240 Plug/Receptacle choices?

[Chris Padilla]

Patrick,
Sorry if this seems like piling on, but something was bugging me and it took a few minutes to register. If you pull 12/3 wire (H-H-N-G) from the panel and are operating in 120V mode on your switch with the 2 resulting 120V circuits each pulling 20A (:: 2 tools running), the breaker will support this just fine. However, the neutral WILL be conducting and trying to carry 40A. And the neutral does NOT have current protection on it.

I realize you may feel you'll never load the circuit like this, but it will never meet code, pass inspection, or be safe. I strongly recommend you do not pursue this option.

--Malcolm

Malcolm,

If I'm following things and interpreting them correctly (dubious sometimes with the written word), running X/3 wire off a double-pole breaker should be fine. Because the hots are out of phase 180 degrees, the neutral between the two hots will carry current that is the DIFFERENCE (not ADDITION) between the currents of the two hot legs. So if hot leg 120V A is pulling 20 A and hot leg 120V B is pulling 10A, the neutral will carry 10 A. If both legs are pulling the same current, then the neutral will carry no current at all.

[Patrick Whitehorn]

Malcolm,
That does not seem like piling on at all - I appreciate the consideration. With that in mind, isn't that how it works for 240V 20A clothes dryers, etc? They have two hot legs, one neutral, and one ground - wouldn't the neutral be forced to carry up to 40A back in that situation too?

And if it turns out that this is the wrong way to do it - excellent! <laughs> I will have learned ... and I can return that spectacularly expensive DPDT switch. <grins>

[Chris Padilla]

... if these are both true ... then why do my 240V machines' power cords only have three conductors? (hot, hot, and not-hot <- to avoid the neutral versus ground issue) If the neutral does carry current (return voltage) and the ground does not ... and the ground is required ... but we only have one ... doesn't that mean that the ground (as you said before hot-hot-ground) is effectively carrying current? And then cannot be used to 'ground' the chassis? <scratches head>

240V is fed into our houses from the pole/transformer out in the street. The neutral is split in the middle of the transformer to pull out 120V legs of opposite phase to each other. At this entry point into your house, the Main Panel, the neutral and ground (earth ground) are connected together. They are one and the same...AT THE MAIN PANEL.

From the Main Panel into your house, things change. The neutral and the ground are now separated everywhere in your house. The neutral will now serve to carry current BACK to the main panel for items that require 120 V. For items that require 240 V, the current simply returns on the other hot leg. The ground should not have any current on it. It is there for SAFETY. Should a hot wire or even the neutral ever touch the metal body of an item (like the metal casing of a motor), the ground will now be there to safely carry that current back to the Main Panel. If that ground is not there, guess where the current might flow to get back to the Main Panel (current always flows in a loop...it must return back to its source...always)? It could flow through YOU should you come into contact with that metal case. You don't want the metal case of some piece of equipment sitting at 120 or 240 V because the current is just waiting for a nice low-resistance path back to the Main Panel and/or earth and YOU could become that wire instead of the ground wire.

Does this help?

[Dan Friedrichs]

... if these are both true ... then why do my 240V machines' power cords only have three conductors? (hot, hot, and not-hot <- to avoid the neutral versus ground issue) If the neutral does carry current (return voltage) and the ground does not ... and the ground is required ... but we only have one ... doesn't that mean that the ground (as you said before hot-hot-ground) is effectively carrying current? And then cannot be used to 'ground' the chassis? <scratches head>

Your machines have 3 wires because they have a hot, hot, and ground. All the current is flowing on the hot wires. There is no neutral connection at all. If you were to follow that 3-wire cord into the motor junction box, you'll see that the two hot wires go to the motor windings and the ground goes to the chassis. There is nowhere to connect a neutral (and you don't need it).

In a 4-wire connection, such as an electric clothes dryer, the inclusion of the neutral allows you to run 120V control circuits or light bulbs, etc.

Your machines do not have ancillary 120V loads. They do not need a neutral. They DO need a ground.

(afterall, what good is a 20A switch that is only really 'good' to 16A?)

