Running NMD-90 wires through a conduit

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  #1  
Old 11-12-13, 02:15 PM
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Running NMD-90 wires through a conduit

Hi folks!
I live in Montreal, QC, Canada. Until last Friday I didn't know much about derating and ampacity. I've been researching quite a bit (seems information on the American standards is a little bit easier to get but I hope this info applies here) and I think I have a good idea of how to do this but I would like someone to check my figures.

A couple of years ago while doing renovations on the ground floor of my bugalow (basement not finished), I ran a 2.5" metal conduit from the basement to the attic (the wall is sound insulated with Roxul Sound&Safe and I've been told PVC conduit would be a no-no in insulation. Is that correct?). I did this in case I needed to fish some wires up for possible additional lighting in the future. I didn't check code requirements at the time and now, the future is here and I have wires to run. The conduit has one 90� bend at the base, goes up about 10 feet inside a wall, comes out in the attic, has another 90� bend and extends another 6 feet on top of the ceiling joists. The conduit is not connected to anything (open both ends -- I'll put bushings to protect the cables).

My need now is for a 5000W garage heater. I got 10-3 NMD-90 cable (cheaper than the 10-2 and not returnable) for the heater and was planning to also run a few 14-2 NMD-90 cables but the calculations tell me I can't do that [based on CEC 310.15 (B)(16) formerly 310-16 (Allowable Ampacities of Insulated Conductors)]. I'm assuming that NMd-90 14-gauge has an ampacity of 20, 12-gauge ampacity of 25, and 10-gauge ampacity of 35. Is this true (couldn't confirm that Canadian info)? Check http://www.barr-thorp.com/wp-content...NEC-Tables.pdf

From looking at the NEC (I'm assuming Canadian derating rules are similar???), I can only have 3 conductors in the conduit without having to derate (I don't want to use Multi-Wire Branch Circuits). Although I'm not going to use the 3rd wire of my 10-3 cable, I should calculate 3 conductors in case someone in the future sees it as an opportunity to connect something else (you never know what someone might install in a garage). Since I already have the 10-3 cable (not returnable), I would only be able to conduit that 1 cable to feed my 5000W garage heater (5000W x 125% = 6250W / 240V = 26A) with a 30A breaker (what a waste of 2.5" conduit). Am I understanding this correctly? I might as well fish my 3-10 cable through the wall (any problems with going through Roxul Sound&Safe sound proofing wool?) and use the conduit for lighting circuits. I'm calculating that (without the 10-3 and looking at EC Table 310.15 (B)(3)(a) - Adjustment Factors ) I can run up to 3x 14-2 cables (6 conductors): ampacity 20 x 80% = 16... still good with 15A breakers. Or up to 3x 12-2 + 1x 12-3 cables (9 conductors): ampacity 25 x 70% = 17.5... still good with 15A breakers. If I run my 12-gauge wire through the conduit to a light fixture, can I then use 14-gauge wire to the next light fixture? Does using electrician's putty at the upper end of my conduit (to stop condensation from dripping down) have any effect on my calculations?

Any help or comments would be greatly appreciated. Thanks
 
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  #2  
Old 11-12-13, 03:50 PM
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Welcome to the forums!

Your calculations may be correct, but I'm only familiar with the NEC requirements. There are members who are familiar with the CEC who will likely be along later.

Here are a few questions and comments I can offer:
I don't want to use Multi-Wire Branch Circuits
Why not? They can be handier than beer in a can when faced with a re-rating challenge because the neutral doesn't count as a current-carrying conductor.

Check the fine print on the trigger for de-rating. It might not apply to a sleeve.

I might as well fish my 3-10 cable through the wall (any problems with going through Roxul Sound&Safe sound proofing wool?)
None that I'm aware of.

I've been told PVC conduit would be a no-no in insulation. Is that correct?).
Um, if it is it's something I've never heard of. But you've got the metal now, so go ahead with that.

Does using electrician's putty at the upper end of my conduit (to stop condensation from dripping down) have any effect on my calculations?
It shouldn't. Here, we're required to seal both ends of a conduit sleeve that bridges floors with a fire-resistant compound - usually fire-rated caulk - from the day it's installed. It gets pulled out to run the wires, then done again.
 
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Old 11-12-13, 04:03 PM
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Welcome to the forums!

