SDMB Electricians, I Need Your Help

Factual Questions
1

If you are not an electrician the following thread will be mind-numbingly technical. But it should have a factual answer.

I have reviewed the National Electrical Code (NEC) as well as looked on-line for my answer. I have scads of tables and PDF’s that provide information, but have not enlightened me. So I think I’ve made an appropriate effort before turning to the All Knowing Source of Human Knowledge.

For this SDMB thread, THHN awg #12 solid wire will be used throughout. The information I have says that a half-inch EMT conduit can contain up to 9 #12 THHN conductors. The various NEC tables qualify this somewhat by stipulating that if 4 to 6 conductors are present the amperage must be derated by 20%.

I get this. Makes perfect sense as the goal is to make sure the wires don’t overheat from lack of surrounding air, acting as a coolant.

Here is where my question comes in. From what I have been able to discern, there is no compensation allowed for large conduit. Suppose I run 4 (#12) conductors in a 1/2 inch EMT conduit. I have to derate them by 20%. Okay. But if I have the same 4 conductors in a 1 inch conduit, do I still derate them by 20%? Do you see the issue here? There is a LOT more room in a 1 inch conduit than in a 1/2 inch conduit. Does the NEC really mandate the same derating for 4 conductors, even though there is 4 times the area they are contained in?

I’m baffled. I’m also irritated because I couldn’t figure this out by doing a diligent review of the NEC or the internet.

Thanks for any assistance.

2

Yes you can run more conductors in a larger conduit…

BUT! Don’t ask here, rather ask your local electrical inspector before doing any work. These are the folks who are going to pass/not pass your work, so it is their opinion which matters.

Also many local areas have their own modifications to the NEC. THAT is what matters, not what is in the NEC!

Anyway many electrical inspectors have local “question asking” hours. You can go and ask questions during these times. Take pictures of where you will be doing work. Might want to go over everything you are going to do.

3

I would ask myself if the financial saving on the cable is worth the effort. As you have seen, there are a lot of variations apart from the size of the conduit, like sharp bends and enclosed spaces. I would go for the heavier cable regardless.

4

In the UK we use the term ‘cable factors’ for this calculation.

What it means is that you can occupy a specific percentage of space with insulated conductors, or cross section, this also applies to trunking too.

You generally have two ways of doing this - you can use the charts and tables from authorised manufacturers specifications and perhaps those from the relevant safety body (in the UK this is the IEEE) or you can do the cable calcs by inputting the numbers into the cable calc formula.

The length of the conduit also has an input to the calculation, the number of bends also has an input. There are also additional considerations for the ambient operating temperature - but you would not be surprised about that, I guess, and very long conduit lengths.

In practice it more convenient to go with the charts, you’ll find that you can only get a certain number of cable through a conduit anyway, and even if you are potentially allowed more - well you find it very time consuming and difficult to haul more cores though anyway, plus you increase the risk of damage which means that when you test all the cables, you have to strip the whole lot back and start over with fewer cables.

The other reason is that you’d want to leave plenty of space is for future alterations, its generally good practice to leave enough room for reasonable additions - I have noted that some contractors that work right on maximum profit and minimum prices jobs leave no room for future proofing - result is that any additions require another much more expensive installation job with further runs being installed - the cheapest contractor is not always the best.

It may also be necessary to consider the impedance of the conduit if it is standard galv or stainless since this can affect the earth loop impedance - in the UK we operate with a wired earth system -PME - so we have to work out the potential earth currents and select protection that will achieve an allowable disconnection time.

If you do want to go through the cable factor calculations, you really have to be operating in difficult or unusual conditions, such as a high ambient, or extremely long runs or some other factor that will require a significant de rating of conduit cable factor, and in those conditions you might be better off choosing another method of installation anyway, such as multicore steel wire armoured or high temperature resistant cable - so for my money stick with the charts.

If you have an exam to pass then its different, you’ll be expected to make the cable factor calculations, but once you have found the answer - go home after the exam and look it up on the charts, you’ll find an afternoons calculating can be replaced by 20 minutes of chart look-up

5

I think you’re taking two concepts and combining them. Derating ampacity due to temperature (what you’re actual question is about) and conduit fill (what I think you’re confusing it with).

They cross paths, but not for the reasons you want them to.

