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Old 04-15-2009, 09:03 PM   #16
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GFCI question: can line and load wires be run inside the same conduit?


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Originally Posted by jbfan View Post
Sorta! It should be derated 80% of the max ampacity of the wire, not the circuit.
If the wire is 14 thhn/thwn then the ampacity is 20 amps, so you can still put the wire on a 15 amp breaker.
I believe you are mistaken, as Table B.310.1 states:

*Unless otherwise specifically permitted elsewhere in this Code, the overcurrent protection for these conductor types shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 AWG amperes for 10 AWG aluminum and copper-clad aluminum.

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Old 04-15-2009, 09:07 PM   #17
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GFCI question: can line and load wires be run inside the same conduit?


True, but for derating you use the max ampacity of the wire, not the max breaker size.
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Old 04-15-2009, 09:30 PM   #18
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GFCI question: can line and load wires be run inside the same conduit?


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Originally Posted by jbfan View Post
True, but for derating you use the max ampacity of the wire, not the max breaker size.
That's an interesting point. I'll have to ask around at work tomorrow about that one. I was always under the impression that for derating purposes (conduit fill) the permitted overcurrent ampacity was used to calculate the derating, not the maximum theoretical ampacity of the wire. I don't have my Ugly's with me, but I doubt that the theoretical ampacity of 14 guage wire in FMC is 20 amps, since it's free air ampacity is 18.
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Old 04-15-2009, 09:31 PM   #19
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GFCI question: can line and load wires be run inside the same conduit?


So basically even with the derating based on the ampacity of the wire the 15a 14g & 20a 12g wire

is OK

Even at 18 = 14.4 derated & my understanding you can round up...no?

25 = 20 derated...

Last edited by Scuba_Dave; 04-15-2009 at 09:33 PM.
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Old 04-15-2009, 09:32 PM   #20
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GFCI question: can line and load wires be run inside the same conduit?


The circuit already has "too many" things on it, but that's how it was when I found it. I'm just trying to give it an added margin of safety. Realistically, there's a 32 inch CRT TV, a Playstation 2, a digital cable box, a DVD player, 5 CFL light bulbs, two incandescent table lamps, and a Pentium 4 setup with an inkjet, an external harddrive, 19 inch LCD, cable modem, and a wireless router running on the circuit. There's never been an issue, so I'm leaving well enough alone. If in the future that becomes inadequate, we'll cross that bridge when we arrive at it.

My father and step mother purchased the home in 2005. I'm not sure how the house would have passed the rough-in inspection all those years ago without the grounding conductor if it wasn't up to code when it was built. However, code or not, it's built and has been like this for 48 years.

The reason I'm working on this house is because they want to replace all of the two prong outlets with three prong outlets. At most of the outlets in the house, using an analog meter with the respective circuit off, I get about 1-3 ohms of resistance between the box and the neutral. There's no bonding wire or strip that I can see, but the clamps are all still tight. (There have been a few other issues that I've found and fixed as I go along--basically, anywhere a certain clod of a previous homeowner messed with the system, something was bound to have been done wrong. Anything that appears original is really in great shape.)

It's not using BX armored cable, it's definitely flexible conduit. The outer diameter of the stuff is about 3/4 inches, which lead me to believe that it is 1/2-inch internal diameter flexible conduit.

I already put the four 14 ga. conductors in this morning, based on advice that the layout and colors were acceptable. If I'd need to derate to 13 amps, that's not good. Would replacing the four 14 ga conductors in that particular conduit with 12 ga. solve that issue? Now, the 14 ga. was not too hard to pull, but 12 ga. might be, because it curves over 180 degrees total (maybe 210 degrees, the way the flex lays).

Last edited by thegonagle; 04-15-2009 at 09:43 PM.
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Old 04-15-2009, 09:48 PM   #21
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GFCI question: can line and load wires be run inside the same conduit?


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Originally Posted by Bigplanz View Post
That's an interesting point. I'll have to ask around at work tomorrow about that one. I was always under the impression that for derating purposes (conduit fill) the permitted overcurrent ampacity was used to calculate the derating, not the maximum theoretical ampacity of the wire. I don't have my Ugly's with me, but I doubt that the theoretical ampacity of 14 guage wire in FMC is 20 amps, since it's free air ampacity is 18.
It is actually 25 amps for thhn/thwn.
Table 310.16 05 code.
This is for 3 current carring conductors in a raceway, cable, or earth.
This is based on the 90 degree column.

BTW sorry to hijack the thread.

I corrected my other post to change from 20 to 25 amps.
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Old 04-15-2009, 09:50 PM   #22
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GFCI question: can line and load wires be run inside the same conduit?


