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.
Best Answer
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.
"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...
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).