Is it me or is this a bit misleading?

[PassnThru]

So I saw this on a floor heating site browsing around:

Is it true that 240V products generate more heat, heat up faster, cost less to run, and work more efficiently than 120V?

No, no, no and no. This is a big misconception that pops up in Internet forums. Let’s say you take two mat systems of the same square footage, one 120V and the other 240V. If they are designed with the same watts per square foot (the industry standard is 12 W/sf), both products will use the same amount of wattsand use the same amount of power. This is because a 120V system draws twice the amps per square foot of a 240V system, and that a 240V system draws half the amps per square foot of a 120V system. Ohm’s law tells us that when voltage decreases, amps increase proportionally. For example, let’s say you want to heat a 100 square foot area. According to the manufacturer’s specs, we know both systems use about 1200 watts, and we know the voltage, so we find the amps by using Ohm’s law: V x I = P, or P / V = I. (Numbers are approximate here):

1200 watts / 240 volts = 5.0 amps 1200 watts / 120 volts = 10 amps

They finish up with this which counts as a save I guess:

So while the 240V sounds like it has twice the power, it generates the same amount of heat (watts) per square foot as the 120V system, heats up at the same rate, and would cost about the same amount per square foot to heat the floor. And for standard power supplies of 120V and 240V, matching products are equally efficient (120V to 120V vs. 240V vs. 240V).

I know what they are trying to say and I believe they are right with my limited knowledge - but I hope the bolded portion was just an editing mistake.

 

[Bert_]

If the watts are the same a 240v heater draws 1/2 the amps of a 120V

2000W/120V=16.6A

2000W/240V=8.3A

Same amount of power used, one method just lets you use smaller wire and control gear.

 

[PassnThru]

Yep. I still get confused sometimes. I still have trouble with watts versus amps. I still want to think 240 is the same as 120 with two wires sharing the load.
So - If I understand correctly, I can double the load on my 100 amp subpanel in my garage if I only run 240 devices? Or is the fact that it's a 240 subpanel even it out?

 

[Glacial_Speed]

No, you can't get something for nothing. nice try

Your panel is already 240V (at 50 amps). Or you could think of it as 2 lines of 120V, (50 amps + 50 amps).

Maybe think of the transformer out on the street.......2 hot lines with a center tapped ground....

 

[Mr. T]

v=ir

 

[PhysicsDude]

Yep. I still get confused sometimes. I still have trouble with watts versus amps. I still want to think 240 is the same as 120 with two wires sharing the load.
So - If I understand correctly, I can double the load on my 100 amp subpanel in my garage if I only run 240 devices? Or is the fact that it's a 240 subpanel even it out?

No, think of 240V appliances as two 120V appliances stuck together. Or a 120V appliance as half a 240V appliance.

If you have a 1200W 120v heater on phase 1 (first breaker spot) and a 1200W 120v heater on phase 2 (2nd breaker spot), that's the same as running a single 2400W 240v heater. Each 120v heater would draw 10 amps, which is 10 amps on each phase. The 240v heater draws 10 amps, but its draw is across both phases, so its the same power draw, and the same amperage on the wires from the perspective of the electric company.

Don't worry, it can be a somewhat confusing subject. About half of the Journeyman Electricians (in Texas) I've asked don't truly understand the concept. They think that 277v lights are twice as efficient as 120V lights, because a fixture will draw about half the amperage on 277v compared to 120v. A lot of people don't understand amperage vs. wattage. And don't even get me started on watts vs. VoltAmps (VA)!

 

[TRWham]

There is another variable in the case of resistance heat- impedance. If you just change the voltage across the same heater, it will draw more current with higher voltage. At the same time, it will produce more heat and consume more power. Power is equal to voltage times current, and also equal to the current squared times the resistance (impedance). So, the 240 V heater will need to have a higher impedance (by a factor of 4) than the 120 V system to produce the same heat at half the current.

 

[PassnThru]

So now I'm seeing conflicting information.
My panel capacity (ignoring physical size) is measured in amps. At 100% load versus capacity I could run ten devices that draw 1200W at 120V or 10 amps. If I use a 240V device that draws 1200W then I am at 5 amps so I can run twice as many.
If that is correct, I would assume that would be because of the lesser load on the wires feeding my panel which would increase the capacity.
Or does my logic fail because I would have to use a 10 amp breaker in either case?

