Applying 110.14 (C) & Correction factors

[rjc3po]

I purchased the 2005 Electrical NEC Exam Preparation review book from the Mike Holt site. While reading it, I found something in CH6 that does not seem correct, or at least I do not understand it.

The confusion is concerning the application of 110.14 (c). It may be easier to ask the question in the form of an example.

If for example, I am trying to size a conductor for a 50 Amp load where, I do not know the terminal temperature rating and I would like to use THHN.

110.14(c) tells me that, because I do not know the terminal temperature rating, I must use the 60deg C column of 310.16 therefore, I need #6AWG with a capacity of 55Amps. This I understand.

If I need to consider ambient temperature, because I expect it to be in excess of 30 deg C, then I would apply the correction factor in the table below 310.16 based on the column for THHN. For example ambient temp of 100 deg F would be a 0.91 correction factor. This I understand.

If I need to have more than 3 current carrying conductors in a conduit, I need to consider the correction factor in 310.15(B)(2)(a). For example 4 current carrying conductors would be a correction factor of 0.80. This too, I understand.

Now, what if I put all of this together, what would the correct size be?

70*.91*.80=50.96 Amps, meaning I must use a #4AWG

Or

75*.91*.80=54.6 Amps, meaning I must use a # 6AWG.

You see the difference?
In the first solution I am using the rule of 110.14(C) and applying the correction factors to the capacity in the 60 deg C column.

In the second I am applying the correction factor to the capacity in the 90 deg C (THHN) column.

It would appear based on the examples in CH6 that when I do not need to consider anything other than terminal temperature ratings, I should use the 60 deg C column. But as soon as I need to consider anything else, I should use the column associated with the insulation type.

Which is correct?

 

[bob]

Since THHN has a 90C rating, you would use that figure in derating the conductor. If you need a 50 amp conductor 50 amp/(.91 x .80) = 69 amps.
If you need a 60 C rating look in that col for 69 amp. If you can use the 75 C col look there of a 69 amp rating. The 75 C rating yields a smaller conductor.

 

[jim dungar]

It is a three step process.

First use the 90?C column for performing all of your adjustments.
Second choose a conductor from the 60?C column (or the 75?C if you know the device rating) without derating.

Finally compare these two sizes. The conductor you choose must never be smaller than the one you would have used without derating.

 

[ramsy]

IMO, Bob & Jim explain better than the NEC itself, how to keep terminations & equipment within 110.14(C), temp. limits.

Unfortunately, NFPA 70 table 310.16 adjustments for 90c insulation, ambient, or CCC's, fails at expecting reasonable people to switch back and forth, ending with 60c columns to avoid overheated equipment.

Most contractors avoiding engineering supervision will stick with 90c columns, and blast 110.14(c), much less should their exploited laborers or undesirable employees be expected to do any better.

Whats worse, inductive loads are not listed as table 310.16 adjustments. Therefore, 75c equipment can be overheated when ambient or bundling-table adjustments combine with motor loads missing this 125% inductive adjustment for excess current.

IMO, avoiding engineering supervision with NFPA table-adjustment complexity may practically support the cottage industry of IR camera diagnostics, perpetuate judicial practices of States like Oregon, which assume all contractors are Guilty until proven innocent, and perhaps provide proof of negligence for bad faith insurers that deny claims.
http://www.badfaithinsurance.org/publications.html

Except for Bob & Jim of course.

 

[SAC]

My understanding is that you need to pick a conductor that satisfies both the terminal temperature requirements, and the insulation temperature requirements - meaning a conductor with an allowable ampacity that is not less than either condition would require if taken alone.

Pick a conductor that would meet both #1 and #2, below, and you are good to go.

1. The terminal temperature conductor ampacity requirement is determined from table 310.16 by choosing the conductor with an allowable ampacity at least as great as the required load, from the temperature column that is not greater than the temperature rating of the terminal to which it is connected.

2. The insulation temperature conductor ampacity requirement is determined from table 310.16 by choosing the conductor with an allowable ampacity, which when corrected and adjusted as appropriate, is at least as great as the required load, from the temperature column that is not greater than the temperature rating of the conductor insulation.

What has had me thinking a little in the past is the lack of ambient correction for the terminal temperature rating - but maybe those are built into the device ratings via maximum allowable ambient?

 

[bryonb]

What has had me thinking a little in the past is the lack of ambient correction for the terminal temperature rating - but maybe those are built into the device ratings via maximum allowable ambient?

I, too, have had this same question. Let's take it to the extreme: If you have a 500kCMIL conductor connected to a 75?C rated terminal installed in an ambient environment of 70?C would it not stand to reason that if we put 380 amps on this wire that the actual temprature of the terminal would be greater than 75?C? It helps me to think of being able to use Table 310.16 "forwards and backwards". If you put 380 amps on a 500kCMIL wire in an ambient environment of 30?C with two other current carrying conductors, the copper will be in the neighborhood of 75?C.

 

[bob]

I, too, have had this same question. Let's take it to the extreme: If you have a 500kCMIL conductor connected to a 75?C rated terminal installed in an ambient environment of 70?C would it not stand to reason that if we put 380 amps on this wire that the actual temprature of the terminal would be greater than 75?C?

That would be correct. You would need to go to the table below 310.16 and use the correction factor for 70C.

It helps me to think of being able to use Table 310.16 "forwards and backwards". If you put 380 amps on a 500kCMIL wire in an ambient environment of 30?C with two other current carrying conductors, the copper will be in the neighborhood of 75?C.

Right on.

 

[ohmhead]

Well very good question makes you think and yes no one in the field after they pass there exam ever look.
This is why we have issues like this i agree the code should include or explain this more so it is a problem with the states that dont have a license electrician doing electrical work .

Most never look at the notes or any further .

Most electricians always go to the 90 col. and never think twice about it .

Most electrical inspectors never look at insulation they see wire size only .

Good post