Conduit fill and derating wire. - Electrician Talk - Professional Electrical Contractors Forum

rod213 · 03-16-2007, 07:55 PM

What I had:
3 big machines that take a 100A, 3 phase 480v Circuit each.
Already had bought about 1000' of #2 to take care of these.

I had intended running seperate conduits for each. Well, it turned out there was an existing and empty 2 1/2" EMT comming from the I line running really close to where I need to get my circuits. So plans started changing. Checked the fill for the 2 1/2" and #2 and found I could put 20 #2s in that 2 1/2"!

I only needed to put 9 wires through it with a ground, so I thought it'd be all good. Welllll, I got to thinking I'd heard about this derating thing. So I started checking into it in the code book and found Table 310.15(B)(2)(a).

For 7-9 wires it says to make it 70 percent of the rated value. The rated value of 3 #2's in a raceway is 130A based on ambient temps. So having 9 #2's would take it down to 91A, correct? (Which I could have easily upsized the wire, or made it a tap to my Junction box comming off of a 300-350A breaker, but I already had the wire on the jobsite).

Now, what I've been wondering most is, does it matter what size conduit is used when figuring derating values? I mean, if I would have pulled the 9 hots and 1 ground I would have only taken up about half of the rated conduit fill for the 2 1/2". Which means there is more air space in that 2 1/2" than a 2".

I didn't figure in for voltage drop because I've always heard it's really unnoticable until you get over 100 feet or so, and this was only around 70-80 feet.

Also, how much emphasis do you guys put behind the ambient temperature adjustment? And why is there the adjustment? Is it because the insulation can get that hot in addition to the heat of the resistance of the wire inside?

I'm sorry this is so long, but this is the biggest job I've been in charge of on my own and I'm learning alot of new things on this job. I want to be in the right with things I do. I ended up only running two of those circuits into a 2x2 jbox through the 2 1/2" which derated it to only 104. I'm running another seperate conduit for the other one. I asked another electrician that works with us his opinion on it and he told me, "I ain't ever derated no wire". He's been doing electrical work way longer than I have. I've only been at it for around 6 years or so.

Thank you all in advance for any insight.

brian john · 03-16-2007, 08:10 PM

Derating IS IMPORTANT, I do not have an NEC handy so I can't check your calculations.

My company does IR scanning and we have plenty pictures of conduit that is exceeding the conductor ratings and have several jobs where the installation faulted from too many conductors in a conduit running at full load with oput derating.

YOU FRIEND IS NO ELECTRICIAN....

Strive to be the best you can and that starts with doing quality work and following the NEC. I have made my employees, correct sloppy work and NEC violations at the expense of loosing money. Quality first.

**MDShunk** · 03-16-2007, 08:39 PM

Assuming these conduits are running near the roof in a factory, I like to use 100° F as an ambient (38° C).

130amps * .91 for temp correction = 118 amps

118 temp corrected amps * .70 for 7-9 CCC's = 82 amps when corrected for temp and so many conductors in the pipe.

Somebody double check that, but I think I'm okay.

Don't ask where the 100° comes from, because I pull that out of the sky. Pretty close for many factory ceilings in the summer. Sometimes higher.

Joe Momma · 03-16-2007, 08:47 PM

My teacher told us he had an HVAC contractor thank him(or us electricians rather) for never derating our wire we run to an RTU.
This tinner knew our hot wires in pipes on the roof running AC equipment in the summer was causing a serious voltage drop thusly giving him plenty of service work.

rod213 · 03-16-2007, 08:51 PM

So the oversized conduit doesn't matter in the matter of derating wire? I bet they'd figure you could pull more wire in it to fill it up, which would negate the extra space. Figure for the worst type thing I suppose.

That figure looks correct to me MD.

**MDShunk** · 03-16-2007, 08:56 PM

Quote:

Originally Posted by rod213

So the oversized conduit doesn't matter in the matter of derating wire? .

Nope. All that wire will be laying in one fat bundle in the bottom of the pipe, no matter how much "too big" it is. Heck, you even need to derate for bundles of cables hanging in bridle rings.

