MWBC for shop machines?

[signcrafter]

Getting ready to start running electrical to shop equipment and wondering if MWBC are allowed or are a bad idea? In one area I have a tire machine(9 amps), balancer(can't check right now but think it's around the same 9 amps), and parts washer(11.9) amps, and then a couple outlets for 5" hand grinder and anything else I might need. I'm just a home shop guy so will never be using them all at the same time. Only time would be using the grinder to clean a rim while the tire machine is spinning. So thinking two 20 amp circuits would be good for this area. Everything is in a row along the wall and sub panel is roughly 30' from the first machine and I will be running conduit and THHN to this area. Is running a MWBC a bad idea to share the neutral? It's not much in cost savings but trying to plan out my conduit runs and wire fill. I have just started the planning so nothing set in stone, but if I figure it out and there is room in the conduit I might be able to run another circuit for something like compressor that is close. Compressor is 240 with F.L.A, of 15 amps so I have had it hooked up with 12 gauge wire and 30 amp breaker, but if I ran it in same conduit then I believe that would be 4 power carrying conductors and need to derate. Starting to get over my head but if I understand the derating thing it says #12 at 90 degrees is 30 amps and for 4-6 conductors, think this is only counting power carrying conductors and not neutrals and grounds? If so then 80% derate would still allow 24 amps?

Sorry for all the rambling but tried to get all the details out. Actual questions are below,

Is a MWBC allowed and would it make sense to do in my situation? Or since it's a short run and I believe I have room in the 3/4" conduit just pull another neutral?

Can I run two 120V/20A circuits or one 240V/20A MWBC in 3/4" conduit and then also run my compressor wiring in there? Think the wire fill would be fine but not sure about the derating part for number of conductors? Thanks

 

[dave*99]

How do you plan to accommodate GFCI protection? A MWBC can be run from the panel to a pair of GFCI receptacles, but after that you will run separate neutrals off the 2 GFCIs Does that save you anything?

 

[micromind]

The service to the shop is a multiwire circuit.........

There is nothing wrong with them, I run them all the time, even if the service is 3Ø.

As far as GFIs go, I don't like to use a GFI and have protected outlets downstream. I use GFI outlets at the point of use. Yes, it can be a lot but at least you don't have to go searching if one trips and kills a bunch of outlets downstream.

 

[dave*99]

The service to the shop is a multiwire circuit.........

There is nothing wrong with them, I run them all the time, even if the service is 3Ø.

As far as GFIs go, I don't like to use a GFI and have protected outlets downstream. I use GFI outlets at the point of use. Yes, it can be a lot but at least you don't have to go searching if one trips and kills a bunch of outlets downstream.

Sounds like a good approach, but the OP has a 30' run from the panel to the start of a row of machines. Best I can tell from his post---- one may be able to spit from one end of machinery row to the other. Hopefully he posts a diagram with more info.

 

[signcrafter]

Sounds like a good approach, but the OP has a 30' run from the panel to the start of a row of machines. Best I can tell from his post---- one may be able to spit from one end of machinery row to the other. Hopefully he posts a diagram with more info.

I never thought about problems with GFI circuits on MWBC. I just googled it and if I understand correctly you can do it but you can't use the load side of the GFI for downstream outlets? This would kind of make it pointless to do. Would be better off running 2 circuits each with GFI and then another outlet downstream off the load side.

I don't have a diagram, but picture the back wall of the garage. Sub panel is in the right most corner and then going to the left about 25' is the air compressor. Then a couple more feet is the line of machines, parts washer, wheel balancer, tire machine. Each machine has about 2' of space in between them. This is all seldom used stuff besides compressor and why it's stacked right next to each other to be able to fit.

