310.16 clarification...

eric dolphy

Member
Location
California
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

Moderator
Staff member
Location
Tennessee
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

Member
Location
California
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

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