neher mcgrath calcs

[mshields]

Am reviewing a project which shows 9 - 750kcmil for a 3000A, 4160V feeder.

I think we're fine with that quantity. BUT would like to be able to be more scientific about it. I understand that Neher McGrath calcs in SKM require an add in and that it is very expensive. Short of that, how can I go about determining exactly what the rated ampacity of this feeder is?

 

[charlie b]

I know of two ways that are cheaper than the SKM add-in. But one of them might not work in this application, since it includes 9 conduits. I generally use Table 310.60(C)(77) for underground installations. But that table only has information for 6 conduits. Nevertheless, for MV105, 5kV cable, and 6 ducts, you are at 395 amps per conductor. You only need 333 amps, in order for 9 of them to equal 3000 total. You can make an engineering judgement on that basis, if you are so willing. You can include in your basis the values that decline from 630 to 495 to 395 as you go from 1 to 3 to 6 ducts, and see if extrapolation can help.

The other approach is to buy AmpCalc from Calcware. I used it many times while working for companies that were willing to pay the cost of the software. The last time I asked my company to buy it was over a decade ago. The price was around $2K then. It is very easy to use. But (again the last time I used it) it had no other capability than to do underground ampacity calculations. That may have changed in recent years.

 

[Julius Right]

You may purchase for 33$ this article:
The Calculation of the Temperature Rise and Load Capability of Cable Systems by Neher and McGrath
and calculate using Microsoft Excel.
https://ieeexplore.ieee.org/document/4499653

 

[mshields]

Awesome!

Awesome!

Thank you Both

 

[charlie b]

You may purchase for 33$ this article:

That article was published in 1957. It's a good bet that it did not include an Excel spreadsheet. :happyno:

The difficulty that an EE would face in trying to use the Neher McGrath method to calculate ampacity comes from assigning numerical values to the variables.

 

[mshields]

hmmm

hmmm

Julius - Were you suggesting that this, the original Neher McGrath, paper would render it immediately obvious how to devise an excel spread sheet to my ends? At first glance, it looks rather a daunting task. Am I missing something?

Thanks,

Mike

 

[junkhound]

re: render it immediately obvious how to devise an excel spread sheet to my ends

Nope, don't think you are missing anything, not intellectually challenging but would be tedious

was Just curious how difficult transcribing the paper into spreadsheet would be so took a look.

Company has IEEExplore subscription so downloaded the paper in pdf, found even a big table like Table III could be copied and parsed directly onto spreadsheet fairly easily, as could the large tables of constants.

One would have to be familiar with lookup tables and such in excel to make use of the parsed data.

OTOH, nearly all of the formula are in common paper notation, and would have to be hand transcribed - very tedious. Having done 500,000 line spreadsheets for some electrical projects in the past, have some idea in the effort involved.

Am estimating that it would take at least 4 or 5 days to fully translate that paper into a workable excel spreadsheet.
Could probably do it from scratch using basic thermo and electrical existing FEA programs in about the same time, -- maybe just a half-day for the one example posted by OP.

 

[Julius Right]

I am sorry.
For an engineer using excel for more than 20 years, I thought this work might be interesting.
However you may take a look in IEEE 835 page 350 for 9 ducts

 

[Julius Right]

I spent 2 hrs. in order to calculate the ampacity for 9*3 750 mcm copper cable in underground ducts.
I think it is close to IEEE 835 for 100% LF 90 RHO 25oC Earth [1/18 shield].