I can only refer to the US NEC, but from what I understand, the US and Canada electrical codes are close to the same, (Some of ours changed in 2011 to match up with Canada's) but you should double check to make sure.

I've been told PVC conduit would be a no-no in insulation. Is that correct?
I have never heard that and it is not an issue in the US. However, since you ran steel you have a new issue. If you are running the raceway for electrical cables the steel conduit is required to be bonded with a ground wire on both ends of the conduit. You can do this with pipe clamps like you would use to bond your water service to ground. The ground wire should be based the size of the largest conductor in the conduit, in your case #10.

For the derateing, you only count the current carrying conductors. So the unused wire in the cable does not count, but you might want to include it anyway as you never know what the future holds. I assume that NMD is the same as NM-B which has THHN wires in it. For derateing you start at the 90 degree column which is 40 amps for #10, 30 amps for #12 and 25 amps for #14. So, based on your plan of one 10/3 and two 14/2's you would have a total of 7 current carrying conductors. Table 310.15(B)(3)(a) (2011) shows you need to derate 70%. Duct seal has no effect on any of this.

70% of 40 amps (#10) is 28 amps
70% of 25 amps (#14) is 17.5 amps

There is no 28 amp breaker so you can go up to the next size which is 30 amps. Lastly NM-B (And likely NMD) cable is limited to the 60 degree column which limits #10 to 30 amps and #14 to 15 amps.

Long story long, your fine with those three cables. If can even add one more two wire cable for a total of 9 current carrying conductors as the derateing is still at 70%. If you go over that, you will need to derate to 50%.

Your other calculations are also not quite correct. Your heater is 5000 watts. Divide that by 240 volts and you get 20.8 amps. You may not load a circuit more than 80% continuous load (30 amps x 80% = 24 amps) Yes, you can still use a 30 amp breaker

You could fish through the Roxul with no issues to the cable, but it might be kind of tough.

If it was me, I would pull in a larger cable, like 6/3 on a 60 amps breaker, and install a sub panel in the garage. Then feed the heater off that with the 10/3. You would then have extra capacity for other things in the garage.
 
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Old 11-12-13, 04:23 PM
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Tolyn, NMD90 = RW90 jacket.
 
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Old 11-12-13, 05:03 PM
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Old 11-12-13, 05:47 PM
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So NMD90 = NM-b. Go it!
 
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Old 11-12-13, 05:59 PM
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I had to dig out my school texts for this one...

Tolyn pretty well nailed it. But a few things to point out...
CEC has a new rule which states you must use the termination temperature of the breaker to determine ampacity. If you're dealing with a 75 degree term. temp., then you're looking at the 75 degree table in the codebook.
CEC flat out states that 14 AWG is only good up to 15A, 12 AWG up to 20A, and 10 AWG up to 30A. Regardless of what it says in Table 2 (which should really be changed).
Your 70% derating applies to between 7 and 24 conductors in a pipe. This is only current carrying conductors, bond wires do not count.
You are only allowed 40% fill in your application, which leaves you to run 36 14AWG wires (18 cables)
You can use 12 AWG for your lights but it is overkill. Residential lighting circuits cannot be on a breaker higher than 15A. 14 AWG will suit you and yes, if in the ceiling you can jump from light to light.

If possible, I would mount boxes on either end of your pipe, run loomex to the box, and run single wires between boxes.

And now my brains hurt.

So long story short, you can run your 10AWG (30A max) for your heater and a few 14AWG (15A max) for lighting circuits (or power if you wish) in your 2 1/2'' pipe.
 
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Old 11-12-13, 06:22 PM
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Ok, a few things might be different then the US book but here is how it is done here:

If you're dealing with a 75 degree term. temp., then you're looking at the 75 degree table in the codebook.
The termination temp is after you derate. You derate according to the wire rating first (90 degree), then after all that, you would take the termination temp rating BUT, NM cable is limited to 60 degree column, so that is where you would end up.

Your 70% derating applies to between 7 and 24 conductors in a pipe.
According to our table 310.15(B)(3)(a)
4-6 = 80%
7-9 = 70%
10-20 = 50%
21-30 = 45%
31-30 = 40%
41 and over = 35%

Residential lighting circuits cannot be on a breaker higher than 15A.
Is this in the Canada code? It is not in the US NEC.
 
