Electric question for dust collector

[Rod Sheridan]

A long run of wire actually reduces the current drawn - it does not increase the likelihood of a tripped breaker.

Let's say the resistive equivalent of our tool is 10 ohms (around 1.3 HP or so). If we plug this tool into a power source that has an infinite current supplying ability without any voltage drop, we can calculate that this load will dissipate 1440 watts (P = V^2/R). The current flowing through the tool is 12A (I = V/R).

Let's say we now take this same tool and plug it into 100' of orange BORG 16ga extension cord. The resistance per foot of 16ga wire is .00473 ohms/ft (which may be optimistic considering how many splice points there are if you use 4 25' cords instead of 1 100' cord for example). Multiplying this by 200' (need to count the round trip distance of the wire) gives us a total resistance of .946 ohms.

This gets added to our tool load, so now our effective resistance of the tool plus wiring is 10.946 ohms. Using Ohm's law again, we find that our current has actually dropped from 12A without counting the wiring losses to 10.9A. Obviously a 10.9A load is less likely to trip a breaker than a 12A load.

The reason for using large wire and short runs is so that we aren't wasting power in the wiring and starving our loads. Again taking the 100' extension cord example, we can find that our tool which really wants 120V of input power for maximum performance will only be operating on 109V, consuming 1188 watts, due to the losses of the small wiring. We've lost 250 watts of potential work output due to the higher load impedance of the circuit and the resistive losses of the wiring.

Tom, reducing the voltage to an induction motor causes an increase in motor current during normal operation, with a corresponding increase in motor heating.

Reducing the input voltage during start up can reduce the inrush current, however it will prolong the duration of inrush current.

Starting an induction on a circuiut with too much voltage drop can cause the breaker to trip, or fuse to melt because the inverse time curve of the protective device has been exceeded.

Regards, Rod.

[John Coloccia]

Personally, if I were going to rerun anything, I would run 10/3. If you ever want to upgrade to a larger collector later, you'll be good for 240V/30A and you'll have the neutral for running your relay. I'm not recommending you rerun anything but if I were going to take the time to do it, I would do it once and never revisit it again.

[Andrew Howe]

I appreciate everyones input and calculations. I ended up salvaging the 14/2 wire run for my overhead filtration unit and ran the 12/2 with a 20 amp breaker.

[Dave MacArthur]

I have a Delta 50-760, and I ran a 12ga for it.
(DOH! I see that while I typed all this, you made the right call and posted! Well, it took me too long to write, I'm not deleting it, and I can kinda tell you are an astute reader of the more discerning shop wiring threads anyways! Read on!)

This question comes up all the time, so I took the time to write a thread detailing how the NEC treats putting a motor, especially a DC motor, on a circuit, and stepping you through all the calculations for what would actually get permitted, not just the "that should work!" WAG.
You may find it useful to read for this and future wiring projects:

Sizing Wires and circuit breakers for 3HP and 5HP shop motor circuits

If you spec'd it all out, here's what I get:
1.5HP motor must be use NEC table 430-148 , Full Load Current for single phase AC motors. This says the number you MUST use for that motor is 20A. For continuous duty, which you must use for any motor if it's POSSIBLE to run it continuous (which it clearly is for a DC), you must use 125% of FLC for your minimum wire ampacity. That means 25 Amps. This is primarily for heat dissipation, not expected current flows.

You must size the wire for termination criteria as well as ampacity. Using 14-2 or 12-2 NM cable, you must use the 60C table for termination criteria, from table 310-16. The table shows that 25A at 60C can NOT be run on 14ga wire, but only on 12 ga and above. Therefore, you must use at least 12 ga wire, due to termination criteria of the wire hooking up to the circuit breaker itself. Why? Because the NEC knows that a it will take a 12 ga wire heated by possibly a 20A FLC, running continuously, to keep the wire heat low enough to not damage the CB it's screwed onto.

THEN you look at ampacity ability, and derate the wire for both temperature and bundling of wires (conduit etc.). NM cable has THHN inside it, so you could use the 90C table for ampacity. You can see that in fact, 14ga wire will run 25A current and stay below the 90C insulation damage point for THHN wire. However, if you had 4 or more wires through a conduit of > 2ft, you'd have to derate that by 80% multiplier, and if your wire ran through any ambient temp > 86F, you'd have to derate it further. So you likely have > 86F in the summer, which would derate the wire by .96 multiplier... which would drop 14ga below it's required 25A the NEC requires it to carry for FLC*125% of a 1.5HP motor. And the NEC doesn't care what the "real" or faceplate amps are, if it says 1.5 on that thing (it does, I have one), then you must use it's numbers. So, short answer is that in all liklihood if your shop gets > 86F, you have to use 12ga anyways for ampacity of the wire. Both required wire checks say you need 12 ga, so 12 ga it is.

If it was a dedicated circuit just for that DC, then you could protect the circuit by a 25A CB, all the way on up to 250% of the 25A FLC from table, 62.5A, and then bump it up to next normal CB of 70. This is because that DC motor is thermally protected for overload, so CB is not required to protect for that. HOWEVER!!!! This is not likely the case, or required for your setup--it is ONLY a special case for dedicated motor circuits properly calculated and attached.
If you are running that circuit as a general circuit that is not absolutely dedicated to the DC (which 99% you are, since it's just a plug in for a normal 20A circuit...) then you will be limited to 20A CB on that 12 ga wire due to the 240-3d "small conductors" requirement that 12ga must be protected by 20A CB (unless excepted as in the example above). Since the 20ACB is expected to protect the wire from the actual load you expect to run, (here you can use the nameplate one), 15A, you're good.

Whew!

OK..... Allll that being said, here's the REAL answer: that machine can be re-wired to 230V with two screw changes, and the directions for how to do so are on the inside of the elec box. DO IT! Now your amperage is just 7.5, and you actually COULD run it on 14ga if desired (bad plan for circuits), but you can certainly run it on a 12ga circuit with no concerns and plenty of circuit power left over to plug your jointer in there too! I changed mine over to 230V, took 2 min.

Good luck, happy NEC reading!

[Bud Millis]

For the little extra cost - go big! 10/2 wiring thur the shop. That will meet the needs for pretty much anything in the future. 20 amp circuits, also dedicated circuits for things are always a plus.

If your going to do it, do it right the first time.

[Derek Stockley]

My local electrical inspector told me I should be running my delta 50-760 on a 20 amp dedicated circuit with a t-slot receptacle because the natural intention of a machine like this is to run it for a considerable length of time and while other machines are running.