It is only good for a 16A motor load. It is good for a 20A resistive (lighting, etc) load. When you disconnect a motor, inductance of the motor and back EMF generate a large voltage that is often enough to arc or weld the contacts of switches. Thus, switches intended for motor loads have physically larger space between the contacts, open more "snappily", etc. It is common for a switch to be derated for a motor load.

[Malcolm McLeod]

Malcolm,

If I'm following things and interpreting them correctly (dubious sometimes with the written word), running X/3 wire off a double-pole breaker should be fine. Because the hots are out of phase 180 degrees, the neutral between the two hots will carry current that is the DIFFERENCE (not ADDITION) between the currents of the two hot legs. So if hot leg 120V A is pulling 20 A and hot leg 120V B is pulling 10A, the neutral will carry 10 A. If both legs are pulling the same current, then the neutral will carry no current at all.

Chris,
In life, I deal with industrial control system's integration. Lots of control voltages (24VDC) for devices, interlocks, and logic; some 3ph power switching and monitoring - - but way shy on electrical theory. Along the way I've had my head in more electrical control panels than I care to count, and (I hope) picked up a basic understanding of electrical applications (especially safety). So all in - - I've never measured current in the neutral of a circuit like Patrick describes. Can't even swear how the hot legs in typical 240V USA-household power are 'phased' - it's never been important. Neither do I know for certain the result of 'combining' 2 phases in a single wire (...saying it like that makes it sound remarkably like a short).

You've certainly made me think about it and I will have to fall back on the basic application of power here. In the worst case scenario I posed, assuming PF=1, 120V x 20A x 1 = 2.4kW, so the hot wires HAVE to be carrying a combined 4.8kW of power. The neutral has to carry this as well.

In spite of my ignorance, I am betting Patrick's switch-scheme is a no-no in any code in use today. ...I wouldn't do it in any situation I am involved in - personal or professional.

[Malcolm McLeod]

Malcolm,
That does not seem like piling on at all - I appreciate the consideration. With that in mind, isn't that how it works for 240V 20A clothes dryers, etc? They have two hot legs, one neutral, and one ground - wouldn't the neutral be forced to carry up to 40A back in that situation too?

And if it turns out that this is the wrong way to do it - excellent! <laughs> I will have learned ... and I can return that spectacularly expensive DPDT switch. <grins>

Not how a dryer does it. The neutral only carries the power dissipated by the 120V control circuit (lights and timers).

Edit: Dan, I am a couple of posts behind - - you got there first!

[Patrick Whitehorn]

Chris,
Thank you for continuing to try and help me. <laughs>

240V is fed into our houses from the pole/transformer out in the street. The neutral is split in the middle of the transformer to pull out 120V legs of opposite phase to each other. At this entry point into your house, the Main Panel, the neutral and ground (earth ground) are connected together. They are one and the same...AT THE MAIN PANEL.

100% with you here ... until you say at the main panel. Every white wire in the house runs back to the same place in the panel - a screw on the buss bar just beside the screw holding down the ground wire. So, unless there is a break in that continuity within the house's wiring, doesn't that mean that they are the same - electrically speaking, at earth potential - throughout the entire house while the circuit is not carrying current? And, if the circuit is carrying current, only then is that specific wire not at earth potential (until it is dragged down at the buss bar in the panel)?

From the Main Panel into your house, things change. The neutral and the ground are now separated everywhere in your house. The neutral will now serve to carry current BACK to the main panel for items that require 120 V. For items that require 240 V, the current simply returns on the other hot leg.

Elsewhere in this thread you said this:

Because the hots are out of phase 180 degrees, the neutral between the two hots will carry current that is the DIFFERENCE (not ADDITION) between the currents of the two hot legs. So if hot leg 120V A is pulling 20 A and hot leg 120V B is pulling 10A, the neutral will carry 10 A. If both legs are pulling the same current, then the neutral will carry no current at all.

This makes sense - your part about the current returning on the other hot leg does not ... or do you mean that instead of returning it is cancelled out? (Being 180* out of phase, after all)

The ground should not have any current on it. It is there for SAFETY. Should a hot wire or even the neutral ever touch the metal body of an item (like the metal casing of a motor), the ground will now be there to safely carry that current back to the Main Panel. If that ground is not there, guess where the current might flow to get back to the Main Panel (current always flows in a loop...it must return back to its source...always)? It could flow through YOU should you come into contact with that metal case. You don't want the metal case of some piece of equipment sitting at 120 or 240 V because the current is just waiting for a nice low-resistance path back to the Main Panel and/or earth and YOU could become that wire instead of the ground wire.