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Old 11-12-13, 06:35 PM
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Tolyn,

I'm not 100% positive when I say this but I think there is mention we only need to derate once. And I think you basically take all your derating options and use the one that brings you to the lowest amperage.

Our derating is:
1-3 = 100%
4-6 = 80%
7-24 = 70%
25-42 = 60%
43+ = 50%

And yes the resi lighting rule is CEC. 20A max is allowed in commercial buildings (mostly to accommodate many fluorescent fixtures in offices running at 347V. I worked at one of the Targets being built here and the entire main floor ran off of only 16 circuits).

This is a good thread, its bringing up info I haven't used in months lol.
 
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Old 11-12-13, 06:46 PM
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Ok, so it is becoming more apparent that there are more differences between US and Canada's electrical code than I first suspected. Horse, please make sure to check your info. I would suggest to follow Mr. Awesome's info more then my own as he is more versed in Canada's electrical code.

And yes the resi lighting rule is CEC. 20A max is allowed in commercial buildings (mostly to accommodate many fluorescent fixtures in offices running at 347V. I worked at one of the Targets being built here and the entire main floor ran off of only 16 circuits).
We mostly use 277 volts here for office lighting, and yup, that is why I love wiring with high voltage!

This is a good thread, its bringing up info I haven't used in months lol.
That is why I like being here, keeps me working with the code book.
 
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Old 11-12-13, 07:00 PM
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Tolyn your understanding of the formula is far better than mine.
The only thing to watch for is our book has a table which states 14 - 10 AWG can be used for x amount of amperage, but in fine print underneath states each is only good for 15, 20, and 30A.

And to make things a little less confusing horse, generally derating is moreso for doing runs of many wires (ex: 100 circuits in an office building). Running a small handful of loomex up some well thought out pipe to an attic likely doesn't require a ton of math put into it. Essentially if you don't have enclosures on either end of the pipe, it is almost like running it through a wall full of insulation.
 
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Old 11-12-13, 08:17 PM
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The only thing to watch for is our book has a table which states 14 - 10 AWG can be used for x amount of amperage, but in fine print underneath states each is only good for 15, 20, and 30A.
The NEC ampacity table has the same footnote. And yes, it gets overlooked a lot.

Essentially if you don't have enclosures on either end of the pipe, it is almost like running it through a wall full of insulation.
The NEC actually has restrictions now on how many cables can be bundled when running through insulation. Added fairly recently, I think.
 
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Old 11-13-13, 11:09 PM
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Thanks a lot guys. It was interesting and educational to see the debate.

Thank you very much Tolyn. When reading your input, it took me a while to understand the way to do the calculations. I started to write a whole new paragraph using my initial (wrong) way but I kept going back to your text and, after a while, I think it sank in.

If it was me, I would pull in a larger cable, like 6/3 on a 60 amps breaker, and install a sub panel in the garage.
I should have done my homework before buying the wire and the breaker.

If you are running the raceway for electrical cables the steel conduit is required to be bonded with a ground wire on both ends of the conduit.
Tolyn, since I'm using cable w/ ground, why would I need to ground the conduit if I'm just using it as a way to fish wires through? Do you know the NEC specification for that so I can try and find the CEC equivalent?

70% of 40 amps (#10) is 28 amps
70% of 25 amps (#14) is 17.5 amps
There is no 28 amp breaker so you can go up to the next size which is 30 amps.
Tolyn, are you saying that even though the calculation for the wire amparity does not reach 30A, I am allowed to use the nearest higher breaker, the 30A breaker (according to NEC of course - CSA hides the Canadian code so it's harder to find the information)? If the 30A breaker disconnects, wouldnt that represent the wiring being overloaded?

On several blogs, I have seen many electricians use the value of 9 conductors before derating (could that be from an older NEC version?), but that doesn't jive with the tables from Tolyn and Mr.Awesome which seem to say that derating starts with 4 conductors in both countries:

CEC
1-3 = 100%
4-6 = 80%
7-24 = 70%
25-42 = 60%
43+ = 50%
NEC
4-6 = 80%
7-9 = 70%
10-20 = 50%
21-30 = 45%
31-40 = 40%
41 and over = 35%
Boy!... the American code is way more restrictive. It must be because the weather up here keeps our wires cooler

By the way, NMD90 wire is rated to 90C.

