# Conduit fill and derating wire.



## brian john (Mar 11, 2007)

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.


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## MDShunk (Jan 7, 2007)

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.


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## Joe Momma (Jan 23, 2007)

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.


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## rod213 (Mar 16, 2007)

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.


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## MDShunk (Jan 7, 2007)

rod213 said:


> 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.


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## MDShunk (Jan 7, 2007)

Looks like you're running another pipe, huh?


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## Joe Momma (Jan 23, 2007)

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


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## rod213 (Mar 16, 2007)

MDShunk said:


> 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......


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## MDShunk (Jan 7, 2007)

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.


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## Joe Momma (Jan 23, 2007)

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?


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## MDShunk (Jan 7, 2007)

Joe Momma said:


> 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.


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## Joe Momma (Jan 23, 2007)

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


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## MDShunk (Jan 7, 2007)

Joe Momma said:


> #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.


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## rod213 (Mar 16, 2007)

Joe Momma said:


> 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.


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## MDShunk (Jan 7, 2007)

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.


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## MDShunk (Jan 7, 2007)

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.


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## rod213 (Mar 16, 2007)

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).


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## Joe Momma (Jan 23, 2007)

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.


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## rod213 (Mar 16, 2007)

MDShunk said:


> 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.


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## MDShunk (Jan 7, 2007)

rod213 said:


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


Right. The plant engineer or head of maintenance may already know about how hot it gets up in the bar joists in the summer. It's an odd sensation, as you may well know, when you're going up in a man lift in the summer time. You can feel the dramatic temperature change as you go up. If you've been working up there for a while, when you come down, it starts to feel really cool. Kinda neat, but not so good for wire in the summer. I just saw an IR picture the other day that showed an RMC on a black rubber rooftop with a measured temperature of about 200 degrees F. I'm not sure what the ambient of the black roof was. Maybe 150?


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## Joe Momma (Jan 23, 2007)

So how do you think 'unc' is gonna take the news of tripling the size of his wire?


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## MDShunk (Jan 7, 2007)

Joe Momma said:


> So how do you think 'unc' is gonna take the news of tripling the size of his wire?


This puzzle about high ambient temperatures reminds me of an old job. It was some lighting in a section of a building that was pretty much designed as some sort of oven. It was to get up to about 300 in that oven, as I remember. We used some sort of nickel plated copper wire with some type of white braided jacket. Don't remember what the wire designation was. Porcelain wire nuts, too. They're a real treat to use. They don't draw up very well.


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## Joe Momma (Jan 23, 2007)

That white braided jacket was fiberglass insulation. It fits the wire kinda loosely right?

We used it once about 8 years ago while rewiring a commercial oven, along with the porcelain nuts.

Very spendy stuff!


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## brian john (Mar 11, 2007)

Joe: the rule is 3 or more current carrying conductors in a conduit.

Rod: You only have to derate for one issue not multiple times.


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## Pierre Belarge (Feb 3, 2007)

brian john said:


> Rod: You only have to derate for one issue not multiple times.


 
Brian
What do you mean by this comment?

Marc showed two ampacity adjustments, one for # of conductors and one for temperature correction. 

Are you saying that is incorrect?


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## faber307 (Jan 22, 2007)

Joe Momma said:


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


perfect explanation Joe...............


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## faber307 (Jan 22, 2007)

brian john said:


> Joe: the rule is 3 or more current carrying conductors in a conduit.
> 
> Rod: You only have to derate for one issue not multiple times.


the rule is more than 3


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## brian john (Mar 11, 2007)

Tab Faber said:


> the rule is more than 3


 
Tab Sorry for the misquote it is more than 3.


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## brian john (Mar 11, 2007)

Pierre Belarge said:


> Brian
> What do you mean by this comment?
> quote]
> 
> ...


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## Joe Momma (Jan 23, 2007)

I think the apology is due to me(for what i said, I have no idea)

And I appreciate Tab has got my back:thumbup:

(just kidding about the first sentence by the way)


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## MDShunk (Jan 7, 2007)

brian john said:


> I was taught that you only had to apply one (the larger of the two) adjustment factors when derating conducotr ampacities.


(scratching head)....

It seems you don't have to upsize for voltage drop if your conductors are upsized already for temp or fill. I'm just not seeing where you pick either temp or fill (the worse of the two) for ampacity adjustments when both apply. I'm still reading. I like it when people drop these little nuggets that cause me to question what I thought for years was true. If you happen to have any text that addresses this, Brian, pass it along.


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## brian john (Mar 11, 2007)

Joe:
I think you said or wrote "more than 6" Then I incorrectly wrote "3 or more". When in reality as Tab corrected both of us it is "more than 3". I don't think anyone said anything that required an apology.


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## rod213 (Mar 16, 2007)

MDShunk said:


> (scratching head)....
> 
> It seems you don't have to upsize for voltage drop if your conductors are upsized already for temp or fill. I'm just not seeing where you pick either temp or fill (the worse of the two) for ampacity adjustments when both apply. I'm still reading. I like it when people drop these little nuggets that cause me to question what I thought for years was true. If you happen to have any text that addresses this, Brian, pass it along.


 
I'd like to know more about this too, please.

Thanks for all the help ya'll.


