I’m running conduit along the walls in my shop (in a commercial warehouse) to provide power to some machines and additional outlets. The service is 3 phase, 208 V, 100 A. One of the machines requires 3 phase 240 V @ 30 A but the VFD manual calls for 8 ga conductors. This machine also has a computer attached that needs 120 V.

The other two machines are 240 V @ 20 A. In addition, I want two branches of 120 V @ 20 A outlets.

I'm facing a situation where I'd like to run 5 (or 6) circuits through 1" EMT, but my calculations are exceeding the 40% conduit fill limit by less than 1%.

  • Circuit 1 (50 A 3p4w): 4x 8ga + 1x 10ga ground (stranded)
  • Circuits 2&3 (30 A 240 V): 4x 10ga + 2x 10 ga ground (stranded)
  • Circuits 4&5 (20 A 120 V): 4x 12ga + 2x 12ga ground (solid)

That totals 40.7%. A workable alternative (the neutral in the 3ph circuit 1 above is only needed because I have a 120V load on that machine):

  • Circuit 1 (50 A 3p): 3x 8ga + 1x 10ga ground (stranded)
  • Circuits 2&3 (30 A 240 V): 4x 10ga + 2x 10 ga ground (stranded)
  • Circuits 4&5&6 (20 A 120 V): 6x 12ga + 3x 12ga ground (solid)

That totals 40.44%.

Can I round down the percentage values? Is there any other way to meet code?

Solid wire instead of stranded meets code, but It’s a pain to work with in the larger gauges.

  • Are your ground wires insulated or bare? Is this conduit run living on a roof where it can get stepped on? Is there some other reason you want ground wires inside a metal conduit, even? Jul 4, 2019 at 13:11
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    Are you sizing your wire correctly to account for the derated ampacity of running 5 and 6 circuits together in a single conduit per 310.15(B) of the NEC based on the expected load on each of those circuits? If not, there's another reason to use separate conduit runs. Jul 4, 2019 at 13:18
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    I’ll have to check the derating.
    – Rick
    Jul 4, 2019 at 13:23
  • 4
    If the computer is mostly standard, there's a good chance it'll take 240V without complaints. Most computer power supplies are universal, though older ones have a switch on the back. Not sure how much that helps you. Jul 4, 2019 at 16:03
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    You have the all the required tools out right now... don't un-future proof yourself. That picture was in the same building that I arbitrarily ran a second, empty conduit in a cramped drop ceiling. Guess what? It has wires in it now. And running those wires disturbed the client 75% less and made my job 99% easier. Which was good because having to listen to 20yo telemarketer's in-office banter, from an attic, is a trying experience.
    – Mazura
    Jul 5, 2019 at 16:49

3 Answers 3


Too many circuits per pipe

All this is about the derating rules in 310.15(B)(3)(a), see ThreePhaseEel's answer.

When I first saw your other question, I thought "Oh boy, I smell a too-many-circuits problem coming". I thought I should warn you to run multiple conduit, but I didn't mention it because it was out of scope.

Now, if you were strictly working in single-phase (counting split single-phase 120-240V), the rule-of-thumb is "4 circuits per conduit", a simple rule for complex reasons. However if you're working in 3-phase also, the simple rule boils down to 3 circuits per conduit for the same reasons.

  • 4 single-phase circuits (8 wires we care about)
  • 1 three-phase circuit + 3 single-phase circuits (9 wires we care about)
  • 2 three-phase circuits + 1 other circuit of any type (8-9 wires we care about)

"We care about" means we do not count neutrals in 3-phase circuits, split-phase 240V circuits, or multi-wire branch circuits. Nor do we count grounds.

Run 2 pipes - it's cheap

Your junction boxes typically have 3 knockouts on a side. I like to run conduit on two of them. Now, if you use the "elbow" method from my other post, use the top 2 knockouts so the bottom one is free for pull-offs. If you use the "tee" method, use top and bottom to make more clearance. Feel free to mix it up.

Remember you also have a thru-wire limit on boxes. Each #12 wire counts for 2.25 cubic inches, each #10 for 2.5 and each #8 for 3 cubic inches. All grounds together count as 1 wire of the larger size. It's easy to very quickly over-cube a 4" square box when making splices therein; that's why I prefer 4-11/16" as they have twice the cubic inches. Buy them at electrical supply; big-box is overpriced.

