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NEC ampacity and ambient temperature adjustment

NEC ampacity and ambient temperature adjustment

NEC ampacity and ambient temperature adjustment

(OP)
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.

RE: NEC ampacity and ambient temperature adjustment

Is this caculation for high voltage cable?

RE: NEC ampacity and ambient temperature adjustment

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

RE: NEC ampacity and ambient temperature adjustment

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.

RE: NEC ampacity and ambient temperature adjustment

(OP)
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.

RE: NEC ampacity and ambient temperature adjustment

ΔTD 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, ΔTD 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 ΔTD=0

RE: NEC ampacity and ambient temperature adjustment

(OP)
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)

RE: NEC ampacity and ambient temperature adjustment

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

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