Tom.Margillo
Member
If someone can explain derateing to me I would appreciate it. What is the maximum number of#12awg or #10awg current carrying conductor allowed in a 2 1/2" conduit?
Number of Conductors After Derateing
- Thread starter Tom.Margillo
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winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The amount of current that a conductor can safely carry is pretty much limited by the _temperature_ of the insulation.
The temperature of the insulation is pretty much set by the amount of heat generated in the wire from electrical resistance heating (there are other heating terms, eg. dielectric heating of the insulation itself, but for the present discussion we can ignore this), combined with any other heat generated in the vicinity (from other conductors, for example), versus the rate of heat flow to the outside world and the temperature of the outside world.
When you put multiple conductors in the same raceway, then each conductor adds heat to the raceway, and the allowable current that any one conductor can carry must be reduced in order to prevent excessive temperature.
To 'derate' you simply find the conductor ampacity from the appropriate table (usually 310.16), adjust for ambient conditions, and then adjust the ampacity again using table 310.15(B)(2)
The number of conductors that you can place in a conduit is set by the physical size of that conduit. However when you fill a conduit up you may find that the allowable ampacity on a given conductor is too low to be useful.
A nominal 2 1/2" EMT has an internal cross section of 3783mm^2 (from table 4) and may be filled to a cross section of 1513 mm^2. A #10 THHN conductor has a cross section of 13.61mm^2 (from table 5). This means that you could fit 111 conductor into such a conduit.
But the ampacity of these conductors would be derated as follows. Presume 30C ambient and since we are using THHN, we look at the 90C column. The un-adjusted ampacity of the #10 conductor is 40A. Now look at table 310.15(B)(2) and find that with 41 or more conductors we must derate the ampacity to 35% of its single conductor value. This gives an ampacity of 14A. We could use these conductors for defined loads of 14A or less on 15A breakers, but could not generally use 15A breakers. Pretty well limits the utility of packing lots of wire into a single conduit.
-Jon
The temperature of the insulation is pretty much set by the amount of heat generated in the wire from electrical resistance heating (there are other heating terms, eg. dielectric heating of the insulation itself, but for the present discussion we can ignore this), combined with any other heat generated in the vicinity (from other conductors, for example), versus the rate of heat flow to the outside world and the temperature of the outside world.
When you put multiple conductors in the same raceway, then each conductor adds heat to the raceway, and the allowable current that any one conductor can carry must be reduced in order to prevent excessive temperature.
To 'derate' you simply find the conductor ampacity from the appropriate table (usually 310.16), adjust for ambient conditions, and then adjust the ampacity again using table 310.15(B)(2)
The number of conductors that you can place in a conduit is set by the physical size of that conduit. However when you fill a conduit up you may find that the allowable ampacity on a given conductor is too low to be useful.
A nominal 2 1/2" EMT has an internal cross section of 3783mm^2 (from table 4) and may be filled to a cross section of 1513 mm^2. A #10 THHN conductor has a cross section of 13.61mm^2 (from table 5). This means that you could fit 111 conductor into such a conduit.
But the ampacity of these conductors would be derated as follows. Presume 30C ambient and since we are using THHN, we look at the 90C column. The un-adjusted ampacity of the #10 conductor is 40A. Now look at table 310.15(B)(2) and find that with 41 or more conductors we must derate the ampacity to 35% of its single conductor value. This gives an ampacity of 14A. We could use these conductors for defined loads of 14A or less on 15A breakers, but could not generally use 15A breakers. Pretty well limits the utility of packing lots of wire into a single conduit.
-Jon
tallgirl
Senior Member
- Location
- Glendale, WI
- Occupation
- Controls Systems firmware engineer
Tom.Margillo said:
Derating doesn't change the conductor fill, it changes the allowable ampacity of the conductors.
Derating is about making sure that you don't fill a pipe up with CCC's and then generate so much heat from the wiring that something bad happens. Limiting the ampacity limits the amount of heat that is produced. You can still pack it to the limit for that size of pipe and conductor, but the amount of heat generated from resistance in the conductors will be lower.
For information on conduit fill, see Annex C. Then go to 310.15 (B) for information on derating those conductors.
I think Tallgirl has it exactly right, but permit me to extend the question with an example.
Fifty 12 AWG THW current carrying conductors are installed in a raceway. After applying adjustment factors the ampacity of each conductor is 8.75 amps. (25 x 0.35)
Now, the next higher standard OCPD is a 10 amp fuse. If the building has only circuit breakers, would 15 amp circuit breakers be acceptable?
Fifty 12 AWG THW current carrying conductors are installed in a raceway. After applying adjustment factors the ampacity of each conductor is 8.75 amps. (25 x 0.35)
Now, the next higher standard OCPD is a 10 amp fuse. If the building has only circuit breakers, would 15 amp circuit breakers be acceptable?
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
finhead said:
They make 10 amp circuit breakers. What kind of load would you put on a 10 amp circuit?
tallgirl
Senior Member
- Location
- Glendale, WI
- Occupation
- Controls Systems firmware engineer
infinity said:
240.6? This is a thought problem -- don't confuse it with reality
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
finhead said:
Why did you derate from 75 degees and not the 90 ?
paul
Senior Member
- Location
- Snohomish, WA
I'm going to load that puppy up with 176 THHN #12's and call 'em control wires. However, it's up to y'all to pull and terminate 'em.
Johnmcca
Senior Member
- Location
- Mid-Hudson Valley, NY
Paul, you're being mean
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
paul said:I'm going to load that puppy up with 176 THHN #12's and call 'em control wires. However, it's up to y'all to pull and terminate 'em.
Use # 14 and get even more in it LOL.You make the head and i will put our tugger on it.
Johnmcca
Senior Member
- Location
- Mid-Hudson Valley, NY
Jim, you've taken it a step further, good for you! There should be no limits.
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
roger said:
Nice try but they require 15 or 20 amp receptacles.
paul
Senior Member
- Location
- Snohomish, WA
Jim W in Tampa said:
Solid or stranded?
Jim W in Tampa said:
What does that have to do with the circuit protection? Have you never fed a 50 amp range receptacle with a 40 amp circuit?
Where is feeding a 15 or 20 amp receptacle from a 10 amp circuit prohibited? Not in 210.21(B)(3).
Roger
- Location
- Illinois
- Occupation
- retired electrician
Roger,
Don
But you can't use that 8.75 amp wire on a 10 amp breaker to feed receptacle outlets. 240.4(B)(1)
Don
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
It cant be this easy to get out of afci breakers.I see a great market for a 19 amp and 14 amp breaker,but then why would they want to help us when they make more from selling worthless afci breakers.
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