Ampacity of Conductors in air

[Gregory Cohen]

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 http://www.electrician.com/articles/ampacity.htm
for 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.

 

[Mr. Smith]

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 http://www.electrician.com/articles/ampacity.htm
for 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.

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.

 

[Mr. Smith]

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 http://www.electrician.com/articles/ampacity.htm
for 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.

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.

 

[Mr. Smith]

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 http://www.electrician.com/articles/ampacity.htm
for 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.

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)

 

[Gregory Cohen]

Thank you for your help.
Greg.

conductor

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)