100% with you here and the reason that I want to ensure I do it correctly! All of what you said above makes me think that the HHG is equivalent to HHN - electrically - because of what you say above. Either the way, two hots supply inbound current and one carries the remaining current "back". That wire is also responsible for grounding the machine. If the two hot legs are at equal potential, they cancel each other out and the third wire can be used to safely ground the machine chassis, etc. If they are not then that remaining wire carries the potential difference "back" and supplies that potential to the chassis of the machine ... which seems potentially unsafe. (Yes, the wires are different colored, but a copper conductor carrying return voltage to the panel ends at the same place, regardless of color - the buss bar connected to the eight foot metal spikes - earth. <thinks> Right?

Does this help?

Any time I can learn, I appreciate it - thank you!

[Chris Padilla]

Chris,
In life, I deal with industrial control system's integration. Lots of control voltages (24VDC) for devices, interlocks, and logic; some 3ph power switching and monitoring - - but way shy on electrical theory. Along the way I've had my head in more electrical control panels than I care to count, and (I hope) picked up a basic understanding of electrical applications (especially safety). So all in - - I've never measured current in the neutral of a circuit like Patrick describes. Can't even swear how the hot legs in typical 240V USA-household power are 'phased' - it's never been important. Neither do I know for certain the result of 'combining' 2 phases in a single wire (...saying it like that makes it sound remarkably like a short).

You've certainly made me think about it and I will have to fall back on the basic application of power here. In the worst case scenario I posed, assuming PF=1, 120V x 20A x 1 = 2.4kW, so the hot wires HAVE to be carrying a combined 4.8kW of power. The neutral has to carry this as well.

In spite of my ignorance, I am betting Patrick's switch-scheme is a no-no in any code in use today. ...I wouldn't do it in any situation I am involved in - personal or professional.

Look up "Edison wiring" and read up a bit. You may find it enlightening.

[Patrick Whitehorn]

Dan,

It is only good for a 16A motor load. It is good for a 20A resistive (lighting, etc) load. When you disconnect a motor, inductance of the motor and back EMF generate a large voltage that is often enough to arc or weld the contacts of switches. Thus, switches intended for motor loads have physically larger space between the contacts, open more "snappily", etc. It is common for a switch to be derated for a motor load.

AH-HA! That makes sense. So, regardless of the advisability of trying to do what I would like to do (as explained above), this switch is the wrong type for what I want to use it for <frowns> Thank you for helping me to get to the bottom of that.

[Patrick Whitehorn]

Malcolm,
In the midst of all of this, I managed to forget to reply to this - sorry.

On you original question, I use the locking plugs. I just feel better about them in use.

Agreed - I am definitely leaning towards the locking plugs, regardless of the other parts of the question. Thanks!

Also, I'm fuzzy on the economics of this. You didn't describe your wire run distances, panel/sub-panel, available breaker space, etc., but for the cost of the DPDT switch you can buy 250ft of 12/2 wire at BORG. That's a LOT of dedicated 120V circuits, plus you still have the dedicated 240V circuit...?

Part of what I want to accomplish is saving space. My main panel is 150A and is close to full - if I can install a single (for now) 20A double-pole breaker and effectively get three circuits (one 240V part time or two 120Vs part time), then it seems worth it. That said, it seems I might not be able to do what I was thinking ... at least not in a safe fashion. <laughs>

[Malcolm McLeod]

... two hots supply inbound current and one carries the remaining current "back". That wire is also responsible for grounding the machine. If the two hot legs are at equal potential, they cancel each other out and the third wire can be used to safely ground the machine chassis, etc. If they are not then that remaining wire carries the potential difference "back" and supplies that potential to the chassis of the machine ... which seems potentially unsafe. (Yes, the wires are different colored, but a copper conductor carrying return voltage to the panel ends at the same place, regardless of color - the buss bar connected to the eight foot metal spikes - earth. <thinks> Right?

