ECN Forum Forums General Topics of the Trade General Discussion Area ampacite of wire

guys this may sound stupid but here goes. if a #6 wire THHN or THWN or MTW lists it as 55 amps as TW but THHN list it as capable of 75 amps & THWN as 65. why do you use the lowest amperage? this is in raceway or earth.this cable is capable of these currents. thanks for the answers. FIRE AWAY!

It has to do with the I(squared)t, or amp-squared seconds. ground rods are basically for lightning, and lightning is of very short duration meaning No.6 bare or insulated can with stand much much more current for a short duration than that listed in 310.16.
for example a 2 cycle fault or 0.0333 seconds, using No.8 can withstand 4791 amps. This is from Soars Book on grounding.
IEEE lists No. 6 at 684 Amps for 5 seconds, using the I x I x Sec. is 2.339e6 amps or over 2million amps for 5 seconds, or for the 2 second example 8381.4 amps for 2 cycles.

WOC

It is for temperature de-rating of a conductor. For example lets say you have a 60-amp breaker and want to use type THHN cable with a ambient temperature of 43 C. Follow the steps.

1.Look up the conductor ampacity in the 90C column of 310.16 of a THHN 6 AWG = 75
2.Apply the correction factor for temperature at the bottom of 310.16. .87 x 75 =65.25
3.Apply the adjustment factor for more than 3 current carrying conductors if it applies per 310.15 (B)(2)(a). None in this example
4.Look up the conductor ampacity in the 75C column of 310.16 or 60C if that is the limit of the termination.
5.Use the lower of the results from step 3 or 4.
In this case a # 6 AWG THHN would work. But a 6 AWG RHW, THHW, or TW would not be allowed once you applied the temperature correction factor, or possible more than 3 current carrying conductors.

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