110 vs 220

[Mike Henderson]

Of course there is. A 120v 1hp motor has (approximately) 10a on 1 leg. A 240v 1hp motor has (approximately) 5a on each of two legs.

You are free to use any model that works for you, but there is nothing wrong with the one I like.

Absolutely not. A 120V motor has 10 amps on both wires. There has to be two wires to complete a circuit and the same amperage flows through both wires.

Same with a 240V circuit: the same amperage flows through both wires.

I think you are calling the "hot" wire (the wire that is 120V to ground) a "leg". But even if you do, there is the same current in both wires that make up a circuit. You can't ignore the neutral wire if you want to have a working circuit. The fact that one wire is at approximately ground potential in a 120V circuit is completely immaterial to the function of the circuit.

What you're doing that's illogical is that you're only looking at one wire in the 120V circuit but you look at both wires in the 240V circuit. In any complete, single phase circuit, each wire will carry exactly the same current.

The correct statement would be that there is 10 amps flowing in the 120V circuit and 5 amps flowing in the 240V circuit.

Mike

[Phillip Gregory]

That is interesting since the resistance of the tool remains the same, the amperage drops in half when the voltage goes from 120VAC to 240VAC therefore the wires size could be smaller with 240VAC. For example, a 1½ HP motor draws 20 amps at 120VAC and 10 amps at 240VAC.

The OP was talking about running a dedicated circuit. You must use at least 14 AWG wire for that, which is good for 15 amps. He was going to run a 1 hp motor, which draws 9-10 amps on 120 volts and 5 amps on 240 volts, both of which are below 15 amps and thus both of which require the same 14 AWG branch circuit. (BTW, a 1 1/2 hp motor of typical efficiency draws around 14-15 amps on 120 volts and 7-8 amps on 240 volts. A 10/20 amp motor would be either a very inefficient 1 1/2 hp unit or a 2 hp one.)

One thing to remember when making a decision whether to run your multi-voltage tools on 120VAC or 240VAC is that the 240VAC circuit takes two circuit breakers as opposed to the one a 120VAC circuit uses. Breaker space in your panel may dictate what you can do.

There are 240 volt double-pole half-width circuit breakers out there that take up only one full space in your breaker panel. My shop's breaker panel is a GE unit and I use double-pole half-width TQHP breakers to preserve space, since the original owner used a bunch of full-width single-pole 120 V breakers to add many 120 V circuits and nearly filled up the panel with them. The only caveat to the double-pole half-width is that they "straddle" the normal dividers in the panel to hit both legs and thus leave a half-width opening above and below them. You can either use a single-pole half-width breaker for 120 V or simply put in a blank plug to block off the empty half space.

[Mike Henderson]

Another alternative that I found is a quad breaker that fits into the space of two full size breakers (this was Square-D). This one has two 120V breakers and one bonded 240V breaker. I used this when I had a full sub panel and needed to add a 240V circuit. I was able to take the space of two full size 120V breakers and add the 240V circuit while maintaining the two 120V circuits.

Mike

[Wade Lippman]

In any complete, single phase circuit, each wire will carry exactly the same current.
Mike

My 240v oven had different currents on the two legs when it was set to "broil". The difference came back on the ground. (They were the same on "bake".) You could build a 240v motor or light bulb like that if you wanted to. The difference comes back on the neutral.

It is sort of like arguing whether God exists; people will believe what they want.

[Mike Henderson]

My 240v oven had different currents on the two legs when it was set to "broil". The difference came back on the ground. (They were the same on "bake".) You could build a 240v motor or light bulb like that if you wanted to. The difference comes back on the neutral.

It is sort of like arguing whether God exists; people will believe what they want.

Except that with electricity we can deal with it with mathematics. There's no "belief" in it.

If you want to address a system with multiple loads, served with a 240V wiring with neutral and ground, we have to look at it mathematically. Let me take a simple system, a three wire system supporting two 120V circuits. If we measure the current on the neutral, we find it is the sum, with the sign (or phase) taken into account, of the current in the two loads.

