Ampacity in air

[Gregory Cohen]

Can you provide the formula for calculating ampacity of conductors in air
please.

Regards

Greg.

 

[Mr. Smith]

Can you provide the formula for calculating ampacity of conductors in air
please.

Regards

Greg.

Oh yes, it is quite simple if you know about heat transfer.

I (in kiloamperes) = SQRT((TC-TA+Delta TD)/((RDC)(1+YC)(RCA)))
found in Section 310.15(C) of the 2002 National Electrical Code.

RDC = resistance of one foot of conductor that is .001 in in diameter in
microhms
TC = conductor temperature in degrees C.
TA = ambient temperature in degrees C.
YC = component ac resistance resulting from skin effect and proximity effect
RCA = effective thermal resistance between conductor and surrounding
ambient in thermal ohm feet
Delta TD = Dielectric loss Temperature Rise
That is all there is to it. Just substitute in and calculate.
For more information read the Neher McGrath paper published in 1957.

Also read Understanding the Neher-McGrath Calculation and the ampacity of
conductors at:

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

 

[Gregory Cohen]

Thanks for that.
Do you know where I can get a copy of the Neher-McGrath paper?

Greg.
----- Original Message -----
From: "Mr. Smith" <[email protected]>
Newsgroups: alt.engineering.electrical
Sent: Tuesday, January 13, 2004 3:40 PM
Subject: Re: Ampacity in air

 

[Gregory Cohen]

Thanks for the info.

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.