310.16 clarification...

[eric dolphy]

Mike has a motor calculation example, thus:

Example No. 3: What size THHN conductor do you need for a 2-hp, 230V, single-phase motor?(a) 14 AWG
(c) 10 AWG
(b) 12 AWG
(d) 8 AWG
Let's walk through the solution:
Step 1: Conductor sized no less than 125% of motor FLC
Step 2: Table 430.148 shows the FLC of 2-hp, 230V, single-phase as 12A
Step 3: 12A × 1.25 = 15A
Step 4: Per Table 310.16, you need to use 14 AWG THHN rated 20A at 60°C

in 2011 NEC, 310.16, we find THHN in the third column @90C temp rating...60C temp rating lists 'types TW, UF'...it shows a 15amp copper conductor @ 14AWG

furthermore, a THHN @60C temp does not appear in this NEC edition...and, the 14**AWG is rated at 25amps (again, @90C temp)...

please help clarify, re: the correct reading of table 310.16 for Mike's example...

thank you

 

[augie47]

THHN has insulation that allows it to be used as a 90° conductor so it is good for any temperature below that. For the most part the 90° rating is only useful for derating due to ambient, fill, ect. as 110.14 limits the termination temperature.
As the example noted, you need a conductor good for 15 amps so you could use any of the conducts listed in any of the columns as long as they had a 15 amp rating (a #14 TW, RHW or THHN, etc.).

 

[eric dolphy]

310.16...

310.16...

thank you, Augie...it's understood that anything up to and including 90C temp is covered by THHN...

not to put too fine a point on this, but where did the 14 AWG THHN rated 20A at 60°C derive from?

...the table shows 14**AWG as 15 amps (@60 temp), 20 amps (@ 75 temp), and 25 amps (@ 90 temp)...

that is, where did Mike find the 14 AWG THHN rated 20A at 60°C correspondence?

<< if the answer is obvious... it's 4AM here in the Bay Area, bear with me >>

 

[augie47]

It very well may be the example came from the '08 Code where #14 was rated at 20 amps in the 60° column.

 

[eric dolphy]

310.16...

310.16...

It very well may be the example came from the '08 Code where #14 was rated at 20 amps in the 60° column.

voila! thanks, mate!

<< back to studying... >>