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
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
Correction on the above;
RDC is the resistance of one foot of conductor for the given Circular Mil
Area and is given by:
RDC = (1.02 *pc /cma)* ((tah+tc)/(tah+20))
RDC = ohms
pc = circular mil ohms per foot of conductor at 20 degrees C. (10.371 ohms
for 100% IACS copper, 17.002 ohms for 61% IACS aluminum)
tah = absolute value of inferred temperature of zero resistance. (234.5
degrees C. for copper and 228.1 degrees C. for aluminum)
cma = circular mil area of conductor from Chapter 9 Table 8 of NEC
tc = conductor temperature in degrees C.
* is a multiplier symbol
1.02 is multiplier for lay factor
NM paper and from:
Thanks for that.
Do you know where I can get a copy of the Neher-McGrath paper?
----- Original Message -----
From: "Mr. Smith"
Sent: Tuesday, January 13, 2004 3:40 PM
Subject: Re: Ampacity in air
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
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.