NEC ampacity and ambient temperature adjustment

[TMD]

The NEC (NFPA 70) has an equation for converting cable ampacity at 40 C to ampacities at different ambients (Section 310). The equation has a Delta TD = Dielectric Loss Temperature Rise. Where can I find values for this factor? I suspect it's in the IPCEA document referenced, but I don't have that.

 

[wareagle]

Is this caculation for high voltage cable?

 

[TMD]

Yes it is. For cable less than or equal to 2000 V, there is a table of correction factors.

 

[jghrist]

The formula was presented originally in AIEE paper No. 5-600, "The Calculation of the Temperature Rise and Load Capability of Cable Systems," by J.H. Neher and M.H. McGrath, AIEE Transactions, Part III (Power Apparatus and Systems), Vol. 76, Oct. 1957, pp. 752-772.

 

[TMD]

Has anybody actually used this equation to adjust the 40 C ampacity values in the tables? If so, what did you use for the Delta TD value? ABS (American Bureau of Shipping) and DNV use this formula in their design rules for LV cables, and use Delta TD = 0.

 

[jghrist]

[Δ]TD depends on insulation material and thickness, and voltage. It's calculation is in my previous reference, and also in "Rating of Electric Power Cables", George J. Anders, IEEE Press,1997.

Actually, [Δ]TD is much simpler to calculate that YC and RCA in the formula. What are you using for these? For 15 kV cable and below, the dielectric loss is low and you can probably safely assume [Δ]TD=0

 

[TMD]

I'll have to see if I can track down those references.

I'm not using YC or RCA. I'd need those if I was trying to calculate ampacity from scratch. I'm just trying to adjust for an ambient temperature higher than 40 C. I'm using the equation in the NEC 310-60(C)(4)

 

[jghrist]

I see now. I was looking at the full formula under 310-60(D). [Δ]TD equals the dielectric loss W_d times the effective thermal resistance between the conductor and ambient for dielectric loss R_da. Unfortunately, R_da has the same factors as RCA except only 1/2 of the insulation thermal resistance is used and no load factor is considered. W_d is not too hard to calculate, but R_da is a bear.