SKM Neher-McGrath software indicating 90C insulation required for 3,000A service

[Paul the BIG Finn]

My company purchased the SKM Neher-McGrath software module and I suspect the results are overly cautious. It indicates 90 degree C insulation required for 4,000A feeder in a concrete duct bank ~100' long. Does this sound reasonable?

 

[petersonra]

My company purchased the SKM Neher-McGrath software module and I suspect the results are overly cautious. It indicates 90 degree C insulation required for 4,000A feeder in a concrete duct bank ~100' long. Does this sound reasonable?

You paid big money for this software and now you don't like what it told you so you want anonymous people on the Internet to tell you it came up with the wrong answer?

 

[infinity]

I know nothing about this software but since it's a duct bank would it require the 90° C conductors to account for some type of ampacity adjustments?

 

[Paul the BIG Finn]

You paid big money for this software and now you don't like what it told you so you want anonymous people on the Internet to tell you it came up with the wrong answer?

We did indeed pay big money for the program, but having owned it for less than a week and it being new to module for SKM as well, I am looking for a reality check from some of the experienced professionals that read this board. I won't characterize us as "anonymous people on the Internet," although I appreciate evaluating any advise I get is my responsibility.

 

[Paul the BIG Finn]

You paid big money for this software and now you don't like what it told you so you want anonymous people on the Internet to tell you it came up with the wrong answer?

The program attempts to calculate the heat gain of a duct bank based on the the size of the conductors, the duct bank spacing, the type of insulation, the thermal resistivity of the duct bank and conductor insulation and the amount of current following through the conductors. It is an iterative program with an out put of the maximum end temperature in each of the ducts. This temperature then dictates the insulation rating required. The program output shows temperature ranging from 70 to 85 degrees C depending on the position of the duct in the bank.

 

[smoothops10]

I saw they added this in version 8. Have not tried it yet.

How do the results compare to Annex B Table B.310.15(B)(2)(7)?

Annex does not have a 9 duct, figure 4 column but if you model the 6 duct in skm and get similar values to the table, atleast you can have some confidence in the software.

 

[petersonra]

We did indeed pay big money for the program, but having owned it for less than a week and it being new to module for SKM as well, I am looking for a reality check from some of the experienced professionals that read this board. I won't characterize us as "anonymous people on the Internet," although I appreciate evaluating any advise I get is my responsibility.

I didn't mean to pick on you. Well, maybe just a little bit.

Why don't you just do the calculations by hand and see what you come up with.

 

[Paul the BIG Finn]

I saw they added this in version 8. Have not tried it yet.

How do the results compare to Annex B Table B.310.15(B)(2)(7)?

Annex does not have a 9 duct, figure 4 column but if you model the 6 duct in skm and get similar values to the table, at least you can have some confidence in the software.

That is a really good idea. Thanks, I'll post what we come up with.

 

[ron]

Since the equipment terminations are only tested at 75deg (the terminations may be listed at 90), you need to increase conductor size or # of sets until you get it down to 75 deg temp for worst case temp calc at the ampacity you ant.

 

[JFletcher]

Doesn't all wire in the sizes needed (500MCM Cu or larger, equivalent in Al) readily come with 90*C insulation anyway? :?

 

[petersonra]

Doesn't all wire in the sizes needed (500MCM Cu or larger, equivalent in Al) readily come with 90*C insulation anyway? :?

I think it is underground so it would have to be wet "W" rated wire at 90 degrees. not sure that is quite as common.

 

[mbrooke]

You paid big money for this software and now you don't like what it told you so you want anonymous people on the Internet to tell you it came up with the wrong answer?

Thats a pretty big insult to the guys here I would trust PhDs, prodigies, and 40+ years of experience over any obscure program which may not be factoring all the parameters.


Anyway, most wire to my knowledge is already rated 90*C, so that in itself is a none issue. What might set you back however are the terminals at the breaker which are usually rated 75*C. I would focus on that and not the 90*C requirement.

 

[smoothops10]

OP may not be coming back but I agree it is not a big deal to spec THWN-2, XHHW-2, RHW-2 cost wise vs THWN, XHHW, RHW etc. You'll probably get the x-2 90C rated whether spec'd or not. The terminations however is something not feasibly met. What is the opinion, 110.14c would apply to the terminations here? I think the fact that this is LV and neher only comes form the annex it is conservative and above code to begin with. I have seen this in several facility standard specs of big companies lately though.