GroundS shouldn't be in the pipe at all

This is EMT metal conduit. It is its own grounding path. I have miles of it and I don't use any ground wires. I have 9 colors of #12 stranded - I have purple - and I don't own any green wire. (Well, a 50' spool for a short PVC run). Even if it was PVC, you only need 1 ground wire for the whole kaboodle.

That said, if I fear an EMT section may be vulnerable to being pulled apart by random damage, I'll throw a ground wire just between the two boxes, grounding it to the box. And it can be bare, which saves conduit fill.

Don't run solid wire

It just makes the wire bundle super stiff, and vulnerable to stress damage. There's no requirement anywhere to run solid wire, unless you get a rare device whose connections are listed for solid wire only, in which case get a better device, or just pigtail. I'll grant that putting stranded wire on receptacle screws can be a trick, but I pigtail most of my receptacles anyway. (with stranded because I got good at it, but solid would be fine).

For small 120V loads on big machines, local transformer

First, for Pete's sake check the computer's power supply rating and make sure it isn't multi-voltage 120-240V, because so many are these days. If it's a desktop style Windows PC, the power supply is probably interchangeable to a 240V-friendly one. If it isn't, get its amp or VA rating... if it's modest, fit a small local transformer and manufacture 120V from two of the phases. That eliminates a wire.

Subpanels are cheap too

Note what Retired Master Electrician says about subpanels.

In fact, you could get everything done with 2 circuits (6 wires): a 240V 3-phase circuit (I assume delta?) and a 208V 3-phase circuit. Take each one to the first (or second) place circuits must diverge, and fit a subpanel there. Then continue off the subpanel. If you go to the second place, then double back in the same conduit (or a different one). When feeding all-phases subpanels, neutrals are free.

Close, but no cigar

We can beat the fill issue by dumping superfluous grounds.

Three-phase neutrals are free from a derate POV, so go ahead and send neutral to the big machine.

Now, we could merge circuits 4 and 5 into a split-phase multi-wire branch circuit. However, there's a gotcha. You can only exclude neutral from a multi-wire branch circuit if the hots are 180 degrees opposed. If your 3 sides are 120, 120 and 240 volts, that "triangle" is a flat line, and the hot-hot voltage will be the sum of the two hot-neutral voltages. Any neutral current flow subtracts same flow from a hot.

However, in your case, your supply is 208V 3-phase. That means your multi-wire branch circuit will be 3-phase and thus 3 wires. Even if you only bring 2 hots and neutral, you still have a "triangle" of 120, 120 and 208V sides. While it's not equilateral, it's still a triangle and so it counts like 3 wires. You might as well just bring the third hot along.

On the other hand, if you can source that 120/240V from a true split-phase or "wild leg delta" supply for instance... then it works like this...

  • Circuit 1 (50 A 3p4w): 4x 8ga + 1x 10ga ground
  • Circuits 2 & 3 (30 A 240 V): 4x 10ga
  • Circuit 4A and 4B: (20 A 120 V, 240V between them): 3x 12ga

We can ignore circuit 1's neutral, but we can't ignore circuit 4's neutral because of the abovementioned problem. If only we could!

However even if it were possible, this is shortsighted. Since you're hanging conduit right now, might as well throw in the second pipe, a 3/4" or even 1/2" would suffice unless it needs more physical support for the span. (some local codes require minimum 3/4" for commercial work).

  • 1
    His service is 208Y/120, so he'd need a transformer to get a MWBC that counts as 2 CCCs, otherwise he winds up still getting dinged on the derate due to 4A/4B counting as 3 conductors Jul 4, 2019 at 16:45
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    I didn't spot that in the comments. I saw 240V 3-phase and assumed wild leg. @ThreePhaseEel Jul 4, 2019 at 16:49
  • a) Yes, this computer can accept 240 V, but wanted to keep actual computer and monitor choices flexible. b) Surely there's some upper limit to the amount of current EMT can handle when grounding? Is it always higher than all conductors within? c) I think second conduit is going to be easiest; can I just add box extenders if I hit fill limits? I chose the 4 11/16", but they only have two knockouts per side.
    – Rick
    Jul 4, 2019 at 22:38
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    a) I really don't think that's going to be an issue. b) There is indeed ground current limits for EMT, there's a table somewhere ... but it is presumed an actual ground fault is a rare event, so you don't have to provide enough ground for all circuits to ground-fault at the same time :) Cover the largest and you're all good. Most likely, if it fits, it ships. c) Yeah, box extenders are my go-to! Jul 4, 2019 at 22:47
  • If I understand everything correctly, I can run one branch as 3x 8ga with a 30A breaker, and 3 more branches as 6x 12 ga with 20 A breakers in one 1" conduit, right? 70% derating for 9 conductors, 36% fill (including one 10 ga ground). I realize I was mistaken claiming I needed 50 A above. I only need 21 A, so 30 A breaker will do.
    – Rick
    Jul 5, 2019 at 7:39

