Aerial Ampacity Calculations and the NEC Code

Not only are ampacity calculations permitted rather than the use of the NEC Code Tables, but sometimes application parameters dictate that calculations must be performed. Calculations can produce more precise answers when taking into account actual cable construction, conduit sizes and environmental characteristics. Some of the factors which can come into play which are not handled by the Code Tables include the following:

• Conduit size - Larger conduits with lower fill factors than what is assumed in the Code Tables can significantly raise ampacities because of the greater heat radiating area of the larger conduit.

• Conduit material - The Code tables utilize "Iron" conduit. Other materials, such as PVC, will result in more cable heating because of the higher thermal resistivity of non-metallic materials.

• Conduit coatings - Fireproof conduit coatings intentionally will add thermal blanketing to an installation to add fire withstand capability. However, that thermal resistance works for heat flow in both directions and will result in derated ampacities not provided for in the Code.

• Cable Insulation -  Differences in cable insulation thickness and thermal resistivity will result in different system ampacities.

• Sun and Wind - There is no allowance for the effect of these environmental factors in the Code.

• There are no tables for some aerial configurations - This includes one or two single conductor cables in conduit. Tables 310-20 and B.310.1 and B.310.3 were added to the Code to supplement arrangements not covered by 310-16 or 3410-17 and cover respectively, triplexed 1/c in air, 3/c cable in conduit in air and 3/c cable in free air. However, these tables are generally in accordance with the Neher-McGrath calculation procedures, and accordingly are less conservative and not consistent with Tables 310-16 and 310-17.

• Limitations on derate table for more than 3 cables in a conduit - Table 310.15(B)(2)(a) provides derate factors to adjust table ampacities for four our more current carrying conductors in a raceway. However, the table provides cable quantity ranges so that the derate for 4 conductors is the same as that for 6, 10 conductors as the same as 20, etc. Clearly a significantly different ampacity should apply in these cases for different quantities of cable.

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