120 or 240 for my DC?

[Richard A. Rivera, M.D.]

I have ordered the SteelCity 1 1/2 HP DC from Woodcraft. It is on sale and the price was right for me at $300.

I need some electrical advise. I have both a dedicated 120 outlet as well as an outlet for 240 avalable for the DC. It comes wired for 120v but can be converted to 240v. What is the advantage to changing to 240v over 120v?

Thanks ahead for any advice...Richard.

[Ken Fitzgerald]

Richard,

There really is no advantage to using 240 over 120 vac. One advantage might be if you already have a heavy load on your 120 vac circuits then adding the DC to 240 would spread the additional load to the 240 vac circuit. There is no gain in horsepower or anything like that. The other advantage is large horsepower motors is a smaller gauge wire could be used at 240 vac vs what would be needed at 120 vac.

[Joe Scharle]

For a convertible motor there is no advantage unless the distance to the panel exceeds 50 feet. Internally the motor always runs on 120V. The distance factor is relative because a higher voltage( = lower current) travels farther.

[Scott T Smith]

Richard,

There really is no advantage to using 240 over 120 vac. One advantage might be if you already have a heavy load on your 120 vac circuits then adding the DC to 240 would spread the additional load to the 240 vac circuit. There is no gain in horsepower or anything like that. The other advantage is large horsepower motors is a smaller gauge wire could be used at 240 vac vs what would be needed at 120 vac.

Ken, one thing that you might not be considering is balancing the load across both legs of your service. Granted that a 1.5 HP motor won't make much difference, but in general 240V equipment pulls an equal current from both legs, helping to keep them in balance.

I've been in shops with a lot of 120V equipment that was not balanced on the panel, and seen almost 20V deltas between the two legs.

[Dan Friedrichs]

Basicially, there are two 120V wires coming into your breaker box. If you connect between either of them and neutral, you've got 120V available. If you connect between the two 120V wires (not using neutral), you have 240V available. Ideally, you'd draw an equal amount of power from each of the two 120V wires coming into the breaker box. What Scott was referring to was a situation where you could inadvertantly connect many 120V loads to just one "side" of the box, meaning the power would not be equally shared between the hot supply lines. Unless you are the world's best multitasker (ie - using your dust collector and table saw and huge bandsaw and shaper all at the same time), this is NOT something you need to worry about.

As others said, there is no difference. The only time it would matter is if you were planning on running a new wire from your breaker box for it, and in that case, you'd probably be advised to run a 240V line, because you can get away with a smaller gauge (cheaper) wire.

[Ken Fitzgerald]

Richard,

There really is no advantage to using 240 over 120 vac. One advantage might be if you already have a heavy load on your 120 vac circuits then adding the DC to 240 would spread the additional load to the 240 vac circuit. There is no gain in horsepower or anything like that. The other advantage is large horsepower motors is a smaller gauge wire could be used at 240 vac vs what would be needed at 120 vac.

Ken, one thing that you might not be considering is balancing the load across both legs of your service. Granted that a 1.5 HP motor won't make much difference, but in general 240V equipment pulls an equal current from both legs, helping to keep them in balance.

I've been in shops with a lot of 120V equipment that was not balanced on the panel, and seen almost 20V deltas between the two legs.

Scott...That's part of what I meant by the highlighted portion of my first reply.

[Tom Veatch]

... What is the advantage to changing to 240v over 120v?

Thanks ahead for any advice...Richard.

A subject that, if not beaten totally to death in these pages, has certainly suffered severe trauma.

In opposition to the general tone of the responses so far received, I advise going with the 240v setup. Potential improvement, albeit quite small, at little or no cost makes it an obvious decision to me.

Justification follows:

There will be a certain voltage drop (power loss) in the supply wiring due to the current drawn by the load. For a given motor running at 120v the voltage drop will be double that at 240v. In the 240v case, because of the slightly lower line losses, a slightly larger percentage of the available power will be available to the motor.

Is that significant? Probably not. In some cases it will be noticeable when the load is drawing high current at startup or under other heavily loaded conditions. Whether it's significant or not depends entirely on the wiring installation that feeds the motor. Other than the consequences of the different voltage drop, for the motor itself, there's absolutely no difference between running on 120v vs 240v.

If you have to run a new circuit, it probably isn't worth converting to 240v. But, if 240v is readily available, there's no downside other than the cost of a plug and the time required to swap a couple of electrical connections in the motor's junction box.

[Scott T Smith]

Scott...That's part of what I meant by the highlighted portion of my first reply.

Yup, you're right.

I prefer 240 because it keeps the load balanced; ie utilizing it as much as possible will help prevent problems, as opposed to having to address them later.

But as Dan said, in most small shops this is probably a non-issue, as the overall current draw is not significant enough to cause concern.

[Chris Padilla]

Sheesh! Everyone knows that doubling the voltage means doubling the power!!

P = I * V so if V goes to 2V, then P must go to 2P, right??!!