But mathematically we can show that the measured current flow is actually made up of two current flows that happen to be out of phase. Let's take the situation where the two 120V loads are exactly equal, for illustration let's say 5 amps each. What will be the measured current in the neutral? It will be zero. But we know that cannot be correct because we can see each load doing work. Where is the return current? It's in the neutral wire, 5 amps each, but the two currents cancel each other out. If we shut off one of the loads, we suddenly see the remaining load (5 amps) on the neutral wire. It's always been there, but it was hidden by the other load which was out of phase with it.

This is similar to the rotating magnetic field in the field coils of a motor. Mathematically we can show that the field rotates both clockwise and counterclockwise at the same time. And that's why you can reverse the rotation of a single phase motor. The starting coil enhances the strength of the rotating magnetic field in one direction (CW or CCW) and the motor starts in that direction. Once it it rotating, we remove the starting field and the motor keeps going in that direction because it is following the rotation of the field in that direction. But in actuality, the field is rotating in both direction simultaneously.

You don't work with electricity based on your "belief", you work with electricity with mathematics, and the mathematics has been demonstrated to accurately represent real circuits.

My comment was based on simple circuits, such as the motor load that we were talking about, but it is also true in more complex circuits. You just have to look a bit deeper into the system.

Mike

[I'll give you another example of how physical measurements of current can lie to you. Suppose you have a motor sitting on a bench with no load on it. You connect the correct voltage to it and you measure the amperage into the motor. Let's say the motor is rated at 2HP. You take the voltage and multiply it by the amperage thinking you're getting watts. You convert from "watts" to HP and you find that the motor is taking over 1 HP just to spin with no load on it. How is that possible? The motor simply cannot require over 1 HP just to spin.

And that's correct: the motor cannot require more than 1HP just to spin. So we have to look deeper into the characteristics of electricity to see what's going on.

The answer is that an unloaded motor looks like a big inductor to the electrical system and that causes the voltage and the current to be out of phase. This is known as the "power factor". Current and voltage that's completely out of phase does not produce any power. Only the portion that is in phase produces power. So that motor may be taking 200 watts to spin it, while the measurements you made would indicate over 1,000 watts. Current is going through that motor and producing NO POWER.

You have to look deeper than simple physical measurements, and use mathematics to analyze the system, in order to get the correct results.]

[Julie Moriarty]

Mike, I think what Wade was saying is if you took an amp probe to the hots of the 120v wired motor and then the 240v wired motor, you will get a reading of about 10 amps on the 120v wired motor and 5 amps on each side of the 240v wired motor. If so, that would be correct. Ohms Law.

[Mike Henderson]

Mike, I think what Wade was saying is if you took an amp probe to the hots of the 120v wired motor and then the 240v wired motor, you will get a reading of about 10 amps on the 120v wired motor and 5 amps on each side of the 240v wired motor. If so, that would be correct. Ohms Law.

The point I'm making is that you would have 10 amps on both sides of the 120V motor, and 5 amps on both sides of the 240V motor. Current does not only flow through the "black" wire, it flows through the circuit - which is made up of two wires.

So if I had a 120V motor served by a white (neutral) wire and a black (hot) wire, I could put the amp meter on either wire and I'd see 10 amps.

If I had a 240V motor served by (let's say) a red (hot) wire and a black (hot) wire, I could put the amp meter on either wire and I'd see 5 amps.

A motor is served by a circuit and the same current flows in both legs of the circuit, not just in the "hot" wire.

Mike

[As I said earlier: "The correct statement would be that there is 10 amps flowing in the 120V circuit and 5 amps flowing in the 240V circuit."]

[Mike Chalmers]

So if I had a 120V motor served by a white (neutral) wire and a black (hot) wire, I could put the amp meter on either wire and I'd see 10 amps.

Yes, but only if the circuit between the white and black was complete. Calling both hot is not really an accurate description. Calling something hot implies it has its own electrical supply. In this case, it does not. It only has power when connected to the actual hot wire.