The difference in a neutral and ground is that one caries current and the other (ground) DOES NOT. If the ground is conducting, there IS a problem.

I often heat the analogy of water to electrical: Voltage = pressure; current = flow rate; wires = pipes; power = power. Again back to the 2x 120V circuits, if it's water you have 120psi x 20gpm in each 'hot' pipe. So 40gpm has to return via the 'neutral' pipe. Subject to the vagaries of what I learn in 'Edison wiring'

[Chris Padilla]

100% with you here ... until you say at the main panel. Every white wire in the house runs back to the same place in the panel - a screw on the buss bar just beside the screw holding down the ground wire. So, unless there is a break in that continuity within the house's wiring, doesn't that mean that they are the same - electrically speaking, at earth potential - throughout the entire house while the circuit is not carrying current? And, if the circuit is carrying current, only then is that specific wire not at earth potential (until it is dragged down at the buss bar in the panel)?

You could argue they are the same; the electrons don't care. Their purposes, however, are different as I've explained. They also go by different names so they must be different then, right? I think as long as you understand the similarities and differences, you should be fine.

This makes sense - your part about the current returning on the other hot leg does not ... or do you mean that instead of returning it is cancelled out? (Being 180* out of phase, after all)

Only the 120 V are 180* out of phase with each other because there are two of them. There is only one 240 V circuit and so the current simply returns on the other wire (or other hot leg). We only have hot legs BECAUSE of the neutral otherwise we wouldn't even bother needing to identify it as such. Clearer?

Think of two car batteries, each puts out 12 V. There is a + and - terminal for each one. Let's pretend that the 12 V is really 120 V. So if we put the two batteries in series: + battery 1 - connects to + battery 2 -. If you measure from the + of battery 1 to the - of battery 2, you get 24 V (or 240 V in our pretend world). Now the neutral connects to the - of battery 1 AND to the + of battery 2 (they are the same point). Now measure from the neutral to battery 1 + OR battery 2 -. You get 120 V. In this scenario, battery 1 + would be a hot leg and battery 2 - would be the other hot leg. So in an item using 240 V, the current simply flows from one hot leg to the other hot leg...the neutral isn't used.

100% with you here and the reason that I want to ensure I do it correctly! All of what you said above makes me think that the HHG is equivalent to HHN - electrically - because of what you say above. Either the way, two hots supply inbound current and one carries the remaining current "back". That wire is also responsible for grounding the machine. If the two hot legs are at equal potential, they cancel each other out and the third wire can be used to safely ground the machine chassis, etc. If they are not then that remaining wire carries the potential difference "back" and supplies that potential to the chassis of the machine ... which seems potentially unsafe. (Yes, the wires are different colored, but a copper conductor carrying return voltage to the panel ends at the same place, regardless of color - the buss bar connected to the eight foot metal spikes - earth. <thinks> Right?

Just be careful that you grasp how the 120 V is derived from the 240 V--some of which you write tells me you don't quite 'get it' just yet. I hope my example clears it up. The neutral is expected to carry current in a normal situation. The ground does not in a normal situation. Only 120 V circuits 'care' about the neutral but the whole system 'cares' about the ground. Also keep in mind that current flows...voltage does not.

[David L Morse]

The difference in a neutral and ground is that one caries current and the other (ground) DOES NOT. If the ground is conducting, there IS a problem.

I often heat the analogy of water to electrical: Voltage = pressure; current = flow rate; wires = pipes; power = power. Again back to the 2x 120V circuits, if it's water you have 120psi x 20gpm in each 'hot' pipe. So 40gpm has to return via the 'neutral' pipe. Subject to the vagaries of what I learn in 'Edison wiring'

You'll find what you need to know about multiwire branch (MWB) circuits here.

The problem with your analogy is that your pipes are in parallel. The MWB loads are in series.

[Malcolm McLeod]

Look up "Edison wiring" and read up a bit. You may find it enlightening.

Assuming you meant at Practical Machinist? Interesting! Indeed it is enlightening too!! You could even say a very unique way to explain things. But I'm still NOT going to cut the neutral on Patrick's switch. ...Sorry, just can't do it.

[Malcolm McLeod]

I was hoping you'd find this and bail me out.