Table 310.15(B)(3)(a)

Status
Not open for further replies.
M

MDJ

Member
Location
Post Falls, ID.
I have always used the “no more than 6 current carrying conductors” rule for conduit fill regardless of conduit size per Table 310.15(B)(3)(A). For example 3-120 volt circuits or 3-208 volt circuits or 2-3 phase circuits. As stated it doesn’t matter what size or type of conduit. However, I see many electricians install 8-10 CC conductors in a conduit with out derating past 80%. I can only assume they are thinking of conduit fill only and disregarding conductor ambient temperature ratings. Have I misunderstood this rule or is there some exception I’m forgetting about?

Thanks in advance.
 
texie said:
#12 THHN has an ampacity of 30 @90 degree C. Table 310.15(B)(3)(a) says you must derate to 70%. 30 X .7 = 21. So your adjusted ampacity is 21 which is fine for a 20 amp circuit.
Click to expand...


Just for clarity to the OP-- you are using 70% when there are 7-9 conductors
 
You are seeing one of those grey areas too. General practice is to use the ampacity atthe terminal temperature rating or the highest temperature column AFTER derating for CCC, whichever is lower. So with 90 C wire and 75 C terminals the terminal rating becomes the limit for 1-3 CCC but switches to the sweated 90 C column as the number of CCC grows.

Also auxiliary gutters and cable trays are your friends when it comes to derating. And I high current applications, despite the extra effort, multiple parallel conduits beats derating by a large margin.
 
MDJ said:
Can you explain your calculation a little more?
Click to expand...

What many people don't realize is that the wire ampacities in Table 310.16 are higher for 90C wire than the overcurrent devices allowed in 240.4(D). Many people learn in the field that #14 is for 15A, #12 is for 20A, and #10 is for 30A. That's because of 240.4(D).

But when you do derating calcs, if you are pulling 90C wire such as THWN-2, you can use 25A, 30A, and 40A respectively. For those sizes that allows you to put 9 CCCs in a conduit without having to upsize #14 or #12. If the supply house gave you THWN then you'll need to stick to your 6 CCC rule of thumb because the 75C column has lower ampacities.
 
jaggedben said:
If the supply house gave you THWN then you'll need to stick to your 6 CCC rule of thumb because the 75C column has lower ampacities.
Click to expand...

THWN-2 is the rating, that lets you use the 90C column as a starting point for derating, regardless of wet or dry location. THHN means 90C dry. THWN without a -2 means 75C wet. I've never seen straight THHN, or straight THWN made today, that doesn't carry a combination of these ratings. It is most common for this wire technology to be THWN-2, however in small sizes it may just be THHN/THWN, which means wet locations limit you to the 75C rating.
 
paulengr said:
You are seeing one of those grey areas too. General practice is to use the ampacity atthe terminal temperature rating or the highest temperature column AFTER derating for CCC, whichever is lower. So with 90 C wire and 75 C terminals the terminal rating becomes the limit for 1-3 CCC but switches to the sweated 90 C column as the number of CCC grows.

Also auxiliary gutters and cable trays are your friends when it comes to derating. And I high current applications, despite the extra effort, multiple parallel conduits beats derating by a large margin.
Click to expand...
Do you happen to know where in the code I can find an adjustment chart for ccc in a trough or gutter? I don’t have an annex handy.
 
So does this simply mean I start my adjustment factors once I exceed 30 ccc at any one cross sectional point? At 45% for 30ccc correct?
 

Attachments

  • A758B484-57BD-4593-9B7E-81D73F0208B0.jpeg
    A758B484-57BD-4593-9B7E-81D73F0208B0.jpeg
    759.6 KB · Views: 7
Last edited:
Dsg319 said:
So does this simply mean I start my adjustment factors once I exceed 30 ccc at any one cross sectional point? At 45% for 30ccc correct?
Click to expand...
Yes, that is exactly what it means, however if you have to do an ampacity correction for elevated temperatures, that applies to all cases, even where the number of current carrying conductors at any cross section is less than 30.
 
Dsg319 said:
So does this simply mean I start my adjustment factors once I exceed 30 ccc at any one cross sectional point? At 45% for 30ccc correct?
Click to expand...

Unless your site is in Massachusetts, where the state code has a different bundling adjustment table, that never goes below 50%. But yeah, the 31st wire is the straw that breaks the camel's back on the 30 ccc rule for wireways and wire troughs, and it is treated just as if it were 31 wires in a conduit or tubing.
 
Carultch said:
Unless your site is in Massachusetts, where the state code has a different bundling adjustment table, that never goes below 50%. But yeah, the 31st wire is the straw that breaks the camel's back on the 30 ccc rule for wireways and wire troughs, and it is treated just as if it were 31 wires in a conduit or tubing.
Click to expand...
Up until the 1993 code the ampacity adjustment table had two columns in the NEC. Column A was based on a "50 percent load diversity factor", and column B was without that load diversity. The values in Column A are the values used in the Massachusetts amendment. In the 93 code cycle column A in Note 8 to the 0 to 2000 volts ampacity tables was removed from Article 310, as the CMP could not agree on a definition of "50 percent load diversity"
The load diversity ampacity adjustment factors appear in Informative Annex B as Table B2(11), in the current code and can be used under engineering supervision.
 
Status
Not open for further replies.
Top