Ampacity of Conductors in air

Jan 18, 2004 4 Replies
G

I am having problems calculating RCA. I know how to calculate Ri for the insulation but cannot seem to arrive at the same current value as per NEC table. The example given at

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ampacity of No 2 conductor uses RCA = Ri + Rsd + Re and is 120A but that is for conductor in conduit. I need to calculate RCA for a conductor in air which should be 190A at 90degC conductor and 30degC ambient.



Can you shine some light on the subject.



Greg.


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M

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for ampacity of No 2 conductor uses RCA = Ri + Rsd + Re and is 120A but

You will need the NM paper. Annex B of the NEC will also shed light on the subject since these tables are a computer run of NM calculations. Ri and Re would apply, but Rsd would not.

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M

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for ampacity of No 2 conductor uses RCA = Ri + Rsd + Re and is 120A but

Based on the referenced calculation Rsd would be eliminated with cables in free air. Additonally, the calculation for Re would have to be modified for cables in free air. Re is now for a one inch conduit in free air.

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M

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for ampacity of No 2 conductor uses RCA = Ri + Rsd + Re and is 120A but

Re=9.5N'/1+1.7Ds'(e+0.41) n' is number of conductors = 1 Ds' is diameter of cable = 0.382 in. for No. 2 XHHW e is emissitivity = 0.5 for dark cable Re = 9.5(1) / 1+1.7(0.382)(0.5+0.41)

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