 

[cmandp]

v=ir

or an extension of Ohms law
Power (Watts)=iv

So if you double the voltage (240 vs. 120) while keeping the power output the same you half the current.

1800W = 15A*120V or

1800W = 7.5A*240V

This doesn't take AC power factors into account

 

[shaggyant]

So now I'm seeing conflicting information.
My panel capacity (ignoring physical size) is measured in amps. At 100% load versus capacity I could run ten devices that draw 1200W at 120V or 10 amps. If I use a 240V device that draws 1200W then I am at 5 amps so I can run twice as many.
If that is correct, I would assume that would be because of the lesser load on the wires feeding my panel which would increase the capacity.
Or does my logic fail because I would have to use a 10 amp breaker in either case?

If im reading you right the answer is no. Basically if you have a 100 amp 240v breaker feeding a panel then you have two opposing legs of 120v at 100 amps each. You get 24,000 watts total whether it’s at 240v or 120v.

It’s one 240v 60hz wave form coming into your house. The breakers are either tapping in peak to peak (240v) or peak to neutral (120v).

 

[Angelfire]

Generally and for real world use in a home, you won't save energy using one over the other (voltages). However, if you are in a shop or factory that is loaded with motors, there may be some advantage to the higher voltage. A 240V motor will start up a little easier than a 120V which translates into a small efficiency gain. Another advantage of the higher voltage is circuits can become cheaper to install as wire can be smaller.
Cheers.

 

[TRWham]

If im reading you right the answer is no. Basically if you have a 100 amp 240v breaker feeding a panel then you have two opposing legs of 120v at 100 amps each. You get 24,000 watts total whether it’s at 240v or 120v.

It’s one 240v 60hz wave form coming into your house. The breakers are either tapping in peak to peak (240v) or peak to neutral (120v).

Peak is more like 340 V or 170 V. 240 and 120 are the root mean square (RMS) values for voltage.

 

[PassnThru]

If im reading you right the answer is no. Basically if you have a 100 amp 240v breaker feeding a panel then you have two opposing legs of 120v at 100 amps each. You get 24,000 watts total whether it’s at 240v or 120v.

It’s one 240v 60hz wave form coming into your house. The breakers are either tapping in peak to peak (240v) or peak to neutral (120v).

So - thinking logically about that. A 100 amp 240 breaker touches both feeds. So there is a 200 amp potential there but only a 100 amp breaker is needed because the feeds are out of phase with each other thus the breaker never sees more than a 100 amp potential at any given moment. Is that reasonable?

 

[rlitman]

So now I'm seeing conflicting information.
My panel capacity (ignoring physical size) is measured in amps. At 100% load versus capacity I could run ten devices that draw 1200W at 120V or 10 amps. If I use a 240V device that draws 1200W then I am at 5 amps so I can run twice as many.
If that is correct...

It is, and it isn't. Your 100A breaker is nominally rated for 100 amps of current, REGARDLESS of the voltage (not that it will trip at 100A, or that it should be used at 100A for continuous loads; those are issues for another post).

Anyway, the part you're missing is that the 240V breaker has two poles, while your 120V circuits are on one pole. IF you happened to manage to put all of your hypothetical 120V 10A loads on the same pole, then you could reach your breaker's 100A rating with just 10 circuits. However, since each pole of a single phase panel connects to every other breaker, 10 consecutive 120V circuits drawing 10A each would only be pulling 50A from each pole of your 240V main.

Here, I must note that 240V loads are naturally balanced between the poles, whereas 120V loads are not always well balanced.

 

[rlitman]

So - thinking logically about that. A 100 amp 240 breaker touches both feeds. So there is a 200 amp potential there but only a 100 amp breaker is needed because the feeds are out of phase with each other thus the breaker never sees more than a 100 amp potential at any given moment. Is that reasonable?

NO. There is no 200A current potential on a 100A circuit (short circuits notwithstanding). They don't add up like that. Here, you need to study up on how the neutral works on a single phase system.

