Do I need to derate wiring?

[jdsac]

Proposed: Need to run a 1 1/2" pvc conduit up the side of the panel & into the attic thru the eve vent to pull new circuits into the attic.

Total length of the conduit in the area of 6 +-feet. Would be open at the attic end & panel end so no trapped heat.

Need to pull at max 5 # 12/2 uf-b romexes. (uf-b because of conduit outside classified as "wet area") More likely only 3-4.

Figuring the fill, it looks like I could do it - less than 40%. however I think I read that I would have to derate the current capacity if it was over 24" in the conduit. Is this true?

As an alternative, If I used thhn wire from the panel through the same conduit to a junction box in the attic would that also have to be derated?

A little confused here, not trying to "get away" with anything, just want to do it correctly . Can anyone explain this to me or point me to a site that does explain it?

Thank You for your help!

 

[whitedogone]

Technically it's a violation. Check with your ahj. You don't use fill calls with nm in conduit. It's a bundling issue.

 

[jdsac]

If I can't do it with the uf-b, then how CAN I do it?

Still need to come up the side of the panel & through the eve vent to the attic.

Please describe what is correct/ code Thank You

 

[rockwithjason]

yes it has to be derated but you may still be ok if your cable carries a 90deg c rating. you can use this rating for the derating calculation. 90deg c 12ga is good for 30a so you derate from that. as long as you don't drop below 20A at the end of the calculation then you are ok. remember that neutrals count on circuits that have dedicated neutrals.

ex. your 5 12/2 cables is 10 current carrying conductors. 10 current carrying conductors in a single conduit must be drated 50%. 50% of 30a is 15 A. no go unless you change the breakers to 15A

ex2 4 12/2 cables is 8 current carrying conductors. 8 conductors must be derated 70%. 70% of 30A is 21A. ok for 20a breakers.

you do use fill calcs for cable in conduit. if you are below the 40% fill for the pvc as calculated by adding the cross sectional area of all the cables and subtracting from the cross sectional area of the conduit.

if you run into problems, put in another conduit.

 

[pattenp]

Technically it's a violation. Check with your ahj. You don't use fill calls with nm in conduit. It's a bundling issue.

If running multiple NM cables through conduit you do need to figure fill. See NEC chapter 9 Notes to Tables (9).

 

[sberry]

This is a place for 12/3

 

[sberry]

Is it really wet? An open hole from a conduit above isn't wet.

 

[Beemer533]

Just use THWN for the conduit portion into a Junction box.. Just make sure the Junction box is sized correctly. Then NM from there..

Not sure why you would bother with NM inside the conduit?

Is the conduit outside, is that why you need the weather rating?

 

[alfredeneuman]

All outdoor conduit is classified as a wet location

 

[jdsac]

Thank you for your help,could you point me to a chart that shows the derations?
Also, if I were to run #10 wire from the panel to a junction box ( through the conduit as described) how much would that be derated? From the junction box on it would be normal 12/2 nm romex. Is that possible & correct?

Just trying to learn and do it correctly.

 

[Beemer533]

You can't run 10awg from the panel then switch to 12awg... You will need to stick with 12awg for the whole run.

How long is the total run?

 

[teamextreme]

You can't run 10awg from the panel then switch to 12awg... You will need to stick with 12awg for the whole run.

How long is the total run?

There was a debate on this in another thread recently. I don't think it was ever resolved, at least not to my satisfaction. I'm not so sure this is true, but I haven't seen a code reference one way or the other.

 

[alfredeneuman]

You can't run 10awg from the panel then switch to 12awg... You will need to stick with 12awg for the whole run.

How long is the total run?

That's just plain wrong.
It's common to have smaller conductors attached to larger conductors in cases of voltage drop with long runs back to the panel, or when you distribute conductors that required derating because of the number of wires in one conduit to another conduit where the lesser number of wires don't require derating.

 

[sberry]

What do these circuits feed?

