220VAC Power Questions

[Anthony Whitesell]

I have some 220VAC power questions I wanted your corrections, clarifications, and expert opinions on.

1) When installing a switch to cut power to the system, both legs should be switched, right?

2) When measuring 220VAC current, is the current measured in one leg the total current and maximum current drawn by the system, is it the current measured half the total current and maximum current drawn by the system?

[Harvey Pascoe]

#1. Yes, switch both legs, which means you use a 220V switch, not 115V.

#2. No, the amperage used by both legs is the total. Assume the motor uses current equally on each.

[David Castor]

The current in each leg is the same current, so they will be equal. It's similar to a 120 V circuit - just twice the voltage and neither leg is at ground potential. In some situations, such as an electrric dryer or range, there are 120 V loads in addition to the 240 V loads, so that is a slightly different situation and the currents in each 240 V hot leg may not be equal. But in a two-wire 240 V circuit, the current in each leg has to be the same.

[Rick Christopherson]

I have some 220VAC power questions I wanted your corrections, clarifications, and expert opinions on.

1) When installing a switch to cut power to the system, both legs should be switched, right?Yes and no. Opening both legs is common, but not required for control circuitry. It depends on what you are doing, and we don't have enough information to answer otherwise. Functionally, breaking one leg is all that is required. Breaking both is only required for a disconnect.


2) When measuring 220VAC current, is the current measured in one leg the total current and maximum current drawn by the system, is it the current measured half the total current and maximum current drawn by the system?As Highlighted, yes. I am amazed by the number of licensed electricians that wrongly believe half the current comes from one wire and half the current comes from the other wire. That's an excusable mistake from someone outside of the industry, but if you ever hear it from an electrician, it is a sign that he is merely a mechanical installer with no electrical knowledge.

[Ole Anderson]

Yes, switch both legs, which means you use a 220V switch, not 115V


Agreed, typically you will switch off both hot wires. You would ask for a two pole switch which will have four screws. Make sure the amperage of the switch is well above the current of the motor you are switching. I would limit a 15 amp 2 pole switch to a 2 horse motor wired for 220V, for a 3 hp motor I would go to a 20 amp switch, but that is just me.

[John Coloccia]

You're breaking an inductive load so you need a switch that can handle your inductive load.

The best way to do this, though, is to put the unit on a magnetic switch. They're available through Grizzly, and elsewhere. It's not easy switching a large, inductive load off. To do it reliably long term, easiest is to use a contactor, i.e. a magnetic switch.

[George Bregar]

Breaking both is only required for a disconnect. Ok, so I ran a dedicated 240V 20A circuit under the floor and up to a box mounted on my 3HP Unisaw and DJ-20. I want to install a switch simply for disconnect...changing blades, general safety. I have two Leviton CSB2 DP 15/20A Switches that were listed as handling motor loads. I'm good?

[Anthony Whitesell]

The purpose of the switch will not be to shutdown the equipment, but to turn off the Mag Switch and act as as a Master Switch. Some mag switch designs draw power all the time(vampire circuit). My jointer draws 2kW per day plugged in and not running! The fancier circuit for my new cyclone should draw the same if not slightly more.

[Rod Sheridan]

The purpose of the switch will not be to shutdown the equipment, but to turn off the Mag Switch and act as as a Master Switch. Some mag switch designs draw power all the time(vampire circuit). My jointer draws 2kW per day plugged in and not running! The fancier circuit for my new cyclone should draw the same if not slightly more.

Anthony, are you sure about that number?

My jointer has a low voltage control starter and only draws 7 watts when the jointer isn't running and the disconnect closed.

My saw/shaper is the same because it has a control transformer.

My cyclone starter draws zero as it has a line voltage contactor coil.

On the saw/shaper and jointer/planer I simply open the built in disconnect switch at the end of the day.

What exactly is running on your jointer to draw 83 watts when it's off? Does yours have digital displays?

Regards, Rod.

[Rick Christopherson]

Ok, so I ran a dedicated 240V 20A circuit under the floor and up to a box mounted on my 3HP Unisaw and DJ-20. I want to install a switch simply for disconnect...changing blades, general safety. I have two Leviton CSB2 DP 15/20A Switches that were listed as handling motor loads. I'm good?If the saw is connected with a cord and plug, then you're OK. If it's hardwired, I am not sure the switch (which I did not look up) would be classified as the required disconnect. Your switch does serve the purpose you want it to do, and that is an extra safety measure for changing blades.

Note: Because I don't know if you have a cord-and-plug connection, I did not take the time to look up the requirements for a device that is serving as a primary disconnect. (A cord and plug qualifies as the primary.)

[George Bregar]

If the saw is connected with a cord and plug, then you're OK. If it's hardwired, I am not sure the switch (which I did not look up) would be classified as the required disconnect. Your switch does serve the purpose you want it to do, and that is an extra safety measure for changing blades.

Note: Because I don't know if you have a cord-and-plug connection, I did not take the time to look up the requirements for a device that is serving as a primary disconnect. (A cord and plug qualifies as the primary.) Well, there is a cord (needs a plug), but my intention was just to run the cord to to a box on the side of the saw and tie it into the feed from under the floor (I guess that's another question...okay?). I just want the switch to cut power. The magnetic switch will be used of course for stop start. Same deal with the jointer. The feed under the floor is THWN through PVC/LiquidTight to the box. I just want to eliminate cords and plugs but still need to cut power with something other than the magnetic switch.