If I had a 240V motor served by (let's say) a red (hot) wire and a black (hot) wire, I could put the amp meter on either wire and I'd see 5 amps.

This is correct.

A motor is served by a circuit and the same current flows in both legs of the circuit, not just in the "hot" wire.

As I said before, in 120, the neutral only has power in it by virtue of being connected to the real hot wire.

Mike

[As I said earlier: "The correct statement would be that there is 10 amps flowing in the 120V circuit and 5 amps flowing in the 240V circuit."]

10 amps on one line in 120, 5 amps on two lines in 220 therefore 10 amps.

[Mike Henderson]

Let me address your comments in your text.

Yes, but only if the circuit between the white and black was complete. Calling both hot is not really an accurate description. Calling something hot implies it has its own electrical supply. In this case, it does not. It only has power when connected to the actual hot wire.

​You seem to be confused by the way electricity is delivered in residential installations in the United States. In the United States, the center tap of the distribution transformer is connected to ground (for safety reasons). That, however, is immaterial to the circuit.

When we look at a circuit, we look at the potential across the circuit, not the potential of any wire to ground. In a 120V circuit, there is a potential of 120V across the two circuit wires. We only call one wire "hot" because it has a potential to ground. The circuit can provide energy because current flows through the circuit (both wires), not because of any potential to ground.

Perhaps it would clarify things if you pretend for a minute that the center tap is not grounded. In that case, the circuit would be isolated from ground and no current could flow between any wire and ground. Just think, you could touch any electrical wire and you wouldn't get shocked! But there are problems which I won't go into here - the engineers who designed our system did so for safety reasons.

The net is that it takes two wires to make a circuit and the same current flows through each wire.


This is correct.



As I said before, in 120, the neutral only has power in it by virtue of being connected to the real hot wire.

No, there's really no concept of power in one wire. There is only a concept of power that can be delivered to a load via two wires.



10 amps on one line in 120, 5 amps on two lines in 220 therefore 10 amps.

No, No, No!!! Absolutely not! This is the thinking I'm trying to show is completely wrong. There will be 10 amps flowing through the 120V circuit and 5 amps flowing through the 240V circuit. In no way can you add the 5 amps on each wire of the 240V circuit to come up with 10 amps. And if you were to use that logic, you would have to say that the 120V circuit has 20 amps because there are 10 amps in each wire of the circuit supplying the motor.

If you were to take the position that the 240V circuit has 10 amps, you would then have the problem of explaining how the power just doubled (240*10 verses 120*10) but it's the same motor with the same HP. You get the same power only if, when you double the voltage, the current is reduced to half.

[John Coloccia]

Yeah, listen to Mike, guys. This is pretty basic electrical engineering. ALL current...not some, not when it's convenient, and not only with motors, but with ovens and RF circuit boards...ALL currents flow in loops. Period. If 2 Amps leave a node, 2 Amps flow back into the same node, whether by 1 wire, 2 wires, arcing through the air, or through some dummy standing in a puddle of water. Wire sizing is generally a function of current, but the POWER is a function of Current X Voltage. So for twice the Voltage, you need half the current in order to get the same power, therefore you can use smaller wires. But it's not because the wires "share" the load. The current in both wires is the same, and the configuration just doesn't matter.

I really hate the word "neutral" as it pertains to residential wiring. It's called neutral just out of convention, and it happens to be bonded to ground. There's nothing more or less neutral about that wire than the "hot" wire.

[Ken Fitzgerald]

Listen to John and Mike Henderson.

Let me demonstrate with a simple formula where P = power in watts, I= current in amps and E=voltage in volts

One of the formulas for power is: P = I x E

So at 120 vac, if you need 10 amps of current, P = 10 x 120 or P = 1200 watts of power.

So keeping the power requirement the same but using 240 vac... solving for I (current), I = P/E or I = 1200/240.... I = 5 amps

[Mike Chalmers]

Here is what I know for sure. In a 120V outlet, I can stick a knife in two of the three holes with no danger. Why? Because only one is live.