 

[exranger06]

So now I'm seeing conflicting information.
My panel capacity (ignoring physical size) is measured in amps. At 100% load versus capacity I could run ten devices that draw 1200W at 120V or 10 amps. If I use a 240V device that draws 1200W then I am at 5 amps so I can run twice as many.
If that is correct, I would assume that would be because of the lesser load on the wires feeding my panel which would increase the capacity.
Or does my logic fail because I would have to use a 10 amp breaker in either case?

You have it correct. The whole point of having appliances that run on 240V is so you can use a high amount of wattage while still keeping the current relatively low. If you replace all of your 120V appliances with 240V appliances (assuming the wattage consumed is the same) you will have half the amount of current going through the panel and wires.

This is one of the reasons utilities distribute electricity at such high voltages - you have thousands of people getting their electricity from a single wire, each person using perhaps 1,000+ watts at the same time. How does the wire not burn up? High voltage. Back in the day, it was common for utilities to distribute electricity at around 2,400 V or 4,000 V. Then technology progressed and new appliances and electronics were invented, and everyone started using more electricity. Utilities had to upgrade to 13,800 V or higher voltages to handle the additional wattage. Same size wires that can carry the same amount of current as before, but they more than doubled the amount of wattage they can supply.

 

[cybrdyke]

Commercial lighting is nearly all 277 volts simply because you can hang more fixtures on a circuit using smaller wires, and smaller conduits, due to reduced current. The cost savings over using 120 volt circuits is huge.
No difference in efficiency, though.
CD

 

[Norcal]

Commercial lighting is nearly all 277 volts simply because you can hang more fixtures on a circuit using smaller wires, and smaller conduits, due to reduced current. The cost savings over using 120 volt circuits is huge.
No difference in efficiency, though.
CD

If a building has a 480Y/277V service, transformers are required for 120 volt loads, or 208V for that matter so it makes sense to have 277V lighting. So there is some gain in efficiency not having to pay for extra transformer capacity & losses.

 

[TractorJeff]

I've never studied a home electric meter but assuming it will spin whether you are on one leg or two?
This ties in with balanced loading.

 

[PassnThru]

NO. There is no 200A current potential on a 100A circuit (short circuits notwithstanding). They don't add up like that. Here, you need to study up on how the neutral works on a single phase system.

Potential was probably the wrong word - if I am drawing 75 amps off of one phase how much can I draw off the other phase before the breaker trips (assuming 100 amps is a hard stop).

 

[rlitman]

Potential was probably the wrong word - if I am drawing 75 amps off of one phase how much can I draw off the other phase before the breaker trips (assuming 100 amps is a hard stop).

I think I see what you're asking (and I'll try to answer that, because the real answer to your literally interpreted question involves "breaker trip curves", and may leave you way more confused).

You cannot just add up 75 + 125 to equal 200, and assume that's the same as 100 + 100. This is where I said "balance" comes into play. Neither leg should be above 100. Period. That's why it says 100 on the handle.

The difference in current between the two legs should be carried by the neutral (or if something is wrong, the ground).

So, if you have 100A on one leg and 100A on the other, the neutral should have zero current. If you have 100A on one leg, and 0A on the other, the neutral should have 100A. In any case, the neutral (on a single phase system) can never carry more current than either leg, so it is not protected by its own breaker.

And back to your example with 75A on one leg. The other leg could still be no more than 100A, and then you'd have 25A of neutral current.

 

[PassnThru]

I think I see what you're asking (and I'll try to answer that, because the real answer to your literally interpreted question involves "breaker trip curves", and may leave you way more confused).

You are correct - that would confuse me more.

I just backed up to the breaker to understand after being told I couldn't put twice as many 240V loads on a main breaker if they drew half the amps for the same wattage as the equivalent 120V device.
My feeble mind tells me that a 100 amp main breaker will feed 10 1200W loads at 120V and 20 1200W loads at 240V. I know the numbers aren't applicable in the real world since I'm not sure if anyone sells a 5 amp 240V breaker. But for theory, 10 10A breakers versus 20 5A breakers. If that's not the case then the number on the 240 breaker isn't the number I'm supposed to use when I figure out how close I am to maxing out a panel.

 

[rlitman]

... a 100 amp main breaker will feed 10 1200W loads at 120V and 20 1200W loads at 240V...

Again, no. You're conflating amps and watts.

 

[PassnThru]

Again, no. You're conflating amps and watts.