 

[rockwithjason]

Google 310.15.b.3.a. the thing about not being able to switch wire sizes is wrong. As long as the breaker is sized to the smallest wire size you are good. Article 240 is clear on that although i dont have the specific reference in front of me

 

[Beemer533]

Well, that certainly surprises me. So if you run 10awg out of the panel and switch to 12awg down the line, what is to stop someone else from switching to a 30a breaker because they see a 10awg wire in the panel?

I can see starting with 12 at the panel and switching to a larger conductor for a long run..

 

[rockwithjason]

Well, that certainly surprises me. So if you run 10awg out of the panel and switch to 12awg down the line, what is to stop someone else from switching to a 30a breaker because they see a 10awg wire in the panel?

I can see starting with 12 at the panel and switching to a larger conductor for a long run..

You cant stop people from doing stupid or illegal things.

 

[jdsac]

yes it has to be derated but you may still be ok if your cable carries a 90deg c rating. you can use this rating for the derating calculation. 90deg c 12ga is good for 30a so you derate from that. as long as you don't drop below 20A at the end of the calculation then you are ok. remember that neutrals count on circuits that have dedicated neutrals.

ex. your 5 12/2 cables is 10 current carrying conductors. 10 current carrying conductors in a single conduit must be drated 50%. 50% of 30a is 15 A. no go unless you change the breakers to 15A

ex2 4 12/2 cables is 8 current carrying conductors. 8 conductors must be derated 70%. 70% of 30A is 21A. ok for 20a breakers.

you do use fill calcs for cable in conduit. if you are below the 40% fill for the pvc as calculated by adding the cross sectional area of all the cables and subtracting from the cross sectional area of the conduit.

if you run into problems, put in another conduit.

Thank You very much- I just want to do it correctly & safely.

So if I'm reading your example #2 in your reply, as long as I use 8 or less conductors then it will be correct and it could be done with either the uf-b or individual wires, using #12 all the way with no size changes. Either would be less than 1/2 of allowable fill, so that's not a problem.

Dumb question- are the grounds counted in this or not?


House has 200 amp panel with 10 open slots.
The reason I need to add circuits is it's an older house (mid 50's) &
the kitchen counter only has one recepticle.
Plus need to run dedicated circuit for over stove microwave
The other one (or possibly 2) circuits are to take loads off of circuits that various handymen have overloaded through the years.

 

[rockwithjason]

Correct. Grounds are not counted because they dont carry current under normal operation and thus dont put off heat.

 

[jdsac]

Thank YOU! I wasn't trying to get away with anything, just needed an education! I want this job to be correct.

In my opinon , your trade is the most important one in the safety of the house.

Thanks again and have a great 2015...

 

[Jolie-Rouge]

Another thing to consider is what is the liklihood that all circuits will not be energized at he same time. If not, or there are circuits with short duty cycles, then the conduit fill issue becomes moot.

 

[alfredeneuman]

Another thing to consider is what is the liklihood that all circuits will not be energized at he same time. If not, or there are circuits with short duty cycles, then the conduit fill issue becomes moot.

While that's true to a certain extent, it has no bearing on how the system needs to be installed to be Code compliant.

Someone could change the loads in the future

 

[rockwithjason]

Another thing to consider is what is the liklihood that all circuits will not be energized at he same time. If not, or there are circuits with short duty cycles, then the conduit fill issue becomes moot.

That has nothing whatever to do with being code compliant

 

[rockwithjason]

Thank YOU! I wasn't trying to get away with anything, just needed an education! I want this job to be correct.

In my opinon , your trade is the most important one in the safety of the house.

Thanks again and have a great 2015...

anytime

 

[600SL]

There was a debate on this in another thread recently. I don't think it was ever resolved, at least not to my satisfaction. I'm not so sure this is true, but I haven't seen a code reference one way or the other.

