Calculation of Higher Ampacities for 0-2000
 Does the NEC Code Allow Alternate Ampacity
 Determining the Effects of Application Factors
Not Accounted For in the Code
 Organization and Reporting of Circuit
 Calculated Ampacities and Local Governing
Aerial Ampacity Calculations and the NEC Code
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Determining the Effects of Application Factors Not Accounted
For in the 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.
- 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
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
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