**MDShunk** · 03-16-2007, 08:57 PM

Looks like you're running another pipe, huh?

Joe Momma · 03-16-2007, 08:57 PM

You can run a 4 inch pipe with 6 #12 ccc's, but once you hit 7 you have to derate.

rod213 · 03-16-2007, 09:18 PM

Quote:

Originally Posted by MDShunk

Nope. All that wire will be laying in one fat bundle in the bottom of the pipe, no matter how much "too big" it is. Heck, you even need to derate for bundles of cables hanging in bridle rings.

Okay that's what I was needing to know most.

And yeah, looks like another conduit for monday......

**MDShunk** · 03-16-2007, 09:28 PM

The rules on conduit sizing are mostly to keep the conductors from getting damaged during installation, and don't have much at all to do with ampacities.

Joe Momma · 03-16-2007, 09:35 PM

After reading the first couple sentences of the original post;
We usually #3 for 100 amp, so perhaps your #2's will meet the derates?
Or is that the calc you already did MD?

**MDShunk** · 03-16-2007, 09:40 PM

Quote:

Originally Posted by Joe Momma

Or is that the calc you already did MD?

Yep...that's the calc I did.

I normally like to use #3 for 100 amp also, but it's often non-stock, so I end up getting #2. It seems like I have a hard time getting #3 and #5 for some reason. #2's better anyhow if you have any old 60 degree terminations involved.

Joe Momma · 03-16-2007, 09:42 PM

#5?
Not sure I've ever handled #5

**MDShunk** · 03-16-2007, 09:44 PM

Quote:

Originally Posted by Joe Momma

#5?
Not sure I've ever handled #5

It's not on any NEC tables that's I'm aware of, but I've seen it a few times on engineered jobs.

rod213 · 03-16-2007, 09:49 PM

Quote:

Originally Posted by Joe Momma

After reading the first couple sentences of the original post;
We usually #3 for 100 amp, so perhaps your #2's will meet the derates?
Or is that the calc you already did MD?

Well what MD did was for all three circuits in one pipe.
What I ended up doing is running two of the machines in the big pipe and a seperate conduit for the third.

So 6 conductors, derated to 80%, and then by .91 (100 degrees F) gives you 94.64 A.

**MDShunk** · 03-16-2007, 09:52 PM

Rod... have you compared the cost of the restocking fee for the #2 and getting larger conductors -versus- the labor and material to construct one or two more conduit runs and pull the #2 you already have? I havn't either, but I'm just thinking out loud here.

**MDShunk** · 03-16-2007, 09:55 PM

Since the other long-time employee admitted that he doesn't derate for multiple conductors, I have a feeling somebody at the office might make you pull all that #2 in that one conduit anyhow. Seems there might be a long standing tradition of breaking the rules.

rod213 · 03-16-2007, 09:55 PM

Let me ask this, how would the 80% of the continuous load play into that 94.64 A? Is that only if the machine would run at full load for more than three hours? That means that it would be good for what? 75A? (75.712A).

Joe Momma · 03-16-2007, 10:01 PM

While looking at the calcs I imagine you could put your conductors in the 95F column giving them a 99.84A current rating?

For future refference, never count on more than 6 current carrying conductors to go in one conduit.
That could save you a lot of hassle.

rod213 · 03-16-2007, 10:05 PM

Quote:

Originally Posted by MDShunk

Since the other long-time employee admitted that he doesn't derate for multiple conductors, I have a feeling somebody at the office might make you pull all that #2 in that one conduit anyhow. Seems there might be a long standing tradition of breaking the rules.

Well, he's not been a long time employee of ours. It's my uncles business, and besides Unc. I'm the longest employee. He's only been with us a few months. Worry not, I'll make mention of it.

See, bossman/unc came in and looked everything over and told me what to pull in and what conduits to put where. The more I look this particular run over the more I'm thinking I don't quite agree here. See the heat is probably going to be more intense than the 100 degrees. In total this facility are installing 3 carbonizers, 2 temperers and 1 parts wash. The latter three are the circuits I've been talking about.

So, now I'm worried about the temperature factors in the ceilings.