Look at this video. Particularly between 14 and 17 min. I believe this resolves it.

 

[Charles (in GA)]

From chapter 9 of the NEC

A multiconductor cable or flexible cord of two or more
conductors shall be treated as a single conductor for
calculating percentage conduit fill area. For cables that
have elliptical cross sections, the cross-sectional area
calculation shall be based on using the major diameter
of the ellipse as a circle diameter.

The above statement means that a section of Romex is to be treated as if it is ROUND with a diameter as the widest part of the Romex. This will eat into your fill very quickly.

Charles

 

[teamextreme]

Look at this video. Particularly between 14 and 17 min. I believe this resolves it.

Yup, that resolves it. If Mike says it's ok, then it must be. So, changing wire size for derating purposes is perfectly fine. Carry on.

 

[Beemer533]

Look at this video. Particularly between 14 and 17 min. I believe this resolves it.

Cool, thanks for the video.. [emoji106]

This post edited by the NSA

 

[sberry]

I have a couple pipes that push the limit of fill/and or number of conductors. If I was going to really bake on it I would change it but its so lightly loaded and near impossible for anyone to come along and apply it. All the recepts on a single 20A circuit and the rest is fixed load of about 5A on on wire and 5 on another, we could add a couple in heat and an amp or 2 in fans down separate wires and a 1/2 garage door opener. The heat never runs and I could add 500 watts for a light I use for a couple hrs a year.
The bundle would need to be derated at above 120A or so. A couple wires run intermittent for power tools, mostly a 4 1/2 on 20A circuit. Its short, about 10 ft. That section was wired by a conservative type with pipe and fittings and generous with wire and circuits,, ha, they used to seeking machines with packed pipes. I got a couple mwbc so its probably legal anyway.
So,,, there is good reason for a code and if I had a choice would use a bigger pipe but I wouldn't loose sleep over 3 or 4 pieces of Romex jammed thru a short pipe especially if they wire not loaded.
I make fun of the losing sleep part but its really serious. I use some electric heaters. The load is so much longer than a hair dryer which is off more than on. Most of the routine equipment is shut off if you are sleeping. My neighbor is a master, could have any pipe or cable he wants for free and still has the same 10 cable to a 6-50 for a buzzer with 50A fuses. He uses it a small rod at a time on occasion if even then and when the switch is off the size of the wire is pretty much irrelevant as long as it is good enough for the short circuit protection which is pretty much the same as the ground wires sizing.

 

[sberry]

New better wire ratings have really increased code margins. While the codes are a minimum it doesn't mean its inadequate or would be significantly helped by increasing every wire or pipe a size. Even fully loaded its not an issue and the breaker on a general circuit is to insure the operator cant plug a heater in every outlet available. With an item at a time the load is 80% on a hog intermittent or its on a dedicated or calculated.
Most of these circuits will be 1 at a time loads with an appliance or power strip even with a cord a size smaller than a 12 and in most cases 2 sizes.
The discussion about the code is great but some of how it works is in order. How come a 10 cable is ok on a 50 to feed a welder etc? Basic reason,,,,, its one size bigger than the cord comes on the machine.

 

[sberry]

The basic concern is heating a bundle of wire, there is a limit to it and many different situations and the code has to cover them all. A little bundled cable doesn't bother me especially if 1 of them is led lights and the rest general recepts and the hi load potential for any time would be counter tops and this is protected with 20A

I just did a job where I replaced 1 circuit with 3 and originally it had a fridge on too so its really 4 and now that I add it up 5 since I added a micro. The code has really made an improvement in the counter tops where so many situations are pitiful. Salesman hate to sell them because its time consuming, invisible etc. There are a lot of them still in service with 15 a general that has a toaster and fridge and a micro on one.

 

[sberry]

Went from 1 number 12 to 5 of them. The "code" in this case doesn't increase te wire size but limit the amount applied. No other load means you have to bring on another source which may also be lightly loaded.