Confusion about wire sizes as they relate to 120/240

[kevreh]

So on my pool subpanel I have 240 going to it, with each leg on a 20 amp breaker from the main panel (so 40amp total going to the pool subpanel). IIRC each leg is 12awg, which is a correct size for 20amp. Keep in mind the pool subpanel is 10' from the house panel so voltage loss doesn't apply.

So my friend buys a new hot tub and is showing me the manual. It wants a 50 amp subpanel, 240v, with all wiring 6awg. Not just the ground but the two hots and neutral too. Googling, I find this is typical gauge for hot tubs. Looking at the wire size/amp table 6awg is rated for 55 amps with minimal runs.

Isn't that 55 amps per leg of the 240, or 110amps collectively? And if so why so oversized??? Only thing I can think of is to dumb it down for most situations so electricians always use the same wire whether its a 10' run to the hot tub or 100' run.

Puzzled.

 

[JasonLion]

Amps are listed for the voltage being used. A 240 volt device that draws 60 amps is drawing twice as much power as a 120 volt device that draws 60 amps, but they are both called 60 amps. While it is technically true that 55 amps at 240 volts is the same amount of power as 110 amps at 120 volts, no one would normally talk about it that way, you would just say 55 amps at 240 volts and use a 60 amp 240 volt (dual pole) breaker.

Also "the correct size" for wire depends on the distance. Longer runs require thicker wire.

 

[kevreh]

I guess what throws me is that in my main panel the feed to the subpanel are 2 circuit breakers connected together and both say 20 (like this one: Breaker). So thats just 20, not 40?

So I look up the amp listing for wiring (ex: http://lugsdirect.com/WireCurrentAmpaci ... 301-16.htm) and find that on my 20 amp 240 circuit I should have 14awg wiring. Right?

And this assumes short runs of less than 20'.

Just checking, thanks Jason.

 

[Bama Rambler]

Isn't that 55 amps per leg of the 240, Correct.

or 110amps collectively? You're confusing yourself when you try adding the amperages in a split phase service because that's not how it works.

And if so why so oversized??? What is oversized?

Only thing I can think of is to dumb it down for most situations so electricians always use the same wire whether its a 10' run to the hot tub or 100' run.

I'd never run #14ga wire on a 20 amp circuit. Besides if your feeding a sub panel you need to upsize the wire 125% anyway.

 

[Samantha Sabrina]

So on my pool subpanel I have 240 going to it, with each leg on a 20 amp breaker from the main panel (so 40amp total going to the pool subpanel). IIRC each leg is 12awg, which is a correct size for 20amp. Keep in mind the pool subpanel is 10' from the house panel so voltage loss doesn't apply.

So my friend buys a new hot tub and is showing me the manual. It wants a 50 amp subpanel, 240v, with all wiring 6awg. Not just the ground but the two hots and neutral too. Googling, I find this is typical gauge for hot tubs. Looking at the wire size/amp table 6awg is rated for 55 amps with minimal runs.

Isn't that 55 amps per leg of the 240, or 110amps collectively? And if so why so oversized??? Only thing I can think of is to dumb it down for most situations so electricians always use the same wire whether its a 10' run to the hot tub or 100' run.

Puzzled.

No this is not correct, it takes 2 legs to make 240 volts, but you do not add them together to get a higher amp rating, it is still only 20 amp service going to that panel, so 2-120 volt legs @ 20 amps each does not total 240 @ 40 amps, it equals 240 volts @ 20 amps

Each breaker is only supplying 20 amps per leg @ 120 volts each, if either leg should draw more than 20 amps the breaker will trip, BTW, are your breakers in the main 2 separate breakers or are they tied together?

If you have anything on the sub panel wired for 240 and the main breakers are not tied together if one side should overload and trip one of the breakers without tripping the other one you will have a very dangerous situation, you could damage the equipment, (it will be trying to run on just 120 volts instead of the 240 it is wired for), not to mention if a sub-panel has 240 supply going to it, and someone works on it and just shuts off one breaker then you still have one side "HOT".

Equipment wired for 240 actually draws fewer amps than 120 does, and operates more efficiently.

Notice that 115 is listed first under voltage, and 14.6 is listed first under Amps, so wiring this pump for 230 uses half the amps.

BTW, notice this says 115/230, this is the same as 110/220, or 120/240, they are all used interchangeably.

20 amps @ 120 and 20 amps @ 240 are the same, they are both only drawing 20 amps, just at different voltages.

Higher voltages comsume less power, that is why industrial equipment operates on 3 phase and much higher voltages, there are a bunch of different configurations in 3 phase, but the simple version is they have 3 legs, (Hot), each is rated for the same amperage, and if only one leg should trip then it does what is called single phasing, and will no longer work properly and most likely burn out.

Anytime you run wire, the farther you have to run power the heavier the wire has to be.

 

[kevreh]

Isn't that 55 amps per leg of the 240, Correct.

or 110amps collectively? You're confusing yourself when you try adding the amperages in a split phase service because that's not how it works.

And if so why so oversized??? What is oversized?

Only thing I can think of is to dumb it down for most situations so electricians always use the same wire whether its a 10' run to the hot tub or 100' run.

I'd never run #14ga wire on a 20 amp circuit. Besides if your feeding a sub panel you need to upsize the wire 125% anyway.

What do you use for a 20amp circuit, 12 or 10 gauge?