But a panel is measured in amps - breakers are marked in amps - not watts. I guess that's my main hangup.

I'll do some research and not drag this out anymore.
Thanks for the feedback.

 

[rlitman]

Yes, the panel and breakers are in amps, which is a measure of current. Your heat loads are in watts, which is a measure of power. Roughly speaking, volts times amps equals watts (or as stated above, P=IV).

So, your 1200W load draws 10A at 120V or 5A at 240V. But let's make this simpler by using TWO loads.

Two 1200W loads at 120V draw 10A each, HOWEVER, if they are on adjacent breakers, 10A will come from each pole on the main.

Now, two 1200W loads at 240V draw 5A each, but both draw 5A from both poles.

So, in both cases, you have 2400W of load, and are pulling 10A from each pole on the main.

Is it starting to make sense? If not, here's a place to start:
https://www.khanacademy.org/science/physics/circuits-topic/modal/v/circuits-part-1

 

[PassnThru]

Yes, the panel and breakers are in amps, which is a measure of current. Your heat loads are in watts, which is a measure of power. Roughly speaking, volts times amps equals watts (or as stated above, P=IV).

So, your 1200W load draws 10A at 120V or 5A at 240V. But let's make this simpler by using TWO loads.

Two 1200W loads at 120V draw 10A each, HOWEVER, if they are on adjacent breakers, 10A will come from each pole on the main.

Now, two 1200W loads at 120V draw 5A each, but both draw 5A from both poles.

So, in both cases, you have 2400W of load, and are pulling 10A from each pole on the main.

Is it starting to make sense? If not, here's a place to start:
https://www.khanacademy.org/science/physics/circuits-topic/modal/v/circuits-part-1

No - that confuses me more. Can you reread and make sure you typed that right?

 

[PassnThru]

Two 1200W loads at 120V draw 10A each, HOWEVER, if they are on adjacent breakers, 10A will come from each pole on the main.

Now, two 1200W loads at 120V draw 5A each, but both draw 5A from both poles.

This specifically. Was the second sentence supposed to be 240?

 

[rlitman]

This specifically. Was the second sentence supposed to be 240?

Whoops, yes. Just edited to reflect that. See, so you are getting it now.

 

[PassnThru]

Whoops, yes. Just edited to reflect that. See, so you are getting it now.

You were trolling me weren't you

Joking - appreciate the patience.

 

[PassnThru]

I will roll that around in my mind for a while while I work on my current electrical project tonight.

Don't worry - it's 12V DC

 

[PNWguy]

So now I'm seeing conflicting information.
My panel capacity (ignoring physical size) is measured in amps. At 100% load versus capacity I could run ten devices that draw 1200W at 120V or 10 amps. If I use a 240V device that draws 1200W then I am at 5 amps so I can run twice as many.
If that is correct, I would assume that would be because of the lesser load on the wires feeding my panel which would increase the capacity.
Or does my logic fail because I would have to use a 10 amp breaker in either case?

Your logic fails, but for the reason you think.

Your 1200W 120v 10amp device runs all that power through one hot wire and one breaker.
Your 1200W 240v 10amp device runs all that power through two hot wires and two breakers (or a double breaker). Each hot wire is 120V at 5 amps, but there's two of them.

Just think of a 240 circuit as 2 120v circuits and it should be more clear.

 

[PassnThru]

Your logic fails, but for the reason you think.

Your 1200W 120v 10amp device runs all that power through one hot wire and one breaker.
Your 1200W 240v 10amp device runs all that power through two hot wires and two breakers (or a double breaker). Each hot wire is 120V at 5 amps, but there's two of them.

Just think of a 240 circuit as 2 120v circuits and it should be more clear.

I've been busy this weekend so haven't given it a lot of thought. To your point yes, the 120V 10 amp device runs on one wire on a 10 amp breaker (probably a 15 or 20 amp breaker but let's assume full loads and perfectly matched breakers). The 240 runs on two wires, 5 amps each. So do we still need a 10 amp breaker or a 5 amp breaker?
So a question - let's say I buy an air compressor that can be wired 120 or 240. Let's assume that at startup a 20A 120V circuit is barely adequate. What would my 240V circuit look like?