I’ve been reading into the NEC quite a bit lately specifically for conduit fill stuff and it’s just easier to look at the online calculators. But as for your question, all it comes down to, specifically regarding derating ampacity due to heat buildup is that that has nothing to do with conduit size. If you have to derate in a 1/2 inch EMT, you’ll have to derate in a 3" EMT.

At least that’s my understanding. From what I gather, the idea is that if the wires are going to build up heat in the 1/2" pipe, they’re going to build up heat in a 3" pipe as well (in some cases) since air is a very good insulator.

Also, I know you said you looked all over the internet, so I’m sure you hit this spot, but Mike Holt is a really good forum. Don’t get me wrong, 10 posters will give you 10 answers, but they cover a lot of topics and I rarely see them being asses to new people like I’ve seen on other boards that cover professional topics (IOW, homeowner type stuff does get covered quite a bit).
ecmweb usually has good articles and southwire is oddly helpful from time to time as well.

6

More cables = more current = more heat. Hence the derating. Conduit fill is restricted for similar reasons. It’s not an either/or rule. You need to follow both. In my opinion, it’s better to make multiple conduit runs rather than derate your cable ampacities by sticking them all in one conduit.

7

Cable factors are one way to handle current carrying capacity, although the carrier fill is about percentage of carrier fill, its not just the physical aspect of how much you can stuff into a carrier and draw through.

If you are considering circuit design, and current carrying capacity then the length of cable run and method of installation also has to be considered, if a demand of 20 amps is required, you have to work backwards from there to determine what size cable will handle it, and that’s where you start to add in the other factors such as cable bunching, length, prospective short circuit current and disconnection times.

So really you have to look at everything that can have a significant effect, and also work outs your costs - does the customer want absolute cheap rock bottom prices or do they want some level of futureproofing

8

I’m not an electrician, but from the thermodynamics of the situation, the area of the conduit wouldn’t be my first guess as to the relevant scaling variable. Actually, it wouldn’t be in my top two, which would be:

(1) No scaling. More than four cables means they’re piled up on one another and one or more will be largely surrounded by others.

(2) Scaling as conduit radius. Heat flow out of the conduit will go as the circumference (or radius) of the pipe.

With no calculations or data to back up my speculation, I’d speculate that (1) is the driving factor for this rule.

9

Yes it does. 1/2 or 6" conduit, it doesn’t matter. For that raceway type derating is required for that number of conductors.

There’s just not a reason to make an exception for that particular senario, not that I can think of anyways.

10

Thanks to one and all.

JoeyP, I did indeed find the Mike Holt site and waded through it for a spell. I agree that some of the other electrical forum sites were forbidding with their disclaimer that they perferred “professional” questions. But at least they state it.

DrCube, “…multiple conduit runs…” Yes, agree with this with the consideration that those conduits might be the next size up so it’s easier to pull wire and leave room for future expansion. But extra conduits also means a signficant step up in time to install which was the initial reason for my investigation.

K2500, “Yes it does. 1/2 or 6” conduit, it doesn’t matter. For that raceway type derating is required for that number of conductors." This is pretty definitive, and basically what someone needed to tell me whether I like it or not.

Thanks again.

– ASG

11

Just like to say that I’d rather slit my wrists than try to pull nine conductors through a piece of 1/2" with any bends in it whatsoever. :smiley:

12

You may be interested to know that since you’re using THHN(90deg conductor) that you can derate from the 90deg column for conduit fill adjustments. That means for a #12 you can legally put 9 CCC in that conduit, derate the 30%, and still be rated at 20A. Which is what the small conductor rule limits you to for a #12 anyways.

As chefguy noted, you probably don’t want to do that in a 1/2".

13

The worst pull I ever had to make was 23 #12 conductors (don’t remember the conduit size, but it was close to max). Straight, short pulls of about 20 feet or so, but what a miserable SOB that was. It was for power to the piers in a small boat marina. The only relief was that three wires terminated in each box, so the pulls got marginally easier as it went along. All because some cheap fucker didn’t want to pop for a second or third conduit run.

14

The only difference between a cheap customer and a greedy contractor is which side of the dollar they’re standing on.

That said, the best way to make your guys hate you is to max out on conduit fill. Especially with solid wire. What a pain in the ass.