Practice exam? Conductor ampacity which column 310.16

[Brent B]

Going over this practice question for my code test. Thought I was right to base it on the 60 degree column because the information wasn’t given if terminals were rated higher, and it was less than 100 amps...

A trade size 1 1/4 Conduit contains 3, 3 phase motor circuits and an equipment grounding conductor. The motors run at the same time and the conductors are number 8THHN copper. What is the allowable ampacity of these conductors?

From nec table 310.16, the allowable ampacity of a number 8THHN copper conductor is 55 A.

The 55 ampere rating is based on not more than three current caring conductors in the raceway and an ambient temperature of 30°C

Because a different ambient temperature is not stated, no correction for ambient temperature is required, the percentages given in NEC table 310.15(c)(1) to the ampacity listed in the table or applied

In this case it is 70% based on the nine current caring conductors.

Note: NEC section 310.15 F provides that the equipment grounding conductor is not counted when applying the provisions of sections 310.15(c)(1).

55×.70 equals 38.5 A

 

[Brent B]

i was thinking it was 40 amps x 70%
Which equals 28 amps

based on the requirements of 110.14(c)(1)(a)

 

[augie47]

It being a THHN conductor, you can de-rate from the 90° column so the de-rated amapcity would be the 55 x .7

 

[Brent B]

So even though there isn’t a terminal rating I base the conductors on the 90 degree column? I always thought it was 60 for under 100 amps, 75 for over 100A. when not knowing anymore info on motor or terminals.

 

[LarryFine]

So even though there isn’t a terminal rating I base the conductors on the 90 degree column? I always thought it was 60 for under 100 amps, 75 for over 100A. when not knowing anymore info on motor or terminals.

You're allowed to start with the 90-deg column; you don't have to, but why wouldn't you?

 

[augie47]

The difference in the 60° and 90° ampacity is due to the insulation. Ampacity is the allowable current without damaging the insulation so you take that into account 1st.
The 60° and 75° rules pertain to termination restrictions and will effect the allowable circuit size (determine by the overcurrent device).
It's a bit tricky but, especially for testing, etc, you need to divorce the two. Check your definition on ampacity.
It only deals with conductors not circuit size.

 

[texie]

So even though there isn’t a terminal rating I base the conductors on the 90 degree column? I always thought it was 60 for under 100 amps, 75 for over 100A. when not knowing anymore info on motor or terminals.

As others noted, you can use the 90 degree value as the basis for your calc as THHN is 90 degree rated. In this case the ampacity is 38.5 amps. And as you correctly stated, you final value could not exceed the 60 degree rating as your breaker is less than 100 amps and not stated otherwise. In other words your final calculated ampacity can not exceed the value of a 60 degree conductor.

 

[Brent B]

The difference in the 60° and 90° ampacity is due to the insulation. Ampacity is the allowable current without damaging the insulation so you take that into account 1st.
The 60° and 75° rules pertain to termination restrictions and will effect the allowable circuit size (determine by the overcurrent device).
It's a bit tricky but, especially for testing, etc, you need to divorce the two. Check your definition on ampacity.
It only deals with conductors not circuit size.

Thank you for the explanation. This is what I needed to understand from the question, I’ve been so fixated on motor calcs that I combined the 2 into this question