I want to run wire inside the 1" pvc conduit that is suppling power to my workshop to control exterior flood lights on the workshop from inside my house. There are four #6 THHN wires in the 1" conduit now for the shop power. Can I run individual 12 or 14 ga THHN wires in the conduit along with the #6 wires or is that to much for the size conduit. Oh, it is a 60 ft. run.

 

> There are four #6 THHN wires in the 1" conduit now ...
> Can I run individual 12 or 14 ga THHN wires in the conduit along with the #6 wires?

You might be able to get some #14 in there.
What type of conduit? Schedule 40 PVC?
Would you be willing to pull out the #6 ECG and replace it with #8?

 

I don't have the table2 in front of me but I think you will find that you can have to 9 conductors before you need to derate from #14=15 amp and #12=20 amps. You don't start at 15 and 20 for this derating.

 

First we need to know the type of conductors that we are dealing with ie... THHN, TW ect..

Then you need to know the terminal temprature ratings (60, 75, or 90 degrees C)

Then you need to take into account any ambient tempratures in excess of 30 degrees C

Then we use 310.15(B)(2)(a) for the number of current carring conductors.

We need to know alot more information to properly do a conductor derating.

Chris

 

> we need to know the type of conductors that we are dealing with ie... THHN, TW, etc.

OP stated THHN.

> Then you need to know the terminal temperature ratings (60, 75, or 90 degrees C).

Why? It's in conduit. There are no terminations. Presumably he needs three wires per three-way switch.


> Then you need to take into account any ambient tempratures in excess of 30 degrees C.

Apparently it is about 60' underground building to building.
Ground temperature above 30°C is rare. But okay, let's ask this.

 

> we need to know the type of conductors that we are dealing with ie... THHN, TW, etc.

Conductors are THHN.

> Then you need to know the terminal temperature ratings (60, 75, or 90 degrees C).

Are you talking about temp ratings written on the insulation or the conduit or what ( I'm not sure what terminal temp ratings are)?


> Then you need to take into account any ambient tempratures in excess of 30 degrees C.

I live in Virginia, ground temperature I don't think it would ever get that warm 24" deep. It would get that warm or warmer in the work shop in the summer when it is closed up

 

> It would get that warm or warmer in the workshop in the summer when it is closed up.

As for the lights, do they draw more than 5A (600W) total per circuit? If not, don't even think about worrying about it.


As for the #6, do you work in the shop using them heavily with the temperature above 86°F? If so, you won't be able to use them for the rated ampacity they have at cooler temperatures.
However, this has nothing to do with your conduit or the new conductors.

 

I only have a 20A circuit for my outlets and a 15A circuit for my interior lights which consist of 2 100W incandesent lights and 2- 4 ft double tube 35W fluoresent shop lights and then a 15 amp circuit left for my outside lights. By no means will I use it heavily. I have a small compressor and a small table saw that I use occasionally, those are the highest amperage drawing items that I use. I already have a 50 amp feed running out to the subpanel in the workshop via the four #6 THHN wires. I was just wondering if it was going to be ok to run the 12 ga or 14 ga wire back to the house in the same conduit as the #6 wires for my 3 way switch.

 

You can't run two or three separate circuits to a detached garage. You must run a sub panel for multiple cirucits. You can run a switch leg back to the house.

You can run two circuits if you run them as a multiwire ciruit with a shared neutral.

 

I missed the THHN in the original post sorry.

I was talking in general for derating of conductors when I stated what I did.

110.14(C)(1) requires that we take into account the terminals that we are using to land the wires ie... the screws on the breaker, receptacle ect...

So if we use THHN conductors and the terminals are rated at 60 degrees C then we cann't exceed the amperage allowed on the 60 degree column of 310.16.

Chris

 

He already did this.

 

Correct. But there are no terminations in the conduit.
So for the purpose of conduit derating, these do not come into play.


> So if we use THHN conductors and the terminals are rated at 60 degrees C
> then we can't exceed the amperage allowed on the 60 degree column of 310.16.

That's always true regardless of whether there is any conduit around. It doesn't prohibit the wires from being hotter elsewhere.

 

Three #14 wires take 18.774mm².
Four #6, 130.84mm².
Total 150mm².

Required conduit: 374mm² minimum.
1" PVC: 535mm² actual.

So you have room for quite a few more #12 or #14 wires.

What type of conduit do you have?

 

In a new installation there is almost nothing rated at 60*c anymore.
About the only thing we use the 60*c column for these days is NM cable.

 

Thanks guys, you have been a lot of help!

 

I agree, what I was posting was a gereral rule.

I agree, but conduit derating is part of the entire process of determining the allowable ampacity of a conductor.

Chris

 

> conduit derating is part of the entire process of determining the allowable ampacity of a conductor.

Sure, two separate considerations, one for the conduit, one for the terminations, use the lower result.