220 vs 110 revisited

[crokett]

Ok, so I always understood that W=V*A and given that, if you could convert a tool to 220V then the amps carried on each leg is cut in half. Given that, the wire sizes used does not have to be as large as it does for 110V. I saw a post (actually a few) on another forum that stated that was not true. Amps is amps whether it is 110 or 220V. There was no explanation given. One of the posters says he is an EE. So...

assuming I have a 100W light bulb that I can rewire for 220V. Plugging those values into our formula and solving for A we get:

100=110*A
100/110=A
1=A (or close enough)

Now using 220V:
100=220*A
100/220=A
.5=A(or close enough)

So what am I missing?

[eccentrictinkerer]

Your math is correct!

Amps ain't amps.

Watts is watts!

[Uncle Cracker]

Your understanding of the relationship is correct, David, even though Ohm's Law for AC circuits is actually a bit more complex than your example. But the fact remains that doubling the voltage will approximately half the relative amperage as long as the work to be done is the same. This is why the big loads in your house all run on 220V, as the wire size needed to carry the increased amps at 120V would be unmanageable.

[LCHIEN]

a lot of things on the internet should not be believed. Do you believe that?

[Tom Slick]

In rough non-electrical terms:

Watts is power
Voltage is pressure
Amps is flow

It takes an amount of power to run a machine, spin a blade, vibrate a sander, etc. That machine always needs that same power to get it's job done. How you get that power is either through lower pressure and more flow of "juice" or higher pressure and lower volume.

1hp is 750w by definition, you can use any combination of volts and amps to get there.
12v X 62.5a = 750w
480v X 1.6a = 750w

[phi1l]

Your theory and math are correct, but I you are using imperfect conductors. You need about the same sized wires in both cases because the energy gradient required to drive the current though the wire will be higher at the higher voltage. .... I think... ... it's been a lot of years since my last EE course.

[cabinetman]

...assuming I have a 100W light bulb that I can rewire for 220V.

Doing that, I would guess would blow the bulb to smithereens.
.

[Garasaki]

I have run up against the same arguement. In fact I think it's possible one of the threads you quoted was one I was participating in and was told the same thing - the amps are the same regardless of voltage.

It does not make sense to me, in any way shape or form. Therefore I choose to believe that either they are talking about a different issue or are just wrong.

I guess I'll stick with what I believe to be right. Sort of a scary concept...

To me, the piece of the puzzle that makes it "make sense" is that the neutral wire, which is always 0 volts on a 120 volt system, actually has a voltage (in my head it makes sense to consider it negative voltage) in the 220v conversion.

I am not an expert on this. I may be way off base.

[Tom Slick]

I can tell you from actual experience that smaller wires are used with higher voltage/lower amperage. Look up how to calculate wire size in circular mills (the way wire size is calculated) and the formulas will determine wire size for you. In farm/home wiring it doesn't make much difference because they are relatively low amperage and small installations but when you start wiring bigger industrial stuff the difference in materials cost becomes significant.

Phi1l
Calculating for inefficient conductors by using the NEC's rules you can have a 3% max voltage drop in a length of wire. So my original calcs would be corrected as
11.64v x 64.4a = 750w
465.6v x 1.61a = 750w
The amperage change probably would not result in a wire size change unless it was right on the edge before. When calculating the wire size you'd probably have rounded the amps up.

Garaski
The neutral conductor stays the same size as the "hot" conductors because in a 220v 1 phase circuit (what a house has) the two "hot legs" are 180 degrees "out of phase" so in layman's terms the neutral isn't carrying the highest current for both legs at the same time.

If you need to "see it to believe it" take a look at the motor data plate below.
Notice the hp rating is a single number, hp is the U.S. way of measuring Watts. If you look at a metric frame motor built in/for Europe it will only have a watt listing, no hp.
Notice the dual voltage rating 115 / 230
Notice the dual amp rating 7.8 / 3.9
115v x 7.8a = 897w
230v x 3.9a = 897w



image stolen from http://www.interschola.com/displayit...items_id=10649

[Kristofor]

assuming I have a 100W light bulb that I can rewire for 220V. Plugging those values into our formula and solving for A we get:

100=110*A
100/110=A
1=A (or close enough)

Now using 220V:
100=220*A
100/220=A
.5=A(or close enough)

So what am I missing?

You used this equation for Power
P= I * V (I=amps)

But remember the other forms of the power equation derived from Ohm's law substitutions
P = R * (I^2)
P = (V^2) / R

A lightbulb is a resistive load. You cannot "rewire" a lightbulb like you can an induction motor to have loads in series or in parallel, the bulb itself is a single resistive load.

Its output in watts is not based on how much work it needs to do but a derived value of how much work it will do at a given voltage based on the resistance of the bulb.

When you double the voltage you're effectively quadrupling the wattage of the bulb as the value of R stays the same* (P = (1V)^2/R = V/R, P = (2V)^2/R = 4V/R) but the bulb isn't going to last very long.

When they make a 100W bulb for a 220V system they take that into account and the resistance of the bulb is much higher so the same light output can be achieved

*This is complicated by the fact that R increases dramatically with temperature and needs to be in a workable range for a given voltage to make the filament hot enough to glow without quickly burning up but you get the idea.

[crokett]

Ok part of my confusing in the thread on the other forum was reading comprehension. Most 220V tool threads start by asking if/how to rewire from 110V. In this case, the OP had a 220V tool that was rated for 21A and was not asking about moving from 110 to 220. If it was 21A at 110V and he wanted to rewire to 220, that would drop it to 10.5A but he was already at 220, so amps stay at 21.

[Mr__Bill]

Ok part of my confusing in the thread on the other forum was reading comprehension. Most 220V tool threads start by asking if/how to rewire from 110V. In this case, the OP had a 220V tool that was rated for 21A and was not asking about moving from 110 to 220. If it was 21A at 110V and he wanted to rewire to 220, that would drop it to 10.5A but he was already at 220, so amps stay at 21.

Unless he wanted to rewire for 110, then about 42 amps.

A lot of confusion comes from people equating amps with power, and if voltage is the same it's true increasing amps increases power. At 110V a 15A motor is more powerful than a 13A motor. Take that mind set and double the voltage. Now if the power is the same then the amps must be the same, which is false, because amps does not = power it's only one component of power.

Now to make things really confusing if we take that 15A motor and wire it for 220V then there are 7.5A per leg, or..... a total of 15A. See it is the same!


Bill.
My work here is done!

[LCHIEN]

I haven't seen the original thread that the OP mentioned.
So I don't know the context.
But, one true statement is that an Ampere is an Ampere whether it comes from 110 or 220V.
Amperes are electron flow, to be specific 1.6x 10^23 electrons per second, thats
160,200,000,000,000,000,000,000 electrons per second.

and the measure of an Amp (or Ampere) doesn't really matter if it comes from 110 or 220V. Now, voltage is a pushing force. It takes twice as much force (e.g. 220V) to push an Amp into a motor than the same Amp into a smaller load.

We do use the analogy of flow and pressure... imagine taking 60psi of pressure to flow a gallon of water per minute into a small hole or only 30 PSI to push a gallon per minute into a larger hole (less resistance).

[cabinetman]

thats
160,200,000,000,000,000,000,000 electrons per second.

I've gotten A/C shock before, and I agree it's about that fast.
.