Nashkat1 asked me why I didn't want to use Multi-wire Branch Circuits. NEC 210.8(A)1-8 says that GFCI protection "shall be provided for all 125-volt, 15 and 20 amp receptacle outlets installed outdoors, in boathouses, garages," etc... I read somewhere else that GFCI can be difficult in a Multi-wire set-up. I will probably install some outlets from the 14-2s so I thought I should avoid that whole aspect. I have no idea what the CEC rules are on this.

Some info I saw in an electricians' chat room... On the chat rooms I visited, a lot of electricians (Mr.Awesome too are complaining about the issue of 14-gauge wire limited to 15A (although the NEC 310.15(B)(16) gives the 90-rated copper wire an amparity of 25), 12-gauge limited to 20A (although 90 copper wire has amparity of 30), and 10-gauge wire limited to 30A (although 90-rated copper wire has amparity of 40) is covered by NEC 240.4(D). An explanation I read for this is that these wire sizes are the most likely to be tampered with (perhaps by non-professionals like me and this gives a greater safety factor (see Electrical Knowledge Repository - nec code 240.4(d) ).
 
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Old 11-13-13, 11:48 PM
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I read somewhere else that GFCI can be difficult in a Multi-wire set-up.
And I read somewhere that the CEC requires multiwire circuits for the kitchen countertop receptacles. Amazing what you can find on the internet.

One trick to providing GFCI protection on a MWBC is to install the protection after the neutral is split to run with each hot. Another is to use a 2-pole GFCI breaker. Neither is particularly difficult.
 
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Old 11-14-13, 08:12 AM
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The gfi protection can be supplied at the point of use. The MWBC would just supply the attic panel.

@ Awesome, the NEC requires us to use the major diameter of a cable when figuring conduit fill, not the conductor count. The conductor count would be used for the creating calculation.
 
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Old 11-14-13, 03:24 PM
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Tolyn, are you saying that even though the calculation for the wire amparity does not reach 30A,
Yes, 240.4(B) allows this 800 amps or less.

Tolyn, since I'm using cable w/ ground, why would I need to ground the conduit if I'm just using it as a way to fish wires through?
Because it is steel. The current traveling through the wires in the conduit will induce voltage onto the steel pipe and cause it to heat up. Basically anything that is part of the electrical "system" that is conductive is required to be bonded to ground.
250.6
250.80
250.86 (The exception in this section may be an "out" but I can't see the conduit so I don't know if 3 and 4 are true or not.)

On several blogs, I have seen many electricians use the value of 9 conductors before derating
That is because, as I showed in my calculations, until you get over 9 wires in a pipe, derating really has no effect if you are using 90 degree wire.
 
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Old 11-14-13, 03:57 PM
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Horse,
-CEC mentions GFCI protection to receptacles installed near a water source, outdoors, in areas prone to heavy moisture, or to replace oldschool two prong receptacles where a ground doesn't exist within the wiring. Doesn't specify anything about GFCI in the garage. In your garage you do require a dedicated plug for your block heater for each car space, and bare minimum one plug for a door opener (within 1m), or a general use which you can run the lights and door opener off of.
I personally would recommend (assuming attached garage) seperating lights onto their own circuit, seperating door opener, at least 2 receptacles per wall (you can add 12 per circuit so go nuts), a receptacle facing outside if easy enough to access (must be GFCI and weatherproof box/cover) and a higher receptacle or two center of the wall adjacent to the door in case you have/plan to have a workbench.
-You need a bond connected to your EMT because it is conductive. If a wire pinches or gets cut and touches the EMT, it is not connected to ground sitting in your wall space. Your bond needs to be connected to ground somehow from the EMT too. Is there a copper water pipe nearby? CEC states you can bond to a gasline too but I would be leary about that. I have to look into this "PVC is a no-no" as I think whoever told you this is on crack.
- No idea about the size and ampacity rule. I would guess they are the most common wire used, pretty well in every building, so they reduce the allowed ampacity as an extra safety precaution.

Nash,
Kitchens require either 20A plugs or 15A split. GFCI if within 1.5M of the sink. No more than two on a circuit.

pcboss,
We have to do that as well for different sizes in a pipe but we have a cheat table if all the conductors are the same size. You just run your finger down from your pipe size to your wire size and poof! tells you how many you can fit.
 
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