21 replies to this topic

[Point Break]

Okay, long story short, I'm not an electrician but I play one on TV. Today I'm pretending to be one in my attic.....but I'm not exactly uninformed on the subject either. So.............I just replaced my thousand year old 80 amp panel.......yep 80........with a 200 amp. I'm pulling some new circuits and am taking advantage of existing conduit. I can't get to the top plate to put in any new conduit where the panel is located because of the roof-line angle.....even if I was willing to open the wall.....which I'm not. I have a mix of 1/2" & 3/4" FMC conduit. Just one of them (a 3/4") has 3 existing THHN #10. I'm pulling THHN #12 for each new circuit. I can find plenty of charts for max conduit fill using all same size conductors, but I can't find any charts for how to calculate max fill for mixed size conductors in the same conduit. I'm not interested in jamming them to the max but I want to make sure I keep within the code requirements and safe operation.

Any resources/help?

[floating dutchman]

It's a heat thing. the reason to not overfill the conduit is to stop the conduit and wires gettin too hot.

"I can find plenty of charts for max conduit fill using all same size conductors"

If you can figgure out how full the conduit will be with this information. ie 2/3. You should be good to go.

[Point Break]

It's a heat thing. the reason to not overfill the conduit is to stop the conduit and wires gettin too hot.

"I can find plenty of charts for max conduit fill using all same size conductors"

If you can figgure out how full the conduit will be with this information. ie 2/3. You should be good to go.

Understood, I get the heat thing (one of the prime reasons I don't want to overfill) but I figure there must be some formula to cover the conversion. I don't want to waste space as I have limited number of conduit, can't get any more in, and want to maximize the circuits I can pull. Where's all these frigging engineers when you need em!!!

[Timo42]

It's a heat thing. the reason to not overfill the conduit is to stop the conduit and wires gettin too hot.

"I can find plenty of charts for max conduit fill using all same size conductors"

If you can figgure out how full the conduit will be with this information. ie 2/3. You should be good to go.

Understood, I get the heat thing (one of the prime reasons I don't want to overfill) but I figure there must be some formula to cover the conversion. I don't want to waste space as I have limited number of conduit, can't get any more in, and want to maximize the circuits I can pull. Where's all these frigging engineers when you need em!!!

France?

I can check with my brother-the-electrician tomorrow, but iirc you can only fill the conduit 40%, look up the crossection of the wires, add them together and subtract from the conduit, bob's your uncle.

[Point Break]

It's a heat thing. the reason to not overfill the conduit is to stop the conduit and wires gettin too hot.

"I can find plenty of charts for max conduit fill using all same size conductors"

If you can figgure out how full the conduit will be with this information. ie 2/3. You should be good to go.

Understood, I get the heat thing (one of the prime reasons I don't want to overfill) but I figure there must be some formula to cover the conversion. I don't want to waste space as I have limited number of conduit, can't get any more in, and want to maximize the circuits I can pull. Where's all these frigging engineers when you need em!!!

France?

I can check with my brother-the-electrician tomorrow, but iirc you can only fill the conduit 40%, look up the crossection of the wires, add them together and subtract from the conduit, bob's your uncle.

Not a bad approach. I'll give that a try. You sure about the 40% cross sectional area? Seems kinda on the light side to me.

Thanks for checking BTW.

[Timo42]

You aren't going to be pulling much more than that anyway, do the existing wires have to be 10ga? might be easier to pull everything out and pull all new wires through the conduit into the junction box in the attic if you can downsize you can get more wires, even if you can't, pulling all new is a hell of a lot easier than trying to pull past existing, especially if you are trying to max out capacity. get some pulling lube, you will need it. That just sounds dirty.

[Point Break]

You aren't going to be pulling much more than that anyway, do the existing wires have to be 10ga? might be easier to pull everything out and pull all new wires through the conduit into the junction box in the attic if you can downsize you can get more wires, even if you can't, pulling all new is a hell of a lot easier than trying to pull past existing, especially if you are trying to max out capacity. get some pulling lube, you will need it. That just sounds dirty.

Its hard to pull out perfectly good #10...........never know when I might need em. I have most of the wires pulled.

I left a bit of room in the one in question. I have 3 #10 and 6 #12 in there right now. Looks at about 2/3's cross sectional area.

