Why? It's already power, not voltage or current.
Fair enough. I've got a 1.5HP dust collector on a 20A 120V circuit. I've been meaning to switch it over to 240V but haven't gotten around to it yet.
Chris
During motor startup the current could be more like 40-60 amps. Breakers are designed to allow for momentary inrush currents of that magnitude, maybe even higher. What does that 6.8% figure change to when you do your calculations on 40 amps instead of 10?
I really don't even understand electrical concepts to know how to determine what the load would be. If I can string a few wires together to get juice where I need it I am capable but only venture into such issues when there is no question of how to proceed. Swapping out a switch, changing a plug to match a socket, etc.
No worries, just a curiosity for now. Would like a better soluti> S> > Thanks guys. This is my home shop but I don't want to do the 4 wire
Okay, but let's use a more realistic wiring length of 30 feet. (20 feet from the panel, 10-foot cord.)
Power loss in the wiring is:
Pc = I^2 * R
For 30' of #12 copper, R=0.048 ohm
At 120V, assuming 60A inrush: Pc = 173W
At 240V, assuming 30A inrush: Pc = 43W
So for a total inrush power draw of 7200W (60*120 or 30*240), at 120V we lose 2.4% of the power to supply losses, while at 240V we lose 0.6%.
We get 1.8% more power delivered to the motor by switching to 240V.
Chris
That's where you make your mistake. Line losses are *not* small, particlarly when the machine is starting. The power lost due to the line resistance is the square of the voltage lost. It is *not* insignificant.
Expensive, it's a good idea if you're going to stick to 120V. Note that the difference between #10 and #12 is less than the difference between 120V and 240V. Going from 120V to 240V takes no more copper; #12 to #10 does. Of course, there is nothing stopping your from doing both.
...harder to find, stiffer, harder to work with...
Volts is volts. Watts is watts. Watts is volts squared. ;-)
I just bought a 2HP DC (came in a crushed box yesterday :(). I went with 2HP because as long as I was going 240V anyway, might just as well do it right. ;-) Still gotta run the line, so replacing the crushed parts isn't an emergency.
You would be surprised.
Don't forget to include the cost of the male plug in your cost comparisons.
Lew
Think molded cord set.
Definitely flexible.
Definitely available at all the big box stores.
Lew
Consider a motor that draws 15A at 120V. Let's call it a 90A inrush current, with a wiring length of 30 feet (20 feet from the panel,
10-foot cord) and #12 wiring.Power loss in the wiring is:
Pc = I^2 * R
For 30' of #12 copper, R=0.048 ohm
At 120V, assuming 90 A inrush: Pc = 389W
At 240V the inrush should be half, or 45A: Pc = 97W
This makes sense, we double the voltage and cut power loss by a factor of 2^2.
So for a total inrush power draw of 10800W (90*120 or 45*240), at 120V we lose 3.6% of the power to supply losses, while at 240V we lose 0.9%.
We get 2.7% more power delivered to the motor by switching to 240V. I wouldn't be able to notice the difference, so I call that "small".
Chris
The inrush current can easily be in the order of 8-10 times the FLA.
Using your example above, a 15 FLA motor could easily have 120A-150A inrush.
Lew
In fact, since the motor is an inductor, the inrush is only limited by the resistance. The lower the source impedance the higher the inrush, for a shorter time. Shorter time =3D> faster start. QED.
OK, I'll give you that one. 8)
It's true that 100 or 120 times per second (50 or 60 cycle) the sine wave model of current amplitude crosses zero, and with zero current, there can be no flow direction.
Tom Veatch Wichita, KS USA
Just remember those "7.5amps each" are not additive. (Robatoy, don't look ;) ) It's the same amps in each leg - one's coming to the motor, the other's returning it from whence it came. Just like 120v, one line brings the amps in, the other line takes it back.
Tom Veatch Wichita, KS USA
Just like 120. The juice comes in one wire and out the other. The difference is, with 120, one wire, the neutral, is connected to ground so there's no voltage relative to ground on that wire. With 240, both wires are isolated from ground and do have a voltage relative to ground, +120 to ground on one wire, -120 to ground on the other, so there's a voltage of 240 between the two wires.
(OK, if we want to get pick nits, the voltage is AC, so it varies in each wire from +120 to -120 and back to +120, 60 times per second in the US - hence 60 cycle AC. But when one wire is at +120, the other is at -120. I would say the voltages are 180 degrees out of phase with each other, but I'd probably be accused of claiming that 240v residential is 2-phase service. And to pick another nit, those are RMS values, not the actual instantaneous voltages. RMS = the square root of the integral of the square of the instantaneous voltage with respect to time over one cycle. Not sure of the actual maximum instantaneous voltage, but seem to recall it's somewhere in the neighborhood of 140 to 150 volts on each hot line.)
Tom Veatch Wichita, KS USA
I believe the resistance of the motor's windings figures into the formula as well as it's part of the circuit.
The line losses of pushing 1000 watts through a given conductor at a higher voltage creates less resistance than at a lower voltage. Hence
500KV power lines. Current is your enemy, voltage is your friend.Speaking of Watts... Charlie did NOT, I repeat, DID NOT quit he Rolling Stones.
Same way. Really. Only difference is that the two wires have a 240V potential between them, not 120V, and neither one is grounded.
Right -- but let's use some different terminology. The common-usage term for what you call "common ground" is "neutral", and for what you call "safety ground", "ground". The official terms as used in the National Electrical Code (NEC) are "grounded conductor" and "equipment grounding conductor" respectively.
Correct. Sort of. It *is* energized, but since it's also connected to ground, there's not nearly the same danger in touching it as there would be in touching the hot wire. The danger comes from the fact that electricity will follow all possible paths, and a person touching such an energized case forms a second, parallel path to ground through his body. Such a path has a much higher resistance than the copper wire, and thus passes only a fraction of the current that the wire does. The problem is that 20 or 30 milliamps can stop a heart.
Yes, that is correct, but as I noted above, the level of current that passes through the body is much lower than in the copper wire.
It's nowhere nearly as dangerous as having the hot wire connected to the equipment chassis. But it's also certainly less than fully safe.
Same way it's completed in any other circuit: by connecting together two points that have a voltage difference between them.
You don't need a neutral for current to flow; you just need the two sides of the circuit to have a voltage difference between them.
The utility company brings somewhere around 4KV to your utility pole, connected to the primary windings of a step-down transformer. This transformer reduces the 4KV from the utility company to 240V. There are *three* wires attached to the secondary windings: one at each end, and one in the middle. The two at the ends are 240V apart, and the one in the middle is 120V from either of them. Let's color the wires connected to the ends of the secondary coil black and red, and the one connected to the middle, white. There's 120V between the black and white wires, 120V between the red and white, and 240V between the black and red. Now tie the white one to a copper rod that's driven into the ground, to ensure that it's always at true earth potential.
There's your 240/120 North American residential electrical service.
The U.S., Canada, and Mexico use substantially the same system. Almost nobody else does.
In article , Tom Veatch wrote: [...]
120V * sqrt(2) = 169.7V
Wow! An AC pissing contest. Don't piss on a live circuit! ;)
nb
#10 is not as flexible as #12.
I've never seen one at the BORG.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.