[Anthony Whitesell]

Yep. Surprise! I measured one leg of the 200 while the motor was off and it was drawing 400mA. I don't know what is in the control box other than the red light. Unplugging it is getting a little annoying, but atleast it is not a difficult reach.

[Dan Friedrichs]

Yep. Surprise! I measured one leg of the 200 while the motor was off and it was drawing 400mA. I don't know what is in the control box other than the red light. Unplugging it is getting a little annoying, but atleast it is not a difficult reach.

That's only 88 watts, but, yeah, that's probably more than that little red light should be drawing.

I would double-check your current measurement (perhaps with a different meter). I have a hard time believing that little light is drawing that much...

[Rick Christopherson]

Well, there is a cord (needs a plug), but my intention was just to run the cord to to a box on the side of the saw and tie it into the feed from under the floor (I guess that's another question...okay?). I just want the switch to cut power. The magnetic switch will be used of course for stop start. Same deal with the jointer. The feed under the floor is THWN through PVC/LiquidTight to the box. I just want to eliminate cords and plugs but still need to cut power with something other than the magnetic switch.I guess a snap-switch is permitted as a primary disconnecting means as long as it is within sight of the saw. However, there are several conditions to this, the most significant is that it be rated as a disconnect for the size of the motor.

[Rod Sheridan]

Now I understand, it was a current measurement.

You don't know the power factor so you can't calculate the power.

The power factor may be as low as .15 which would put you back around 10 to 15 watts.............Regards, Rod.

[Anthony Whitesell]

Power factor of what? The motor is running, so the power factor is either 1 or 0 (don't remember exactly what the equation is). Two meters. Handheld and wall powered, 390 and 380mA respectively.

[Dan Friedrichs]

In practice, power factor is not ever going to be exactly 1 or 0.

Rod is saying that the current you are seeing on your meters may be out-of-phase with the voltage present, meaning that the true power being consumed by the load is less than V*I. The cosine of the angle between the voltage and current phasors is known as the power factor. For a purely resistive load, the power factor will be 1 (or nearly so). For a more inductive load (like a motor), the power factor could be much lower, meaning that the power drawn by the motor is actually much less than V*I.

I'm not sure the problem is really clear at this point. As I understand it:
1) You have a jointer that has a magnetic starter.
2) The magnetic starter has a small light on it to let you know that it's plugged in.
3) When the motor was off, you measured the current through one leg (how did you do this?) of the 220V circuit, and measured appx 0.4A
4) You are concerned because 220V * 0.4 A = 88 Watts, which is quite a lot of power to be wasting 24 hrs a day (I agree).

If all the above is true, I'd be concerned that something was wrong with the starter. When the motor is off, nothing in the starter (except that light) should be drawing power, either. When the motor is on, there will be some small amount of current used to energize the relay coil, but only when the motor is on. I'd consider disconnecting that little lamp and measuring the current again to see how much its contribution is.

[Anthony Whitesell]

Knowing that I was not going to actually run the machine, I pulled back the cord cap, removed the white from the plug and inserted the meter between the plug terminal and the end of the wire.

[Dan Friedrichs]

Well, that's the way to do it...

I'm really surprised - that's a lot of power for an indicator light. Maybe Rod will have another suggestion, but I'd open the switch box, take the light out of the circuit, and repeat the amp measurement. Something doesn't seem right, here...

[Chip Lindley]

Back to the OP's current drain:


Yep. Surprise! I measured one leg of the 200 while the motor was off and it was drawing 400mA. I don't know what is in the control box other than the red light. Unplugging it is getting a little annoying, but atleast it is not a difficult reach.

Understandable IF the magnetic starter is a low voltage-type with a 24V transformer in the control box. That xformer will be powered when the red light is on. Delta's LVC controls used a .040 Kva multi-tap xformer to power the 24v coil.

If your mag starter's coil uses line voltage, nothing is energized and no current should be flowing until you push Start. In this case, there may be a voltage drain somewhere besides that little pilot light.


Ok, so I ran a dedicated 240V 20A circuit under the floor and up to a box mounted on my 3HP Unisaw and DJ-20. I want to install a switch simply for disconnect...changing blades, general safety. I have two Leviton CSB2 DP 15/20A Switches that were listed as handling motor loads. I'm good?

Go whole hawg and use a commercial 30A 2-pole disconnect. They are cheap enough on eBay:

Square D Light Duty Disconnect Switch L211N 30A
181466

[George Bregar]

Go whole hawg and use a commercial 30A 2-pole disconnect. They are cheap enough on eBay:

Square D Light Duty Disconnect Switch L211N 30A
181466 Thanks Chip...I looked at my original post and it's worded poorly. To clarify, both the Uni and DJ are on their own dedicated 240V 20A circuit if that changes anything. These are 30A...why? Because of the 3HP uni? The DJ-20 is 1-1/2 HP. WHy a fused disconnect? Could I use this http://www.homedepot.com/h_d1/N-5yc1v/R-100356941/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053