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#22560 - 02/25/03 05:44 AM Neher-McGrath calculations
George Offline

Registered: 02/23/02
Posts: 380
Neher-McGrath calculations provide an alternative method for computing derating of conductors in conduit.

While an engineer is required to oversee the computations. This shows how I would do the computations.

My goal is to place as many 12g wires in a conduit as possible while maintaining 20amp capacity.

12g THHN (90 degree) has 30amp capacity. 7 to 9 current carrying conductors in conduit are derated to 21amps. 9 current carrying conductors (c) plus 4 grounds will fit in 3/4" PVC-40. 3/4" PVC-40 has an inside diameter (id) of .824" and a wall thickness (t) of .113".

From the Neher-McGrath calculations (or general heat transfer solutions) one can deduce that id/t/c is a constant. In this case the constant is .81.

To compute the number of conductors allowed in a different size PVC-40 conduit use the formula


2" PVC-40 conduit has an 2.067 and a wall thickness of .154.

(2.067/.154)/.81 = 16 current carrying wires.

Fill computations allow up to 99 12g wires in 2" conduit. So fill in not a concern. 8 two wire THHN circuits plus grounds will fit. There might be enough space to use 8 12-2+G NM circuits in 2" conduit.

While I chose 12g wire for my starting point, an engineer is free to start where he wishes. He can be more agressive than I.


There is one detail. Neher-McGrath requires the conduit spacing be the lesser of 3 times the conduit diameter (center to center) and the wall thickness.

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#22561 - 02/26/03 05:11 AM Re: Neher-McGrath calculations
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
>While an engineer is required to oversee the computations. [...]

Why this fuss about engineers? A degree in engineering only shows that you have a five year training to make wild guesses look like the unquestionable truth. Make nice looking CAD's, PowerPoint slides and sound like you know what you are talking about.

#22562 - 02/26/03 08:15 AM Re: Neher-McGrath calculations
Joe Tedesco Offline

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA

I thought you were going to join this board?

Come on, let's see you do your stuff here! You had quite a bit to say about being a PE, and let everyone know that you as a PE separated the men from the boys!!

Now you can shine my!

Come up to the podium and take over the class, or say whatever you want about the issues raised here and in the other threads we have posted during the last few weeks!

[This message has been edited by Joe Tedesco (edited 03-04-2003).]
Joe Tedesco, NEC Consultant

#22563 - 02/26/03 09:49 AM Re: Neher-McGrath calculations
elecRB Offline
Junior Member

Registered: 10/22/02
Posts: 5
Loc: Loveland CO USA
Wow George, Great job. I guess that is why I always end up sticking to the tables and charts. Thanks for the input George. I will go through carefully as I should at least know how these are done. By the way, are there examples of such calcs back in Annex D or where would I go?

Hey Joe, my hats off to PE's as I know they have to take alot of responsibility when they put their seal on that drawing. But if the PE made a mistake of saying he was better than the electrician, maybe he is not around to comment on the forum because he told the guys on the jobsite he was better. Ouch!!!

#22564 - 02/26/03 03:09 PM Re: Neher-McGrath calculations
George Offline

Registered: 02/23/02
Posts: 380
I have done more thinnking about this in the past few days. I have rewritten everything in terms of the heat dissipated by conduit and and generated by the conductors.

It is certainly easier to read and the math is more correct.

I placed a paper in Word format on my recreation site. Sorry it is not HTML.

I found some stuff at


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