Only 9 wires allowed in conduit under 3 feet?

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Nope, in some raceways 24" or less you do not need to derate and the fill jumps up to 60%. Can you be more specific in your details?
 
Eddy Current said:
Is there a code that says anything about a certain number of wires allowed in a conduit 3 feet and under?
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The 9 wires comes up often because with a 90C wire usually the derating that is necessary will not affect the final ampacity at the 75C rating until you have more than 9 conductors.

For instance, #12 THHN is rated 30 amps and must be derated 70% for 9 CCC however that still leaves us with 21 amps so the circuit is not affected by the 9 conductors. It has nothing to do with in the ground unless you have a local ordinance
 
Dennis Alwon said:
The 9 wires comes up often because with a 90C wire usually the derating that is necessary will not affect the final ampacity at the 75C rating until you have more than 9 conductors.

For instance, #12 THHN is rated 30 amps and must be derated 70% for 9 CCC however that still leaves us with 21 amps so the circuit is not affected by the 9 conductors. It has nothing to do with in the ground unless you have a local ordinance
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within the ground?

I would say it has nothing to do with how long the conduit is?
I agree it is just the magic number that derating works out to be.
 
But before anyone jumps too deeply into that thumb rule pool, be advised that for 30 amp circuits and #10 wire, a similar mathematical process will conclude that the "magic number" is 6.
 
charlie b said:
But before anyone jumps too deeply into that thumb rule pool, be advised that for 30 amp circuits and #10 wire, a similar mathematical process will conclude that the "magic number" is 6.
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Yep that one stumps a few because they don't look at the ratio or percent difference between the allowed rating and the 90?c rating, they just look at the 10 amp difference across the board and think they are all the same.

#14= 15 amp allowed, 25 amp 90?c rating = 10 amps but a 60% difference
#12= 20 amp allowed, 30 amp 90?c rating = 10 amps but a 661/3% difference
#10= 20 amp allowed, 40 amp 90?c rating = 10 amps but a 75% difference

so it is this that causes a #10 to have only a 28 amp rating when 9 conductors are installed into a conduit, while #12 will have a 21 amp rating, and #14 will have a 17.5 amp rating
 
hurk27 said:
Yep that one stumps a few because they don't look at the ratio or percent difference between the allowed rating and the 90?c rating, they just look at the 10 amp difference across the board and think they are all the same.

#14= 15 amp allowed, 25 amp 90?c rating = 10 amps but a 60% difference
#12= 20 amp allowed, 30 amp 90?c rating = 10 amps but a 661/3% difference
#10= 20 amp allowed, 40 amp 90?c rating = 10 amps but a 75% difference

so it is this that causes a #10 to have only a 28 amp rating when 9 conductors are installed into a conduit, while #12 will have a 21 amp rating, and #14 will have a 17.5 amp rating
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and at 28 amps,in most cases, you can still use a 30 amp ocpd
 
Dennis Alwon said:
and at 28 amps,in most cases, you can still use a 30 amp ocpd
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I think your right as 240.4(B) allows it for 800 amps or less and 240.4(D) only restricts it to the 30 amp OCPD even if the intent was not to allow this, the words do say so.

Of course 240(B) would not allow it for receptacle loads and if using 240.4(E) or (G) you would have to use the 28 amps as the wire rating for the minimum circuit size for AC units, and motor loads and you would have to use the 22.4 rating when the 125% comes into play when sizing conductors, so that might limit you from feeding an electric water heater with a 30 OCPD.

While in most cases sure you can use 240.4(B) but very few cases would you be allowed to use the rating of the wire and in some cases (125% rule) you would have to reduce the rating even more.
 
Missing a huge factor here gentlemen... This is an old exam question: way off what th

Missing a huge factor here gentlemen... This is an old exam question: way off what th

Actually gentlemen, this question DOES have EVERYTHING to do with the three foot under part of the question. Case in point, see the controlling article 300.5, where a raceway (ANY raceway) is buried below grade, THAT interior is a wet rated location. So then referencing 310.104(A), a lot of insulation types (most actually) are restricted to the 75 degree column where installed in a wet location-even if you have the other two items needed when using the 90 degree column... (Also see 310.10(B) & (C), conductors rated for dry-damp, or wet locations.

Hope my input helps.
 
Mitchell Tolbert said:
Actually gentlemen, this question DOES have EVERYTHING to do with the three foot under part of the question. Case in point, see the controlling article 300.5, where a raceway (ANY raceway) is buried below grade, THAT interior is a wet rated location. So then referencing 310.104(A), a lot of insulation types (most actually) are restricted to the 75 degree column where installed in a wet location-even if you have the other two items needed when using the 90 degree column... (Also see 310.10(B) & (C), conductors rated for dry-damp, or wet locations.

Hope my input helps.
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And I agree but, I don't think the OP's question nor the answers are aimed at buried installations.


Roger
 
Mitchell Tolbert said:
Actually gentlemen, this question DOES have EVERYTHING to do with the three foot under part of the question. Case in point, see the controlling article 300.5, where a raceway (ANY raceway) is buried below grade, THAT interior is a wet rated location. So then referencing 310.104(A), a lot of insulation types (most actually) are restricted to the 75 degree column where installed in a wet location-even if you have the other two items needed when using the 90 degree column... (Also see 310.10(B) & (C), conductors rated for dry-damp, or wet locations.

Hope my input helps.
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You make a good point but I agree with Roger. 3' underground seems to mean that 2' underground is different. Also depending on the insulation such as XHHW-2 it can be rated 90C for wet and dry locations.

And please don't call me a gentleman-- you will ruin my reputation.. :lol:
 
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