Derailed by Derating

Your plan is a non-starter, even if you overcome the fill issues you're having, because of the other limit the NEC places on conduit fill; namely, the derating factors found in 310.15(B)(3)(a) that limit ampacity based on the number of current-carrying conductors. The first two rows of the table are normally not an issue because nobody gets to use the 90°C ampacities of wires straight up for termination temperature rating reasons. However, staying within those rows limits you to a maximum of 9 current-carrying conductors, and you need a minimum of 10: 3 for the three-phase machine, 4 for the two 208-240V single phase machines, and 3 for the 120V circuits (the analysis comes out the same in both scenarios, assuming we put the 120V circuits on a multi-wire branch circuit in either case).

This means that the ampacity rating of your 8AWG wires is limited to 27.5A by the 50% derate imposed by the NEC due to the conductor count. Going up to 6AWG gets you to 37.5A, which would be acceptable for your application, albeit marginal. The other wires are not spared either: the 10AWG wires need to become 8AWG as they are limited to 20A by the derate, and the 12AWG wires need to become 10AWG or else they will be limited to a mere 15A for the same reason.

Even with the fill savings yielded by removing the extraneous equipment grounding conductors and using a MWBC for the 120V circuits, we still wind up needing 3 6AWG, 4 8AWG, and 4 10AWG conductors in the best case, or 32.71*3 + 23.61*4 + 13.61*4 = 247.01mm2 of conduit fill to run this in a single conduit. Since your 1" EMT has 222mm2 of fill available for you to use, we come to a single conclusion: you must split this run into two separate runs, one for the 3-phase machine + its convenience receptacle and the other for the two single phase machines and their associated receptacle, in order to wire this feasibly using "normal" methods.

There is a tricky way out, though...

There is a way to get this run down to 9 CCCs (and thus to manageable derates), while staying within the fill of a single 1" EMT, though. If you can run the computer's power supply on 208V AC (check the power supply's label, if it says "90-264VAC" or something like that, then it will run quite happily on any AC voltage within that range), and the two single phase machines do not mandate specific upstream breaker sizes, we can cram them together to form a multi-wire branch circuit, consisting entirely of line-to-line loads. This forces that circuit to go up to 8AWG wire at a minimum due to the fact we are effectively connecting the loads together in a delta configuration, vs the wye configuration more common in three-phase MWBCs, but lets us run the computer and both single machines off of a single three-phase branch circuit.

As a result, we wind up with 3 8AWG wires for the three phase machine, 3 6AWG wires (why 6AWG? this has to do with three phase power and wyes vs deltas, suffice it to say that the factor is roughly 1.7 times the single phase amps for a balanced load consisting of 3 single phase machines of a given ampacity wired as a delta, and then you have to factor continuous load derating atop that) for the two single phase machines and the control computer wired in a delta configuration across the circuit, and 3 12AWG wires for the convenience receptacles, wired as a conventional MWBC. This gives us 3*32.71 + 3*23.61 + 3*8.581 = 194.703mm2 of total conduit fill, well within the 222mm2 of available fill in your 1" EMT. You can even get a 3rd receptacle circuit in there "for free" simply by pulling its hot wire, as then the neutral on the 20A MWBC stops counting.


+1 for @ThreePhaseEel.

If you do not have any other option but to run 1" EMT then the only other way I can think of to satisfy loading your conductors is to run 1 set of #4 THHN, THWN or THWN-2 copper conductors, which have a capacity of 85A. Then you can set a subpanel at the equipment location and feed individual pieces separately.

Of course you will have to do a load calculation to see if 85A is large enough. I am assuming it is since it would be 85% of the total load of your 100A Panel you are feeding from. It would also be a bonus if you can place the subpanel within line of sight, no farther than 50' and you have unencumbered access to it. You could also use it as the required equipment disconnect.

Hope this helps and good luck.

  • 85A because you're allowed to work out of the 75C column, since this is THHN in conduit. You cannot get 85A out of #4Cu Romex, there you're stuck in the 60C column for 70A. Jul 4, 2019 at 16:23

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