In a three prong 240V outlet, two of the holes present a danger.

I understand that the two lines in a 240v system do not add their amperage together, they are in different phases. At the end of the day, the same amount of power will be used by a 20 amp 120v system as a 10 amp 240v system.

Anyway. I bow to the obviously superior knowledge available on this forum.

[Mike Henderson]

Here is what I know for sure. In a 120V outlet, I can stick a knife in two of the three holes with no danger. Why? Because only one is live.

In a three prong 240V outlet, two of the holes present a danger.

I understand that the two lines in a 240v system do not add their amperage together, they are in different phases. At the end of the day, the same amount of power will be used by a 20 amp 120v system as a 10 amp 240v system.

Anyway. I bow to the obviously superior knowledge available on this forum.

I don't know if this will help, but imagine if the system was set up with one side of the secondary of the distribution transformer grounded instead of the center tap. You would still have three wires bringing power into your home. But let's see what the potential to ground would be on those wires.

On the side that's grounded, the potential to ground would be zero. On the center tap, the potential to ground would be 120V. On the other side of the secondary of the transformer, the potential to ground would be 240V.

Now, let's see what a 120V circuit would look like. We'd have one wire with 240V to ground and the other wire with 120V to ground. But the difference between the two wires would be 120V and that's what the load would see.

For a 240V circuit, one wire would be at ground potential, while the other wire would be at 240V to ground. You could stick a knife in the ground side of the 240V plug and you would not be shocked.

The designers chose to ground the center tap because the maximum voltage to ground is 120V. To get shocked by 240V you have to be simultaneously touching two wires, one from each side of the transformer. It's safer than the grounding I mentioned above.

But as you can see, the voltage to the load is just the difference in potential between two wires, no matter how the system is grounded.

Mike

[Rick Christopherson]

I understand that the two lines in a 240v system do not add their amperage together, they are in different phases.

Actually they are not different phases. It's single phase and it's never correct to refer to it as 2-phase. Even the commonly used expression that the two are 180 degrees out of phase is incorrect, and the result of a mathematical representation of what is actually just a minus sign. Even the minus sign is imaginary, and the result of reversing the polarity of your voltmeter by maintaining one probe on the middle of the measurement.

An analogy would be a long staircase in your home, and you are standing on the middle step. If you look to your left, the stairs are going up. If you look to the right, the stairs are going down. From that description it sounds like there are two different staircases, when in reality, it is just a single staircase and your point of reference is just in the middle of them. There are 240 steps in total, but from where you are standing there are only 120 steps to go up or 120 steps to go down. But to go all the way from the bottom to the top, it is still 240 steps.

Without an external reference, we have no idea where those steps lead. It's just 240 steps from the top to the bottom. Without a reference, those steps could be at the top floor of a highrise building or the bottom of a mine shaft, and we wouldn't know. Instead, we arbitrarily decide to put a landing at the middle step and call it the ground floor. There is nothing different about that step from any of the others. We just picked it for safety so that if you ever fell, you'd only fall half the length of the total steps.

---

Going back to reality for a moment, there is something to explain that most people, including electricians, aren't normally aware of. It is commonly understood that we tie the center-tap of a utility transformer to Earth-ground via a copper rod driven into the Earth. What most people don't realize is that if we disconnect that ground rod, the voltages on that transformer are going to literally float to nearly any voltage. They can float to any value, but in all likelihood, they're going to jump to the 7200 volts present on the primary side of the transformer. There will still be only 240 volts between the two wires, but relative to earth, their voltages will be 7200 and 6960 volts respectively.

It doesn't happen very often, but there have been cases where the center tap on a utility transformer has been completely severed, and the secondary side jumps to 7200 volts. It wreaks havoc on grounded appliances, burns houses down, and has killed a few people.

[Mike Henderson]

Rick - maybe you know the answer to this question. How is the residential distribution done in Europe where they have 220V? Is there a ground in that system? If so, where? I know there are a lot of countries so an answer for only one country, maybe the UK, would be appreciated.

Mike