Derating conduit fill

[donny58]

Why doesn't the code have the conduit fill table already derated ? What good is having all those size conductors listed on a chart if you cant use that chart ?Another way to ask would be , Why cant it be a chart telling you how many conductors you CAN ! put in a conduit ! instead of a chart telling me how many I cant put in a conduit ? I am not talking about ampacity That I get . Just talking about how many # 12s in say a 3/4 emt that is 10 ft. long ?

 

[cadpoint]

The NFPA doesn't have a table because for each size of conductor due to a size of different conduit's, raceways, paths; or because any combininations that needs to reflect a nearly infinite combinations of what and how a circuit is run! One would be looking at log-rythmes.

 

[infinity]

As Cadpoint mentioned the variables are far too great to put all of the information in one table. A rule of thumb is that for #14,#12,#10 AWG conductors you can have 9 CCC's in a raceway before derating will affect their typical 15, 20 and 30 amp ampacities.

 

[ohmhead]

Derating

Derating

Well it would not be that much fun its a lot better to figure it all out the long way !

But then if you think about it what would Mike Holts do from now on !

 

[don_resqcapt19]

In addition to what has been posted already, not all conduits have power conductors that are subject to derating. I often install control conduits with over 100 #14s or 16s. The wire fill tables are needed for applications like that.

 

[Dennis Alwon]

Just talking about how many # 12s in say a 3/4 emt that is 10 ft. long ?

Table C in Annex C in the back of the book . Is that what you mean????

 

[donny58]

Think inside the box !

Think inside the box !

You Dont get it > If the code says 9 # 12 in a 1/2 in emt conduit and you have to derate 40% then why bother having a chart that says 9 conductors ! The chart should say 6 conductors , you savy ??

 

[ActionDave]

You can put nine THHN conductors in a 1/2" emt and put them on a 20A breaker.

There are tables that give max conduit fill. What are you asking?

 

[Little Bill]

You Dont get it > If the code says 9 # 12 in a 1/2 in emt conduit and you have to derate 40% then why bother having a chart that says 9 conductors ! The chart should say 6 conductors , you savy ??

The table is for percentage of conduit fill, not derating. In other words, you are only allowed to fill conduit to a certain percentage , eg,. 40%, 60% etc. The table is for the same size conductors in a raceway, that way you don't have to calculate the fill percentage, just use the table. They don't know what amperage you are needing to have after derating. Someone could already know they only need to achieve 15A after derating and are intentionally using the #12. Also probably not all the conductors are current carrying and wouldn't need derating. But in the conduit fill all conductors whether CC or not have to be counted.
I hope that helps and you are now "savy."

 

[Dennis Alwon]

You Dont get it > If the code says 9 # 12 in a 1/2 in emt conduit and you have to derate 40% then why bother having a chart that says 9 conductors ! The chart should say 6 conductors , you savy ??

Where do you see that you have to derate 40%. The conduit fill is 40% but that is whenever you use 2 or more conductors. Derating is from Table 310.15 (B)(2) for the # of conductors in a conduit. 9 conductors gets derated 70% but that is not for conduit fill but rather for the ampacity of the conductor.

Take #6 thhn- Table 310.16 states that it is good for 75amps. Now if we have 9 #6 ccc in the conduit then we must derate the 75amps by 70%. Thus the wire (#6 THHN) is now only good for 52.5 amps.

 

[suemarkp]

There are basically two things you need to know -- how many current carrying conductors can you use before derating has lowered ampacity too far (and this is 9 in sizes 14-12, 6 in #10, 8, 6, 4 but depends on what you consider a "normal" breaker for that wire size and whether you're rounding up to the next standard size). This number in independent of conduit size.

The other thing is how many conductors can you fit for fill.

It is too complicated to know which are current carrying and which are counted by fill in a simple chart. There could be equipment grounds, isolated grounds, neutrals that aren't considered current carrying, and neutrals that are.