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I already put the four 14 ga. conductors in this morning, based on advice that the layout and colors were acceptable. If I'd need to derate to 13 amps, that's not good. Would replacing the four 14 ga conductors in that particular conduit with 12 ga. solve that issue? Now, the 14 ga. was not too hard to pull, but 12 ga. might be, because it curves over 180 degrees total (maybe 210 degrees, the way the flex lays).
You are fine with the 14 wire you ran.
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Old 04-15-2009, 09:52 PM   #23
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GFCI question: can line and load wires be run inside the same conduit?


So, wait, I'm actually OK with the 14 ga. conductors?

P.S. I didn't pull a green wire because it seemed pointless, as there are no other green wires in the original parts of the house.

EDIT: OK then!
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Old 04-15-2009, 10:04 PM   #24
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GFCI question: can line and load wires be run inside the same conduit?


Quote:
Originally Posted by jbfan View Post
It is actually 25 amps for thhn/thwn.
Table 310.16 05 code.
This is for 3 current carring conductors in a raceway, cable, or earth.
This is based on the 90 degree column.

BTW sorry to hijack the thread.

I corrected my other post to change from 20 to 25 amps.
Table B.310.1 Ampacities of Two or Three Insulated Conductors, Rated 0 Through 2000 Volts, Within an Overall Covering (Multiconductor Cable), in Raceway in Free Air Based on Ambient Air Temperature of 30C (86F) lists the maximum ampacity of 14 guage THHN as 21 amps.
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Old 04-15-2009, 10:08 PM   #25
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GFCI question: can line and load wires be run inside the same conduit?


NEC 2008 310.16 lists the ampacity jbfan indicated

I thought he was running single wires & not a multi-conductor cable?

Last edited by Scuba_Dave; 04-15-2009 at 10:10 PM.
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Old 04-15-2009, 10:19 PM   #26
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GFCI question: can line and load wires be run inside the same conduit?


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True, but for derating you use the max ampacity of the wire, not the max breaker size.
This is how it was explained to me in school. You want the over current protection to be the weak link in the circuit, so you want the breaker to trip before the wire gets anywhere near being overloaded/overheating.

So in this case you're derating the wire which could cause the wire size to increase so that the amp rating of the wire remains greater than the load at which the breaker will trip.

Boy, building electrical design seems soo long ago for this structural engineer.
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Old 04-15-2009, 10:22 PM   #27
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GFCI question: can line and load wires be run inside the same conduit?


But in this case the wire size doesn't have to increase
As it still meets the required ampacity for 14g & 15a breaker
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Old 04-15-2009, 10:48 PM   #28
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GFCI question: can line and load wires be run inside the same conduit?


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Originally Posted by Scuba_Dave View Post
So basically even with the derating based on the ampacity of the wire the 15a 14g & 20a 12g wire

is OK

Even at 18 = 14.4 derated & my understanding you can round up...no?

25 = 20 derated...
10, 12, and 14 are already derated once by the code limiting the breaker size as they have. When derating a wire, you use the actual ampacity of the wire as a starting point.

Where did the OP state he had a 15 amp breaker? I didn't see it anywhere.
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Old 04-16-2009, 05:52 AM   #29
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GFCI question: can line and load wires be run inside the same conduit?


Thanks for so many replies, everyone. It's all very helpful.

Although I neglected to mention that this circuit is on a 15 amp breaker, I did specify that it uses 14 ga., which had become intuitively interchangeable in my mind. Sorry for the confusion.

Why didn't I use a GFCI breaker? The panel is stuffed--overstuffed! It's a 16 space Square D QO that already has two double poles and FIVE(!) tandems.

Since I actually intend to add GFCI protection to each receptacle on the original circuits (that don't have a dedicated ground conductor), I'm using GFCI outlets wherever I can (partially for cost), and will only switch to a GFCI breaker if absolutely necessary, due to there only being a few single pole breakers left to possibly change out. (Further complicating matters is that there are at least two common neutrals still in use on four of the breakers, so I can't just go moving breakers around willy-nilly.)

So really, this whole house is a bigger can of worms than I ever mentioned in the original post. (Wait till I tell you about the copper electric stove neutral that landed under the same screw as the aluminum service neutral... Truly horrifying. The dimming and brightening is greatly diminished since I removed the no-longer-used electric stove wiring from the panel.) I'm just trying to make it a little better while I can.
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Old 04-16-2009, 08:58 AM   #30
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GFCI question: can line and load wires be run inside the same conduit?


Beat me to it, JB!

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