I'm not thinking about this for saving a few bucks on wire, was more thinking about it for less wires in the conduit. I need to do some more homework and learn about if the wiring for my two 120V/20A circuits and 240V compressor circuit, can all go in the same 3/4" conduit. Would be nice to only have to run 1 conduit for all that since it's all in the same area. Would all be 12 gauge wire and be 4 hots(2 for 240V compressor and 2 for the 2 120V circuits) and then 2 neutrals and a ground. I'm not familiar with all that. Thanks for the advice.

 

[NostraThpmas]

Running the MWB and 240 in the same conduit would be 5 current carrying conductors. 3 for the MWB and 2 for the 240.

 

[signcrafter]

Running the MWB and 240 in the same conduit would be 5 current carrying conductors. 3 for the MWB and 2 for the 240.

I thought I read that the neutral doesn't count but I could be wrong on that. But even if it is counted, if I'm understanding things right if you have 3-6 current carrying conductors in a raceway you derate to 80% of the 90 degree column? For 12 gauge 90 degrees says 30 amps, hopefully I understand that right because I always thought 12 was rated for 20 amps. So 80% of 30 amps is 24 amps and still be good for all my circuits? Unless I'm missing something which very well might be the case and why I'm asking.

 

[dscheidt]

About conduit derating, it's based on the number of current carrying conductors. For a normal 120V circuit, there are two current carrying conductors, the hot and the neutral. For a 240V circuit, there are also two current carrying conductors, the two hots. A MWBC is really a 240 circuit with a neutral to carry any current imbalance. Because heat is from power, and power goes up with the square of current, the most heat the MWBC can put in is when two wires are carrying all the current. So a MWBC lets you run 2 120V circuits with three wires, only two of which are current carrying for conduit capacity purposes. That's why they're commonly used in conduit, they save lots of money. In romex-land, the complications only make it make sense for long runs or specialized uses.

As for GFCI protection of a MWBC, there are three options. One is a two pole GFCI breaker. The other is run a MWBC to where the circuits split. Then put a GFCI on both 120V circuits, and treat them as regular circuits after that, with four wires. The last is to use a GFCI at every receptacle. Depending on what the GFCI breaker costs, that's my preferred approach. Then the split at use. GFCI receptacles everywhere would be most expensive, unless there are only a couple receptacles.

 

[micromind]

It's perfectly fine to put both 120 and 240 in the same conduit, it's done all the time. In the industrial world, it's perfectly fine to run 120 and 277/480 in the same conduit.

If you're using THHN in the conduit, according to code, there's no reason to derate until there's more than 6 current-carrying conductors. The neutral doesn't count as a current-carrying conductor.

 

[sparky 1971]

A MWBC is fine but the load sides of the GFCIs will require separate neutrals or a point of use GFCI at each receptacle locaction. When I do it using conduit I take advantage of all the colors of wire. The line sides may be black, red, white. Then one load side might get blue and white while the other might get brown and gray. Hot colors can be black, red, blue, brown, orange, yellow, pink, or purple with neutrals limited to white and gray. I've also been known to shoot a mist of paint over the wires so I know which is which when it comes out the other end d of the pipe. For example; I'm pulling black, red, blue, and three whites through a conduit. The white that goes with the black will get a mist of black paint t, the red neutral gets red paint, and the blue might get blue bit probably won't get anything since it's not either red or black makes it obvious.

Of course, if this were my own shop it wouldn't matter because the likelihood of GFCIs being installed isn't very high.

 

[signcrafter]

About conduit derating, it's based on the number of current carrying conductors. For a normal 120V circuit, there are two current carrying conductors, the hot and the neutral. For a 240V circuit, there are also two current carrying conductors, the two hots. A MWBC is really a 240 circuit with a neutral to carry any current imbalance. Because heat is from power, and power goes up with the square of current, the most heat the MWBC can put in is when two wires are carrying all the current. So a MWBC lets you run 2 120V circuits with three wires, only two of which are current carrying for conduit capacity purposes. That's why they're commonly used in conduit, they save lots of money. In romex-land, the complications only make it make sense for long runs or specialized uses.

