Conductor Type

[lti instructor]

I am an instructor and have come across an issue with conductor type.
I have always been under the understanding that if a question is asked such as " what is the minimum size conductor needed to supply a load of 87.6 amps" that it is assumed that a THHN copper conductor is used, chosen from the 60 degree column based on 110.14(C)(1) under 100 amps

I was taught this rule 30+ years ago and took it for granted however I have recognized a couple of textbooks that seem to state that unless otherwise noted consider the conductors as THW. Of course this may possibly change some of the way that I teach this, but I was wondering if this was noted in the NEC somewhere or used for uniformity in these tetbooks. I noticed Holt slides show this and an ATP text Electrical Systems based on the 2008 NEC.

Any information is appreciated.

 

[TT009]

The way I understand 110.14 (C) (1) (a) is no matter the type of wire, you always go off the 60 degree column, unless the equipment is listed for greater than 60 degrees (ex. 75 degree lugs) or if it is a marked motor B, C, or D.

 

[Pierre C Belarge]

110.14 Electrical Connections.

110.14(C) Temperature Limitations.
110.14(C)(1) Equipment Provisions.
The determination of termination provisions of equipment shall be based on 110.14(C)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(6).
(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:
(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.

 

[TT009]

But if the equipment is not listed or identified with a temp rating and your had a THWN-2 (90 degree) wire, you would base your ampacity off 60 degree column after modifying it with 310.15(B)(2) and table 310.16 correction factors?

 

[kwired]

But if the equipment is not listed or identified with a temp rating and your had a THWN-2 (90 degree) wire, you would base your ampacity off 60 degree column after modifying it with 310.15(B)(2) and table 310.16 correction factors?

most of the time you need to do two calculations. The first calculation always has to be done. It is 125% of continuous load plus 100% of non continuous load. For equipment rated 100a or less or wire terminals 1
AWG or smaller use the 60 degree column to determine wire size, unless ALL terminations are rated otherwise. For equipment rated over 100 A or terminals larger than 1 AWG use the 75 degree column to determine wire size.

This calculation sizes the conductor for the temperature rating of the terminations and will be the smallest conductor allowed for your circuit.

Next calculate using 90 degree column (if using 90 degree conductors) your derations for temperature, number of conductors in raceway, etc.

This calculation sizes the conductor for the insulation of the conductor.

The larger of the two calculations is the minimum size conductor required.

 

[lti instructor]

Conductor type

Conductor type

Thanks to all that replied, however I think the question may not have been fully understood.

Is there a code article that states that a THHN conductor is assumed unless the question provides a different type or was this just my instructors way of teaching this 30 yrs ago? I'm OK with the understanding of articles 110.14(C)(1) & 110.14(C)(2) its just that when students come across questions that ask for conductor sizes that may require derating and sometimes without derating requirements without a conductor type (TW,THW, THHN) they want to know how they decide if its 60,75, or 90 degree conductor they are derating from. As I stated before I was taught to always assume THHN and size based on 110.14 standards. I must have been taught well, I passed my CT Masters license many years ago lol.

 

[raider1]

The NEC does not have any code sections that would require you to assume anything for an electrical exam.

The NEC is written for actual installations not hypothetical questions for an exam.

I would guess that the writers of the exam should tell you what to use for the questions.

Chris

 

[gndrod]

I am an instructor and have come across an issue with conductor type.
I have always been under the understanding that if a question is asked such as " what is the minimum size conductor needed to supply a load of 87.6 amps" that it is assumed that a THHN copper conductor is used, chosen from the 60 degree column based on 110.14(C)(1) under 100 amps

I was taught this rule 30+ years ago and took it for granted however I have recognized a couple of textbooks that seem to state that unless otherwise noted consider the conductors as THW. Of course this may possibly change some of the way that I teach this, but I was wondering if this was noted in the NEC somewhere or used for uniformity in these tetbooks. I noticed Holt slides show this and an ATP text Electrical Systems based on the 2008 NEC.

Any information is appreciated.

In all due respect, a question such posed must include a predicated reference point of condition whether hypothetical or real to avoid academic mindset. rbj

 

[480sparky]

........ what is the minimum size conductor needed to supply a load of 87.6 amps" .........

You are dealing with a single component...... the conductor. 110.14 does not enter into it.


If you want to get into assuming things, what else might you assume? More then 3 ccc's? High ambient temps? Continuous use? Aluminum?

Given the question above, I would answer #4 THHN. If you let the paranoia of assuming take over, you'll end up with 750kcmil.

 

[Pierre C Belarge]

Sometimes the conductor insulation Type and conductor size is not the only determining factor.
110.14 (C)(1)(a) & (1)(b) will be a part of the determining factor(s).

 

[benaround]

I would answer #4 thhn also, the question, "what is the minimum size conductor for a load

of 87.6 amps" without any other stipulations I'm looking for the 'minimum size'. This could

be an a/c unit on a 125a c.b.