Uh, NO. The trick is that while V --> 2V, I --> I/2 and P remains the same.

Tom V. points out the very minor gain in going to 240 V as does Ken in being able to run 240V/20A with #12 wire (where, perhaps, a 120V/20A circuit once lived....).

[Wilbur Pan]

For me, the advantage of converting a machine from 120V to 220V whenever possible is that the lights in the rest of the house don't dim whenever I start it up.

Don't laugh. My wife gets really irritated by this.

[Rick Christopherson]

There will be a certain voltage drop (power loss) in the supply wiring due to the current drawn by the load.
.........
Is that significant? Probably not.

I prefer 240 because it keeps the load balanced.......
But as Dan said, in most small shops this is probably a non-issue...

So let me paraphrase what is being said here. "I strongly believe that everyone should do this because of xxxxxxxx reason, even though xxxxxxx reason is not really significant."

And you wonder why this is a topic that does find itself beat to death so frequently?

So I have a couple of questions:

1. Have you ever calculated (or measured) the voltage drop for a 1-1/2 hp DC on a 20 amp circuit? (Yes, it will require a 20 amp circuit.)
2. Have you ever measured the amperage in your load center to see if there is a potential for imbalance?

A 1-1/2 hp motor is borderline as to being justified in rewiring to 240 volts, but the original poster has not even got the thing home yet to find out if there might even be a problem to begin with. The simple answer that could have been mentioned a long time ago was simply, "plug it in and see how it responds, and then decide if a problem exists or not."

[Joel Earl]

I won't - had the same issue until I balanced the breakers in my panel.

amazing what a difference having the loads balanced makes. Allowed me to save that 220 circuit for another tool when I avoided having to convert the DC back then. Of course more tools meant more circuits meant new sub-panel but at the time it was just the ticket

[Tom Veatch]

So let me paraphrase what is being said here. "I strongly believe that everyone should do this because of xxxxxxxx reason, even though xxxxxxx reason is not really significant."

Perhaps I'm being obtuse in presenting my point.

In most cases there is insignificant to no benefit to using the higher voltage. However, in a few, isolated cases there is discernible benefit. In either case, there is no operational penalty for using the higher voltage.

Therefore, if only insignificant effort and expense is required, there's no reason to not use the higher voltage. You may gain something. But even if you don't, it hasn't cost much either. IMO, a much better gamble than buying a lottery ticket.

If a significant effort or expense is necessary to utilize the higher voltage, then don't do it unless you're sure a problem exists.

[Josh Mountain]

Tom, No issues to your points involving the supply installation feeding the motor. Your points are clearly accurate that given a particular installation with same wire size / distance, the lower current will yield half of the voltage drop. The significant thing here is the percentage drop. So not only will you get the half the Vd from half the current, but your percentage of overall drop will be signifant since it will be percentage of supply voltage (which we'd like to minimize -- if we're gonna nitpick or because it's just good engineering).

But as Dan points out, we can get away with smaller wiring which can be quite significant. Bear with me. Changing two wire sizes will yield a greater change in Vd than halving the current. Meaning if you had a 12AWG 120V circuit pulling 15A running 30' ea way in copper with no other current draws (assume one outlet and standard temperature) you'd produce 1.469V. If you could get away with a 16AWG 240V circuit pulling 7.5A your drop would be 1.874V. Does anyone run 16AWG wire? NO! Just babbling really. But my point being that you would have to go all the way to 18AWG before your percentage voltage drop would encroach on what you were looking at in the 120V 12AWG example (about 1.22%). In practice we don't run 10A circuits and in Richard's case both circuits are already in place and probably 12AWG. In which case not only does the Vd halve but the percentage of voltage drop will quarter.

So are motors happiest seeing the proper source voltage? Yes. Is a couple of volts going to matter in most circumstances? No. But for those running long runs and/or running aluminum for long runs, setting up your power requirements to 240V where applicable will make you some money AND it automatically keeps your panel balanced as already stated. Obviously NEC helps us out with this, but I like playing with the various voltage drop calculators nonetheless.

So, Richard, follow the sound advise already given, can't really go wrong either way. For a nominal run to your circuit panel and a fairly small motor, whichever is more convenient ought to suit you. No disadvantages to setting up 240 (since the circuit is already there and presumeably unused) other than the cost of a new plug and ten minutes of your time!

Enjoy the collector!

[Josh Mountain]

Perhaps I'm being obtuse in presenting my point.

In most cases there is insignificant to no benefit to using the higher voltage. However, in a few, isolated cases there is discernible benefit. In either case, there is no operational penalty for using the higher voltage.

Therefore, if only insignificant effort and expense is required, there's no reason to not use the higher voltage. You may gain something. But even if you don't, it hasn't cost much either. IMO, a much better gamble than buying a lottery ticket.

If a significant effort or expense is necessary to utilize the higher voltage, then don't do it unless you're sure a problem exists.

I think we all agree on these points.

OBTUSE?
That was me. Time for bed in the Middle East. Talk to you all another time.