did not want to do this because table 240.4 (d) (1) thru (d)(7) but now I am home from work and have a chance to look at the code book It says the overcurrent protection shall not exceed that required by 240.4 D1 thru d7 my Forman must of miss read the article and thought it said the conductor cannot exceed the amperage given he that you could not Exceed 30 A on a number 10 Awg copper conductor so he wanted to not pass 9 wires in the conduit so the adjustment could be 70% witch is 30x.7 =21.
xformer
Senior Member
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- Dallas, Tx
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- Master Electrician
IMHO 240.4(D) 1 - D(7) is a general rule that can be trumped by 240.4 (E) or 240.4(G)
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Why 10 conductors, is one an EGC? If so then you have 9 CCC's with a derating of 70% (40 amps*70%=28 amps) that would be your adjusted ampacity. Since you have AC units you need to know the MCA (minimum circuit ampacity) to know if 28 amps is large enough.
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- Licensed Electrician
And if the MCA is small enough it might be possible to run 12AWG and still be compliant. That might really get the discussion rolling.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
If the MOCP is 20a, the MCA might even allow #14 wire.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
Tell him that the wire need only suit the MCA, the breaker must not exceed the MOCP, and no other figuring requiredOhh man! Now this discussion at work tomorrow is going to be good
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
You could run 3/4" emt but that is the max for a thwn-2 conductor. If there are a few elbows and/or offset and any good distance the pull will not be so easy.
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- Chapel Hill, NC
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- Retired Electrical Contractor
Isn't there an engineered drawing? Usually they will have conduit size specified but I agree 3/4" emt will work with #10 on a 20 amp breaker.
The foreman needs to look at 110.14. As long as the overcurrent protective device is not greater than 30 amps this would work but even with that 240.4(D) has a bit of an exception which applies for motors.
Here is part of 110.14(C)
The foreman needs to look at 110.14. As long as the overcurrent protective device is not greater than 30 amps this would work but even with that 240.4(D) has a bit of an exception which applies for motors.
Here is part of 110.14(C)
Sometime the concept is a bit difficult to understand. You need to recognize there is a difference in a conductors ampacity and the rating of the branch circuit. Ampacity is the current a conductor can carry without exceeding it's temperature rating. For the most part it's related to the type insulation and the current allowed without causing damage to the insulation. A #10 THHN can safely carry 40 amps continuously without damage to the insulation whereas the same conductor (#10) with a TW insulation would suffer damage. So your adjustment factors (fill, ambient, etc. take into account factors that can increase the temperature the insulation encounters (more wires in aconduit the hotter temperature the insulation encounters)
The second part of your post addresses Art 240 which is the overcurrent device which determines the circuit size. It is direct;y related to ampacity but takes into account termination temperatures as Dennis notes (110.14) and other factors. 240.4 is a guideline for selecting overcurrent devices to protect the conductor (and, at times, the utilization equipment) in accordance with it's ampacity. 240.4(E) & (G) address situations where those "general" rules in 240 need to be altered due to circumstances particular to those specific situations (motors, HVAC, etc),.
The second part of your post addresses Art 240 which is the overcurrent device which determines the circuit size. It is direct;y related to ampacity but takes into account termination temperatures as Dennis notes (110.14) and other factors. 240.4 is a guideline for selecting overcurrent devices to protect the conductor (and, at times, the utilization equipment) in accordance with it's ampacity. 240.4(E) & (G) address situations where those "general" rules in 240 need to be altered due to circumstances particular to those specific situations (motors, HVAC, etc),.
lordofthisworld
Senior Member
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Ya, even pulling (10) #10 AWG THHN in 3/4' is going to be a suck pull if u don't have any JB's or LB's for the reason u are maxing out the fill
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