 

[sberry]

You could uss the same wire, it would be only 1/2 loaded which wod reduce voltage drop some. It would give a 20a breaker some ore headroom, reduce the tendancy to trip.

 

[American Locomotive]

You're really putting too much thought into this. Think of it this way:

120V loads: Power comes in through the breaker and returns through the neutral.
240v loads: Power comes in through one breaker, and returns through the other breaker.

So if you have a 100A main, and only 120V loads, you can have up to 200A worth of 120V loads (provided they're split evenly between each side of the main breaker). This is because the power from each 120v load returns through the neutral, not the other breaker. (what's actually going on is a bit more complicated than this, but for this conversation, it's a good enough explanation)

If you have a 100A main, and only have 240V loads, you can have up to 100A worth of 240v loads. This is because the 100A from your loads comes out one breaker, and returns through the other.

The total "power" you can get from the breaker is the same in either scenario. Where 240v comes in handy, is that since it has twice the voltage, you can get twice the power through a given wire size. 12 gauge wire can only support 20A. That's only 2400 watts on 120V, but 4800 watts at 240V!

 

[PassnThru]

You could uss the same wire, it would be only 1/2 loaded which wod reduce voltage drop some. It would give a 20a breaker some ore headroom, reduce the tendancy to trip.

This has been my line of thought all along but...

You're really putting too much thought into this. Think of it this way:

120V loads: Power comes in through the breaker and returns through the neutral.
240v loads: Power comes in through one breaker, and returns through the other breaker.

So if you have a 100A main, and only 120V loads, you can have up to 200A worth of 120V loads (provided they're split evenly between each side of the main breaker). This is because the power from each 120v load returns through the neutral, not the other breaker. (what's actually going on is a bit more complicated than this, but for this conversation, it's a good enough explanation)

If you have a 100A main, and only have 240V loads, you can have up to 100A worth of 240v loads. This is because the 100A from your loads comes out one breaker, and returns through the other.

This is what I was asking earlier - do 120 versus 240 loads affect my panel capacity. Although I did not know the neutral did not serve the same function between the two. Why is there a neutral on a 240 circuit if it doesn't function as a return? Is it because some 240 devices pull 120 off the circuit for certain functions?

Too much thought? You can never put too much thought into something if you don't understand it. I've never liked an explanation of 'that's just how it works'. Once you truly understand something then you can answer questions you haven't thought to ask yet

 

[American Locomotive]

Not all 240v devices require a neutral. Only ones that have mixed 240v and 120v loads.

 

[PassnThru]

Not all 240v devices require a neutral. Only ones that have mixed 240v and 120v loads.

I have not run into one yet and thus have learned something - I would guess that most consumer appliances need one.
You have to give me credit though for predicting the answer - when I thought about it that was the obvious answer based on what I knew to that point.
That would also help explain why rlitman said " Here, you need to study up on how the neutral works on a single phase system."

 

[Bert_]

I have not run into one yet and thus have learned something - I would guess that most consumer appliances need one.

Very few 240V devices need a neutral. Stove and dryer are probably the most common because of clocks and timers that run on 120V.

Other 240V stuff, air compressor, welder, electric heaters, only take 2 hots and a ground for safety.

 

[rlitman]

Very few 240V devices need a neutral. Stove and dryer are probably the most common because of clocks and timers that run on 120V.

Other 240V stuff, air compressor, welder, electric heaters, only take 2 hots and a ground for safety.

Lots of stoves use it. Years ago, my parents had a stove that came with 120V convenience outlets (it predated GFCIs). Since it had a 3-wire plug, the neutral shared the wire with the ground. Because of that, you'd feel a tingle if you touched it while running anything plugged into those outlets. I've since upgraded it to 4-wire.

 

[wyliesdiesels]

Lots of stoves use it. Years ago, my parents had a stove that came with 120V convenience outlets (it predated GFCIs). Since it had a 3-wire plug, the neutral shared the wire with the ground. Because of that, you'd feel a tingle if you touched it while running anything plugged into those outlets. I've since upgraded it to 4-wire.

That means you had a poor(high resistant) neutral connection either at the stove terminal block or at the panel.

There are thousands of existing 3-wire 120v/240v stove installations, where the frame is bonded to neutral, that do not shock people.