As for GFCI protection of a MWBC, there are three options. One is a two pole GFCI breaker. The other is run a MWBC to where the circuits split. Then put a GFCI on both 120V circuits, and treat them as regular circuits after that, with four wires. The last is to use a GFCI at every receptacle. Depending on what the GFCI breaker costs, that's my preferred approach. Then the split at use. GFCI receptacles everywhere would be most expensive, unless there are only a couple receptacles.

Thank you for that. So if I run a MWBC to 2 GFIs and then run a hot and neutral from GFI load side to regular receptacle downstream that is good right? This will work perfect. Although if micromind is correct in that you don't derate until more then 6 current carrying conductors I could run 2 regular 120V and 1 240V circuit and be fine in my 3/4" conduit. So either way would work and if I do separate regular circuits it's only 30 extra feet of wire. I wasn't worried about saving wire, more trying to figure out conduit fill and derating issues. But sounds like I should be fine.

It's perfectly fine to put both 120 and 240 in the same conduit, it's done all the time. In the industrial world, it's perfectly fine to run 120 and 277/480 in the same conduit.

If you're using THHN in the conduit, according to code, there's no reason to derate until there's more than 6 current-carrying conductors. The neutral doesn't count as a current-carrying conductor.

I thought I had read with 3-6 THHN current carrying conductors it said to derate to 80%. Table 310-15(C)(1) says 4-6 derate to 80%. Maybe I'm not understanding that correctly. But if I use THHN and can run 6 current carrying conductors in conduit without derating I should be good.

For my own future knowledge, table 310-16 says 12 gauge THHN in the 90 degree column is rated for 30 amps. I always thought 12 was only good for 20 amps?

 

[sparky 1971]

Thank you for that. So if I run a MWBC to 2 GFIs and then run a hot and neutral from GFI load side to regular receptacle downstream that is good right? This will work perfect. Although if micromind is correct in that you don't derate until more then 6 current carrying conductors I could run 2 regular 120V and 1 240V circuit and be fine in my 3/4" conduit. So either way would work and if I do separate regular circuits it's only 30 extra feet of wire. I wasn't worried about saving wire, more trying to figure out conduit fill and derating issues. But sounds like I should be fine.

I thought I had read with 3-6 THHN current carrying conductors it said to derate to 80%. Table 310-15(C)(1) says 4-6 derate to 80%. Maybe I'm not understanding that correctly. But if I use THHN and can run 6 current carrying conductors in conduit without derating I should be good.

That is correct but you're starting off the derating with #12 at 30 amps so 80% is 24 amps. It's got a way to go before you get to the 20 amps a #12 is used on for a normal circuit (motors and air conditioners aside).

For my own future knowledge, table 310-16 says 12 gauge THHN in the 90 degree column is rated for 30 amps. I always thought 12 was only good for 20 amps?

Look at the *.

 

[mm08822]

It's perfectly fine to put both 120 and 240 in the same conduit, it's done all the time. In the industrial world, it's perfectly fine to run 120 and 277/480 in the same conduit.

If you're using THHN in the conduit, according to code, there's no reason to derate until there's more than 6 current-carrying conductors. The neutral doesn't count as a current-carrying conductor.

In the OP's case correct, for his described MWBC where all (both) phase conductors are present. ....Just be careful stating "the neutral doesn't count", b/c it can and does in other correct configs.

 

[signcrafter]

That is correct but you're starting off the derating with #12 at 30 amps so 80% is 24 amps. It's got a way to go before you get to the 20 amps a #12 is used on for a normal circuit (motors and air conditioners aside).

Look at the *.

Ok makes sense now, thanks. So 12 is good for 30 amps but 240.4(D) says overcurrent protection is mandated to 20 amps. I have always just known 12 gauge is for 20 amp circuits so just thought that was the amp rating of 12. So was worried about derating from 20 amps but should be good since you start at 30 amps and will have a 20 amp breaker. Except for the compressor motor that is 240V number 12 THHN on a 30 amp breaker, which is what I did years ago after reading up on motor circuits when I hooked it up. Always good to learn new stuff.