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## rod213 (Mar 16, 2007)

Alright a little update. We're going to just run three seperate conduits, as was mentioned earlier. 1 circuit per. Only thing to worry with now is the ambient temp which shouldn't really be a problem now that they are all seperated.

Let me ask another question, do you always have to figure in your 80% of a circuit when sizing your wires/breakers. Or only when the machine is on for more than three hours straight?

For instance One of these machines has a max draw of 90A according to the blueprints. But it shouldn't be on for more than 3 hours, 1 or 1 1/2 hours at most I think. We're using #2THHN rated at 130A landing on a 100A breaker and the machine itself came from the previous location with 100A fuses in it. So should I need to figure in that 80% or is that only if it's over 3 hours use?

80% of the 130A is 104A then figure in ambient temp

Or

130A to start and figure in ambient temp.

Sorry if that ran a bit long, repetative or scattered. Lol.

And thanks again for the input guys and gals.


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## pfanta (Feb 5, 2008)

*2 boats in a 3/4" pie*



Tab Faber said:


> perfect explanation Joe...............


the easiest way to remember #12's is 
2 boats in a 3/4" pie
--paul


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## pfanta (Feb 5, 2008)

*wiremold pn10*

Anybody working with this material: wiremold pn10

The way I found this thread was looking for derating info on the wiremold pn10 plastic duct. I assume you have to derate it just like any pipe
in Article 300.1 (a) ALL WIRING METHODS: and
ARTICLE 388 covers the Non-metallic surface mount raceways in the 2008 NEC makes no mention of special requirements for derating plastic duct
so I guess it's just 2 boats in a 3/4"pipe...

and I have one other question!!


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## pfanta (Feb 5, 2008)

*last one*

GROUNDING

Years ago we had a residential 400amp service with a 200a main that was on the other side of the house from the meter. The subfeed included a #4 grounding conductor. At the subpanel we pulled a second #4 uffer ground up from the rebar.

The inspector said it was multiple grounding paths and demanded it be cut. No on has ever answered if this was the right call. I bet you guys will have an opinion!

What makes me think of this right now is a similar residential.
100a subpanel has only the rigid pipe as grounding conductor. It is an old pipe. The pool equipment outbuilding is daisy chained out of this subpanel. There is no grounding conductor to the pool house [2x20amp 240v or five wires]. I will pull a #12 thhn wire for the ground and then drive another two rods, six feet apart, at the pool house and bond that to the #12. with #8. Then I plan to bond the pumps together with the #8. Do you say the wire back to the 100a subpanel should also be #8??

I always think the more grounding electrodes, the better. As long as they are all bonded together...

Anyway thanks for you time!

--Paul


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## itsunclebill (Jan 16, 2007)

The table for adjustment factors for fill has a note over the percentage column that says the ampacity should be adjusted for ambient temp before applying the fill deratings. If I'm not reading this right I'd like an explanation or verbage that says you don't apply both.


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## RePhase277 (Feb 5, 2008)

You must derate for both ambient temperature and number of conductor. So, just from conduit fill, your ampacity is at 91 amps. Assuming you have the standard ambient, 30C, you are done. See 210.19(A)(1). The ampacity of the wire must be rated at 125% before the applications of any derating. Ok. Then see 240.4(B). You can go to the next highest size fuse or breaker, which is 100 amps.

InPhase277


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## RePhase277 (Feb 5, 2008)

rod213 said:


> Let me ask another question, do you always have to figure in your 80% of a circuit when sizing your wires/breakers. Or only when the machine is on for more than three hours straight?


NEC 210.20(A) addresses this issue. Yes you must over rate the overcurrent device by 125% for continuous loads. BUT, the exception to that says that IF the assembly is rated for operation at 100% of its rating then you can use a breaker rated at not less than the load to be served.

(I paraphrased there of course)

But, in general, you must rate the conductors for 125% of the load to be served BEFORE applying any adjustment factors. THEN, you must derate for BOTH conduit fill, AND ambient temperature. It doesn't matter which order you do it in, since the commutative rule for multiplication holds true. But if the circuit serves multiple outlets for cord-and-plug connected equipment, you cannot go up to the next highest overcurrent device, as per NEC 240.4(B)(1).

InPhase277


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## bigfactoryrat (Mar 1, 2007)

Rod213, you originally said you had 3 machines that take 100A each. If that is the load of each machine and you run them continuously - then I think #2 wire is undersized for this load.

From NEC 2002:
110.14(C)(1)(a)  Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following: 
(1) Conductors rated 60°C (140°F) 
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used 
(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors 
(4) For motors marked with design letters B, C, D, or E, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity. 

If all your equipment is identified for 75 deg C then you could use the 75 deg column of 310.16 after derating.

(310.16 #2 THHN = 130A) 
130A x 80% x 70% x .91 = 66.25 amps 
Therefore: #2 THHN wire is not large enough to handle a continuous 100A load.

Your conductor has to be able to handle the load.

I think you would have to use a #3/0 conductor.
(310.16 #3/0 THHN = 225A) 
225A x 80% x 70% x .91 = 114 amps

The (6) 3/0’s and ground wire would fit in the 2-1/2 inch EMT.

What do you all think? Did I miss the boat or am I on target with the above calculations?


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