Cable Ampacity Calculations

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Ahmed Abdelfattah

Member
Location
Edmonton, Alberta, Canada
Hello,

I'm fairly new to NEC and I would like to clarify one thing regarding cable ampacity calculations. I'm referring to NFPA 70 (NEC) 2017.

So, we have tables 310.15(B)(16), (18), and (20) for the ampacities of not more than 3 current carrying conductors. For these tables, it is clearly stated that Table 310.15(B)(3)(a) is to be used if we have more than 3 conductors (to allow for grouping effect)

We also have tables 310.15(B)(17) and (19) for single insulated conductors. For these tables, however, there no mention of the grouping factors to be considered if more than one conductor is used.

My questions are;
(1) If tables (16), (18), and (20) are for not more than 3 current carrying conductors, can't we used these tables in case of single conductors?
(2) If tables (17) and (19) are to be used, how to get the grouping factors for more than one conductor?

Thanks
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
310.15(B)(16) must use Table 310.15(B)(3)(a) because it is based on current carrying conductor's in a raceway. The tables for free air do not need to be derated based on 310.15(B)(3)(a) however you may have to derate based on 310.15(B))(2)(a or b)
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Ahmed Abdelfattah said:
Hi Smart,
Tables 310.15(B)(2) (a) and (b) are for temperature derating.
I personally prefer to use the formula 310.15(B)(2)
Regards
Click to expand...
Ummm... he did not say "Tables", at least at the end of the sentence. That threw me off as there is no text 310.15(B)(2)(a) or (b).
 
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