 

[mm08822]

Ok makes sense now, thanks. So 12 is good for 30 amps but 240.4(D) says overcurrent protection is mandated to 20 amps. I have always just known 12 gauge is for 20 amp circuits so just thought that was the amp rating of 12. So was worried about derating from 20 amps but should be good since you start at 30 amps and will have a 20 amp breaker. Except for the compressor motor that is 240V number 12 THHN on a 30 amp breaker, which is what I did years ago after reading up on motor circuits when I hooked it up. Always good to learn new stuff.

In the case of 90C conductor insulation starting at 30A.

If it was 75C or 60C rated insulation, you would derate from 25 or 20 A respectively. This is when the permitted cb rating can become a problem.

 

[NostraThpmas]

In the OP's case correct, for his described MWBC where all (both) phase conductors are present. ....Just be careful stating "the neutral doesn't count", b/c it can and does in other correct configs.

I was thinking more about conduit & box fill. My understanding is that as of NEC 2026 they are counted for derating purposes as well.
EDIT: I just reread the relevant NEC section and it only applies to 3 phase. Sorry about the confusion.

 

[mm08822]

Box fill and conduit fill limits have different accounting than used for ampacity derating based upon # of CCC's.

 

[dave*99]

Regarding the MWBC, note that if you use 2 single pole breakers with a handle tie, they can trip independently which is often desirable.

If you use a 2 pole common trip breaker, overloading one circuit will cause both to trip off. Also note that if you ever attach a 240V load to the MWBC, you would want a common trip breaker. That 240V case would have to be upstream of the GFCIs. It doesn't seem like you plan to do this - but if it comes up in the future it's good to know.

 

[mm08822]

I was thinking more about conduit & box fill. My understanding is that as of NEC 2026 they are counted for derating purposes as well.
EDIT: I just reread the relevant NEC section and it only applies to 3 phase. Sorry about the confusion.

It applies to single phase as well.....NEC310.15 (E)(2)

 

[sparky 1971]

Regarding the MWBC, note that if you use 2 single pole breakers with a handle tie, they can trip independently which is often desirable.

They should trip independently but there is a chance that the tripping breaker jolts the other hard enough to turn it off. It's possible but not probable and I have been a witness to it (at least I don't think anyone turned the other breaker off before I got there). If one does trip, both will have to be turned off to reset it.

If you use a 2 pole common trip breaker, overloading one circuit will cause both to trip off. Also note that if you ever attach a 240V load to the MWBC, you would want a common trip breaker. That 240V case would have to be upstream of the GFCIs. It doesn't seem like you plan to do this - but if it comes up in the future it's good to know.

 

[signcrafter]

Box fill and conduit fill limits have different accounting than used for ampacity derating based upon # of CCC's.

You're talking about total number of wires in the conduit, hots, neutrals, and grounds? I didn't dig to far into it but from what I found you can do 16 number 12s in 3/4" EMT. Even if I do the two 120V and one 240V circuit pulling a ground I would have 7 total wires so should be good, I think? So I didn't dig further into that and was more concerned about derating for CCC after learning about that. But should be good there also.

 

[mm08822]

You're talking about total number of wires in the conduit, hots, neutrals, and grounds? I didn't dig to far into it but from what I found you can do 16 number 12s in 3/4" EMT. Even if I do the two 120V and one 240V circuit pulling a ground I would have 7 total wires so should be good, I think? So I didn't dig further into that and was more concerned about derating for CCC after learning about that. But should be good there also.

Two dimensions of concern:

How much can fit in a raceway volume-wise (max 40% fill) and of that permitted number space occupiers (wires), how much heat is possibly generated from the current carrying conductors, hence the reason to de-rate max permissible current in each circuit.