Wiring: 3/4 EMT capacity

[snojcb]

I am a bit confused by conductor derating within conduit. A snipit of the table is:

4-6 current-carrying wires, derate 80%
7-9 current-carrying wires, derate 70%
10-20 current-carrying wires, derate 50%

These numbers are irrelevent to the size of the conduit. 10 wires derate at 50% in 3/4" EMT and also in 2" EMT and 4" EMT. However, putting 2 sets of 5 wires in two 1/2" EMT runs, and we're up to 80% derating.

Ok, I understand that 10 wires in 3/4" EMT would need to be derated (due to heat), but there's more air in the 2" conduit, so why doesn't the 2" EMT have a higher derating at 10 wires than 3/4"?? Am I missing something??

The reason I'm asking is this:

I want to run two 220V 20A circuits and two 110V 20A circuits from my subpanel using 12 AWG copper. That's 8 current-carrying wires plus a shared ground, which is 9 12AWG wires total. 3/4" EMT can handle 16 12AWG wires, so no problem there. However, that would derate to 70%, which is 17.5 Amps, insufficient for the 20Amp requirements.

So it looks like I need to use two runs of 1/2" EMT OR 10AWG wire ($$$$).

Am I missing something??

 

[Grgramps]

JC
Not being an electrician, I wouldn't attempt to answer your question, but I can refer you to a forum where the professionals lurk. When I was wiring my shop I found them knowledgeable and helpful. The first step is understanding enough to be able to post a question and you've demonstrated that. See: Mike Holt

Good luck, Roy

 

[SteveColes]

#12 THHN wire is rated for 30 amps in theory, so a derating of 70% = 21 amps.:lol: A rule of thumb is that you use #12 for 20 is becuase in real applications like conduit and NM the end result is 20 amps.

 

[junquecol]

I am a bit confused by conductor derating within conduit. A snipit of the table is:

4-6 current-carrying wires, derate 80%
7-9 current-carrying wires, derate 70%
10-20 current-carrying wires, derate 50%

These numbers are irrelevent to the size of the conduit. 10 wires derate at 50% in 3/4" EMT and also in 2" EMT and 4" EMT. However, putting 2 sets of 5 wires in two 1/2" EMT runs, and we're up to 80% derating.

Ok, I understand that 10 wires in 3/4" EMT would need to be derated (due to heat), but there's more air in the 2" conduit, so why doesn't the 2" EMT have a higher derating at 10 wires than 3/4"?? Am I missing something??

The reason I'm asking is this:

I want to run two 220V 20A circuits and two 110V 20A circuits from my subpanel using 12 AWG copper. That's 8 current-carrying wires plus a shared ground, which is 9 12AWG wires total. 3/4" EMT can handle 16 12AWG wires, so no problem there. However, that would derate to 70%, which is 17.5 Amps, insufficient for the 20Amp requirements.

So it looks like I need to use two runs of 1/2" EMT OR 10AWG wire ($$$$).

Am I missing something??

I only see six current carring conductors, four in 220 circuits and two in 110 circuits. Neutrals don't count. The EMT furnishes your method of grounding. This is one of the real advantages to "metal" pipe, either rigid, or EMT. You will need to connect a pig tail between device and boxes though. All though #12 is rated for 30 amps, overcurrent device must be 20 amps.

 

[NZAPP1]

Steve and Bruce have hit the nail on the head # 12 THHN is rated at 30 amps and I also only count 6 current carrying conductors remember the grounding strap from the box the outlet.

 

[cpowell]

I believe that a "white" neutral used in 120 volt applications IS a current carrying conductor and MUST be counted as a current-carrying conductor. It is carrying the SAME current as the "Hot" lead (typically black). A neutral conductor that is designed to carry ONLY the unbalanced current between two hot leads in a two pole 220/240V circuit is a "true neutral" and need not be included. The two "Hot" leads in a two pole circuit with a neutral are 180 degrees out of phase and the currents cancel IF the load is balanced, so the neutral would not normally be carrying a load.

The problem is that the heat generated by a white conductor in a single pole application (resistance * current^2) is just as great as that generated by the "Hot" lead. I feel that it is likely that the conduit size doesn't matter when looking at derating because there is an assumption that the larger couduit will be filled, so that the heat generated per cross sectional area is similar. Most folks aren't going to run six #12 AWG wires through a 2 inch conduit if they're paying for the materials and the guys that write the code KNOW that and make the rules accordingly.

FWIW I have found that the NEC limits on the number of conductors in a conduit are well beyond what I allow, not because of heat, but because increases the risk of damage to the insulation while pulling the wires. Further, if you ever have a problem and need to replace a conductor in a "full" conduit, the risk of damaging the insulation when pulling in a single new wire is pretty high.

I'd always recommend thinking conservatively on electrical sizing. I'm not suggesting overkill. Just apply the factors and size according to NEC guidelines. I realize it may be more costly to "do it right" but would caution that the NEC is developed specifically to prevent much more costly incidents.

Chuck

Edit In: Sorry I posted this so long after the original post. I believe it's important information.

 

[snojcb]

I've done more research and have called the local town inspectors. The answer is...

1) Use the 90 degree rating for the wire guage ampacity (#12 @ 90 = 30 Amps). 70% of 30 is 21 Amps. Good news there.

2) Both hot and neutral count for 110 circuits unless the using a multi-wire situation. If you have two "normal" 110 circuits, then you have 4 current carrying wires (hot, neutral, hot, neutral). If you have two multi-wire circuits, then you have two current carrying wires (hot, hot).

3) In a garage, all 110 outlets must be GFCI protected. A multi-wire outlet cannot be GFCI protected using a GFCI receptical. So, I cannot use multi-wire for my application.

Thanks for all of your replies!!!!!

 

[thrt15nc]

3) In a garage, all 110 outlets must be GFCI protected. A multi-wire outlet cannot be GFCI protected using a GFCI receptical. So, I cannot use multi-wire for my application.

Thanks for all of your replies!!!!!

Doesn't mean anything in the discussion of a "shop", but just throwing this out there. In Johnston county, in a garage, you're allowed one single plug receptacle that is not GFCI protected. This allows the placement of a refrigerator or freezer in the garage where the current will come back on to it after an outage, rather than risking the GFCI to get tripped and then not having juice to it when power is restored, and losing your food if you happen to be gone at the time of the outage.

Tom

 

[Bernhard]

I've done more research and have called the local town inspectors. The answer is...

1) Use the 90 degree rating for the wire guage ampacity (#12 @ 90 = 30 Amps). 70% of 30 is 21 Amps. Good news there.

2) Both hot and neutral count for 110 circuits unless the using a multi-wire situation. If you have two "normal" 110 circuits, then you have 4 current carrying wires (hot, neutral, hot, neutral). If you have two multi-wire circuits, then you have two current carrying wires (hot, hot).

3) In a garage, all 110 outlets must be GFCI protected. A multi-wire outlet cannot be GFCI protected using a GFCI receptical. So, I cannot use multi-wire for my application.

Thanks for all of your replies!!!!!

Oh yes, you can!! Two options: Use GFCI overcurrent devices (aka breakers) in the load center (preferred way). Or use the GFCI DCO's (duplex convenience outlets aka receptacles) and connect any other (regular) receptacles in parrallel for each home run.
PS Dedicated DCO's do not need ground fault protection (i.e fridge, dust cleaner of ceiling, etc).

Actually, there is a third option: Remove the garage door and close it in; then you do not need any GFCI's....just kidding

Happy wiring!

Bernhard