WIRE AMPACITY

[americanele]

WE INSTALLED 6 PARRALELL RUNS OF 500 MCM XHHW AL .THE SERVICE SIZE WAS 2000 AMP.500 XHHW IS A 90 DEGREE WIRE SO I USED 90 DEGREE COLUMN FOR AMPACITY. THE INSPECTOR SAID BECAUSE OF THE TEMP. RATING OF THE LUGS THE WIRE HAD TO BE DERATED TO THE 75 DEGREE COLUMN. MY QUESTION IS,IS THERE AN ADAPTER AVAILABLE THAT WOULD TAKE THE WIRE TO THE 75 DEGREE RATING WITH OUT REPLACING THE WIRE? I KNOW MCADAPT MAKES AN ADAPTER THAT WILL GO FROM 500 AL XHHW TO 500 MCM CU THHN. WOULDNT THIS MEET
THAT REQUIREMENT?

 

[ron]

Re: WIRE AMPACITY

The inspector is correct.
The important rating is not only the lug you terminate in, but the rating that the lug within the equipment is UL listed for.
Very often the lug on its own is rated at 90 degrees, but the switchboard/switchgear/disconnect that the lug lands in is rated for 75 degrees, then the wire is only as good as the rating of the termination in the equipment, 75 degrees.

 

[stew]

Re: WIRE AMPACITY

geezz and you were so close too!!!! 1860 isnt 200 0however . Need a 7th 500 unfortunatly. code only allows 90 deg table for use when DERATING conductors per 110.14 c. This sounds like its gonna cost too! !

 

[mclain]

Re: WIRE AMPACITY

You might look at 310.15 (C) Under engineering supervision; conductor ampacity can be calculated by the general formula given at this section. You are close and it might be that an engineer can approve your installation.

Don McLain

 

[charlie b]

Re: WIRE AMPACITY

There might be another solution. But I just spent a couple minutes trying to look up this concept in the NEC, and could not find it. So if anyone can help with a code citation (or can tell me I am wrong), then please jump in.

If your calculated load is less than the 1860 amps (i.e., the ampacity of six 500 XHHW Al), taking into account the sum of the noncontinuous loads plus 125 percent of continuous loads, then I think you can use a main service OCPD with a variable trip setting, set the device to 1860 amps, and declare that the service is rated for 1860, and not for 2000. I also think that you would need to post a label that declares the service conductors to be limited to 1860 amps. Please note that if the calculated load is above 1860, then you must look for another answer.

Can anyone confirm or refute this notion?

 

[charlie b]

Re: WIRE AMPACITY

Originally posted by mclain: You are close and it might be that an engineer can approve your installation.

That is another possibility. But I would suggest not placing a great deal of hope in it. I have done this type of calculation many times. I have yet to come up with an answer that gives a higher ampacity than Table 310.16. But you never know for sure until you try.

 

[stew]

Re: WIRE AMPACITY

Charlie b . The variable setting type breaker would fall within the provisions of 240.4 C if the total computed load with all variables considered was equal to or les than 1860 amps wouldnt it?

 

[charlie b]

Re: WIRE AMPACITY

I agree in principle. In fact, having the conductors protected at their ampacity (of 1860 amps) is the reason I think it might be OK. But there are two things going on here. First, the service conductors have to be sized to handle the calculated load, including 125% of continuous and 100% of noncontinuous. That is from 230.42(A). Secondly, the service conductors must be protected at their ampacity. That is from 240.4.

But I have a vague recollection of a requirement to post a sign on the switchgear, stating that its rating had been reduced because of the size of the conductors. That is the code section I was hoping someone could help me find.

 

[earlydean]

Re: WIRE AMPACITY

Is it possible for you to terminate your 90 degree conductors in an enclosure close to the switchgear, using 90 degree lugs. Then, after "metering" run 75 degree wiring to the 75 degree lugs in the switchgear.

 

[don_resqcapt19]

Re: WIRE AMPACITY

As long as the wires are large enough for the calculated load, just add an additional service disconnect. See 230.90(A) Exception #3.
Don

 

[j_braddock]

Re: WIRE AMPACITY

wouldn't temperature derating also be required?