*****DERATING*****

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gregsparky

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WHEN DERATING WIRE AMPACITY FOR PARALLELING IN THE SAME CONDUIT, WHICH TEMPERATURE RANGE IS USED?
I'M ATTEMPTING TO GET 600 AMPS IN A SINGLE 3" EMT CONDUIT.
 
steve66

steve66

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Re: *****DERATING*****

You can use the temperature rating of the conductors (often 90C). You have to use the temperature rating of the terminations for straight ampacity calculations, and this usually limits you to the 75C temp (60C for 100 amps and under). However, the 80% derating for more than 3 current carrying conductors is usually applied to the temp. of the conductors.

Steve
 
charlie b

charlie b

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Re: *****DERATING*****

To get 600 amps through one set of conductors, you need size 1500 MCM. 3? EMT will only hold one of these.

To get 600 amps through two parallel sets of conductors, each must be able to carry 300/0.8, or 375 amps. For this, you need size 400 MCM. 3? EMT will hold six of these. If you don?t need a neutral, it might be possible to sneak 6-400?s and a #1 EGC in to a 3? EMT, but you?d have to calculate it by % fill and by cross-sectional area of the conductors. The Annex C Tables don?t give you room to include the EGC.

To get 600 amps through three parallel sets of conductors, each must be able to carry 200/0.7, or 286 amps. For this, you need size 250 MCM. 3? EMT will hold nine of these. Once again, if you don?t need a neutral, it might be possible, as discussed above.

In either of the parallel cases, you're likely to have trouble actually executing the pull. So I?d agree with Bob ? it's not going to happen.
 
iwire

iwire

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Re: *****DERATING*****

I will change my answer to it might happen.

Lets assume either straight 3PH 3W or 1PH 3W.

The largest 6 conductors you can fit in 3" EMT will be 400kcmil THHN

400kcmil = 380 amps at 90C x 2 = 760amps x .8 = 608 amps

So if you only need a total of 6 current caring conductors and all your equipment is rated 90C you could do this.

But if one component of the system is rated 75C or you need more then 6 current carrying conductors it is not going to happen.

Also I did not include a ground wire in the EMT which if you need it would probably push you down to 350kcmil putting you under 600 amps even at 90C.

Sorry, time to run another raceway.
 
iwire

iwire

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Re: *****DERATING*****

Charlie looks like we where typing at the same time. :D

My first post was off the cuff but I was bored and tried to make it work and found the same thing you did, tough pull at best.
 
ryan_618

ryan_618

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Re: *****DERATING*****

240.4 (B) may allow you to round up to 600 and provide less than 600 amps, but I still think the whole thing is pie in the sky. Time to run a new raceway!
 
J

jro

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Re: *****DERATING*****

Parallel conductors makes more sense, smaller conductors are easier to handle than than?larger ones, but why just one 3" emt, it would be easier to parallel the conductors running each set in different conduits, that is if you can run additional conduits, if not, as it has been stated, its not going to happen. :(
 
ryan_618

ryan_618

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Re: *****DERATING*****

JRO: I don't think that not running parrellel is even an option. Notice in table 310.16 how quickly ampacity decreases after 500 KCMIL. In order to provide 600A on a single conductor we are talking about 1500KCMIL!!! :eek:
 
P

pierre

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Location
Westchester County, New York
Re: *****DERATING*****

How about if this is a service? Are not neutral conductors required to be run to the service?
I also say that based on experience of this condition, that the only good thing that raceway will provide after the wire is pulled (if you actually get it in) is to keep water from flowing :D :D

Pierre
 
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jro

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Re: *****DERATING*****

So what is this installation, the main service, which I assume it is, we have not established those facts, good point Pierre, and why can't additional conduits be installed.
 
steve66

steve66

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Illinois
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Re: *****DERATING*****

Two more reasons to run another conduit:

I would bet that since we trying to avoid running another conduit, the first run is probably a long difficult pull.

Also, if I remember right, if the conduit is in a wet location, the wire would have to be rated at 90C for a wet location. I don't know offhand what this would do to the size of the wire (thicker insulation?)

Steve
 
ryan_618

ryan_618

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Salt Lake City, Utah
Re: *****DERATING*****

Hello Steve. Take a look at article 310.8 (C) and you will notice that wet locations must have a W on the insulation. Notice on table 310.16 that many conductors with a W on them are actually rated for 75 degrees and some are rated for 90 degrees.
 
roger

roger

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Re: *****DERATING*****

Table 310.13 shows the characteristics of the conductor types. Please note that some (W) types are the thinest insulations and some are the thickest.


Roger
 
charlie b

charlie b

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Re: *****DERATING*****

Originally posted by iwire: So if you only need a total of 6 current caring conductors and all your equipment is rated 90C you could do this. But if one component of the system is rated 75C or you need more then 6 current carrying conductors it is not going to happen.
Click to expand...
Let me offer to split a hair here. If the conductors are rated for 90, and the terminations are rated for 75, it can still be done. Using the 90C column for 400, as you and I have both done, gives us a limit of 608 amps for two parallel conductors, or 304 amps for each conductor. That value is lower than the 335 amp limit for the same conductor at 75C. Therefore, the terminations are not at risk, and the installation is acceptable.
 
iwire

iwire

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Re: *****DERATING*****

Originally posted by charlie b:
Let me offer to split a hair here. If the conductors are rated for 90, and the terminations are rated for 75, it can still be done.
Click to expand...
Charlie I do not think you are splitting hairs here, this is not one of the gray areas of the code.

I messed up and you should point that out, the idea here IMO is to get the correct info out.

Thanks, Bob
 
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pwhite

Senior Member
Re: *****DERATING*****

don't forget to compensate for the temperature.
i did not see anyone mention this.
 
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gregsparky

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Re: *****DERATING*****

GENTLEMEN---
I THANK YOU ALL FOR YOUR INPUT BUT WE ARE WANDERING OFF OF THE MAIN SUBJECT.
LET ME REPHRASE THE QUESTION AND ADD A LITTLE MORE INFO TO IT.

MY STORY ABOUT INSTALLING 6-400 KCML THHN/THWN'S IN A 3" EMT CONDUIT IS TRUE, IS LEGAL AND IS DONE.
MY OBJECT IS TO FIND OUT WHAT THE SMALLEST SIZE THHN/THWN CONDUCTOR I CAN INSTALL IN THAT SAME CONDUIT (PARALLELED) AND STILL FUSE IT AT 600 AMPS.
SOMEONE AT WORK TOLD ME THAT YOU ALWAYS USE THE 90C COLUMN FOR FINDING AMPERAGES FOR DERATING.

DON'T FORGET ABOUT THE OVERCURRENT PROTECTION RULE. THAT WOULD BE ARTICLE 240-3(b).

NOW LETS SEE WHAT KIND OF ANSWERS WE GET! :)
 
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