conduit fill

[michael a noakes]

What is a rule of thumb for conduit fill? I understand the demand factors.....I was telling my foreman that no matter if you run 1 inch pipe or 2 inch pipe you can't fill to capacity....that is what is states in the Ugly's book. I stated to him that they were running 1 inch pipe and running 9 hots and 9 neutrals. I explained that is over the demand factor and that the pipe will overheat and they would have to derate the circuits. I stated the rule of thumb after going through calculations that you stay under 6 to 7 wires to be safe no matter what size conduit. I also stated you can put that many wires in conduit that is 24 inches or less.

Respectfully yours,
Michael Noakes

 

[haskindm]

Huh?
Take a look at table 310.16.
It is base on three current carrying conductors in a raceway or cable.
You will need to derate the conductors in a raceway anytime there are more than 3 current carrying conductors installed.
310.15(B)(2)(a) gives you the adjustment factors for over three current carrying conductors.
Then look at the article for the type of raceway that you are installing. 358.22 for EMT as an example.
It tells you that you may not fill the conduit more than specified in Table 1 of Chapter 9.
This tells you that if you have over 2 conductors you must not fill it more than 40%. Annex "C" calculates the conduit fill for you if all of the conductors are the same size. You will need to calculate it yourself using Chapter 9 tables 4 and 5 if they are different sizes.
I believe you are trying to over-simplify.
The "pipe" will not overheat, but the conductors may if not derated.
Conduit "fill" and derating of conductors due to the number of conductors in a raceway are two different things.

 

[tryinghard]

Michael,

• The rule for conduit fill begins in 310-15.
• The capacity for 1? or 2? (all) conduit is found in Table 1 Chapter 9
• For multi circuits in a conduit you must use Table 4 Chapter 9 & the 40% column
• To stay within the in of a particular conduit you must use Table 5 Chapter 9 & the in column; i.e. ?? EMT = .122 in @ 40% & #12 THHN = .0133 in approximate area so .122/.0133 = 9 #12?s allowed and Table C.1 in Annex C can qualify this.

This is the route you must take in calculating your wire fill in conduit especially for multiple circuits. I will often use a spreadsheet for multiple circuits in a conduit.

You do have to comply with 310-15 & Table 310-15(B)(2)(a) and adjust conductor amperes [derate] as needed, watch out for 310-15(B)(4)(a) through (c) on counting your neutrals for derating purposes, have fun!