[Slick470]

You can do the calc the fill method. FMC at 40% fill is 0.213 square inches. Wire size depends on the insulation, THHN is common, with #12 is 0.0133 sqin and #10 is 0.0211 sqin. You should be able to find the insulation type on the wire and then look up what your actual area is.

Another method that I use often is to use Annex C in the NEC. It lists how many wires of a size and insulation type you can fit in a particular type of conduit. So FMC is actually table C.3 and using THHN insulation again, I can fit 10 #10's or 16 #12's in a 3/4" conduit. So, based off of that, you should be able to fit the #12's in there no problem.

[amro]

Easy work around would be treat all your conductors as #10 and do conduit fill based on that. You'll be under the fill limit easy.

Don't forget any grounds you may pull count as fill. And you should have at least one in there to your first jb.

[DA-WOODY]

Old Indian Trick

remove roof tiles above panel - open roof sheathing

drill down from above & run new conduit or just romex if all unexposed

Oh and then patch roof

so it does Not look like a patch

[MoeAlfa]

You can do the calc the fill method. FMC at 40% fill is 0.213 square inches. Wire size depends on the insulation, THHN is common, with #12 is 0.0133 sqin and #10 is 0.0211 sqin. You should be able to find the insulation type on the wire and then look up what your actual area is.

Another method that I use often is to use Annex C in the NEC. It lists how many wires of a size and insulation type you can fit in a particular type of conduit. So FMC is actually table C.3 and using THHN insulation again, I can fit 10 #10's or 16 #12's in a 3/4" conduit. So, based off of that, you should be able to fit the #12's in there no problem.

Talk dirty to me, Baby!

[Point Break]

Easy work around would be treat all your conductors as #10 and do conduit fill based on that. You'll be under the fill limit easy.

Don't forget any grounds you may pull count as fill. And you should have at least one in there to your first jb.

Yes on the ground. Pulling a ground in each conduit.. Pulling enough neutrals to share two circuits per return (opposite phases of course).

[Point Break]

Old Indian Trick

remove roof tiles above panel - open roof sheathing

drill down from above & run new conduit or just romex if all unexposed

Oh and then patch roof

so it does Not look like a patch

Standing seam metal roof. That won't work my old Indian friend.

[dreaded]

holy cow, CA allows your panel to be in such a place... no way in dallas.. good luck..

[Point Break]

holy cow, CA allows your panel to be in such a place... no way in dallas.. good luck..

House built in 1958. Existing non-conforming probably. No location fuss on the permit.

[not for nothing 2]

hire an electrician , so if your house burns down , your insurance will pay up, if you do it on your own they may not

[Timo42]

hire an electrician , so if your house burns down , your insurance will pay up, if you do it on your own they may not

He works for the fire dept...there are fringe benefits...


When I relocated and upgraded my panel, I ran 2 1" conduits to a junction box in the attic, little bit of extra work initially, but...

[Timo42]

holy cow, CA allows your panel to be in such a place... no way in dallas.. good luck..

Low rise roof, 8" max at the eaves to fish a conduit through, do-able, but...

[Peragrin]

You aren't going to be pulling much more than that anyway, do the existing wires have to be 10ga? might be easier to pull everything out and pull all new wires through the conduit into the junction box in the attic if you can downsize you can get more wires, even if you can't, pulling all new is a hell of a lot easier than trying to pull past existing, especially if you are trying to max out capacity. get some pulling lube, you will need it. That just sounds dirty.

Its hard to pull out perfectly good #10...........never know when I might need em. I have most of the wires pulled.

I left a bit of room in the one in question. I have 3 #10 and 6 #12 in there right now. Looks at about 2/3's cross sectional area.

so you have 9 wires in a 3/4" pipe

if memory serves you get 15 #12 wires and 9#10's your mixing so your can't get the max of either you can probably pull 3 more #12 wires through and that would be about it. I would watch the amperage and average loads though(for a house it should be fine as long as you aren't putting a full kitchen through it.

[PBO]

Conduit fill table

[v21sailor]

Conduit fill table

Buzzkill

[Timo42]

Conduit fill table

Buzzkill

Doesn't help, that is for same size conductors. Of course a real man would have ripped out the 3/4 and run a couple of 1 1/2 flex conduits while the box was out (insert Tim the Toolman grunting here). Why not punch some holes in the header and push some romex into the attic for the current extra circuits? Leave the space in the conduit for future expansion.