What I meant by oversized is that his hot tub has a heater (I checked and for hot tubs they tend to be @4500 watts for 240....so 18amps) and a pump, guessing around 1 hp. IIRC the pump would consume about 8 amps (pic samantha shows 7.2 amps at 240 for a 3/4 pump). So that adds up to 24 amps. Far short of the 55amp Hot Tub Subpanel. But now that I type this I realized that its possible to have bigger heaters, more pumps, ozonator, etc... So the 55 amp capacity is "future proof" to a degree.

 

[kevreh]

No this is not correct, it takes 2 legs to make 240 volts, but you do not add them together to get a higher amp rating, it is still only 20 amp service going to that panel, so 2-120 volt legs @ 20 amps each does not total 240 @ 40 amps, it equals 240 volts @ 20 amps OK, got it

Each breaker is only supplying 20 amps per leg @ 120 volts each, if either leg should draw more than 20 amps the breaker will trip, BTW, are your breakers in the main 2 separate breakers or are they tied together? Definitely tied together

If you have anything on the sub panel wired for 240 and the main breakers are not tied together if one side should overload and trip one of the breakers without tripping the other one you will have a very dangerous situation, you could damage the equipment, (it will be trying to run on just 120 volts instead of the 240 it is wired for), not to mention if a sub-panel has 240 supply going to it, and someone works on it and just shuts off one breaker then you still have one side "HOT". Good explanation, always wondered why they were connected.

Equipment wired for 240 actually draws fewer amps than 120 does, and operates more efficiently.

Notice that 115 is listed first under voltage, and 14.6 is listed first under Amps, so wiring this pump for 230 uses half the amps.

Right, half the amps 'cause twice the volts. But to be fair that's not more efficient. If that was the case the 240 rating would be half THEN A LITTLE LESS than the 120 to show that it was using power more efficiently. Of course that assumes that the label tells the whole story, which I'm sure it doesn't

BTW, notice this says 115/230, this is the same as 110/220, or 120/240, they are all used interchangeably. Tell me about it....always confusing to know which to use (110, 120, ...)

20 amps @ 120 and 20 amps @ 240 are the same, they are both only drawing 20 amps, just at different voltages.

Higher voltages comsume less power, that is why industrial equipment operates on 3 phase and much higher voltages, there are a bunch of different configurations in 3 phase, but the simple version is they have 3 legs, (Hot), each is rated for the same amperage, and if only one leg should trip then it does what is called single phasing, and will no longer work properly and most likely burn out.

Anytime you run wire, the farther you have to run power the heavier the wire has to be.

 

[Samantha Sabrina]

Isn't that 55 amps per leg of the 240, Correct.

or 110amps collectively? You're confusing yourself when you try adding the amperages in a split phase service because that's not how it works.

And if so why so oversized??? What is oversized?

Only thing I can think of is to dumb it down for most situations so electricians always use the same wire whether its a 10' run to the hot tub or 100' run.

I'd never run #14ga wire on a 20 amp circuit. Besides if your feeding a sub panel you need to upsize the wire 125% anyway.

What do you use for a 20amp circuit, 12 or 10 gauge?

What I meant by oversized is that his hot tub has a heater (I checked and for hot tubs they tend to be @4500 watts for 240....so 18amps) and a pump, guessing around 1 hp. IIRC the pump would consume about 8 amps (pic samantha shows 7.2 amps at 240 for a 3/4 pump). So that adds up to 24 amps. Far short of the 55amp Hot Tub Subpanel. But now that I type this I realized that its possible to have bigger heaters, more pumps, ozonator, etc... So the 55 amp capacity is "future proof" to a degree.

The key here is that sub-panel is "rated" for 55 amps, so it has nothing to do with what equipment is currently connected to it.

It was designed to possibly add more equipment, or they are using a 1 size fits all sub-panel, and it just happens to be overkill for his tub.

 

[Bama Rambler]

What do you use for a 20amp circuit, 12 or 10 gauge?

At least 12 ga but it really depends on the distance and the amount of voltage drop you want to allow.

What I meant by oversized is that his hot tub has a heater (I checked and for hot tubs they tend to be @4500 watts for 240....so 18amps) and a pump, guessing around 1 hp. IIRC the pump would consume about 8 amps (pic samantha shows 7.2 amps at 240 for a 3/4 pump). So that adds up to 24 amps. Far short of the 55amp Hot Tub Subpanel. But now that I type this I realized that its possible to have bigger heaters, more pumps, ozonator, etc... So the 55 amp capacity is "future proof" to a degree.

Most hot tubs have bigger than a 1 hp motor, but your point is valid the requirement is oversized for he actual use. Another consideration is that when you turn it on, everything fires up at once and the inrush current can be a lot. They probably take that possibility into account when setting the requirements.

 

[Samantha Sabrina]

Right, half the amps 'cause twice the volts. But to be fair that's not more efficient. If that was the case the 240 rating would be half THEN A LITTLE LESS than the 120 to show that it was using power more efficiently. Of course that assumes that the label tells the whole story, which I'm sure it doesn't.

BTW, notice this says 115/230, this is the same as 110/220, or 120/240, they are all used interchangeably. Tell me about it....always confusing to know which to use (110, 120, ...)

What do you use for a 20amp circuit, 12 or 10 gauge?

The size wire for any given circuit is determined but what kind of load is going to be put on it, how far away is the load from the power source, and is it a single use line or feeding a sub-panel.

Personally I always increase the wire size just in case, you never know when something might change and you will need larger wire to carry a bigger load, better to go bigger the first time and not have to buy more wire later and redo all the work.

Notice it says "Max Load", it doesn't mean it "Will" draw that many amps.

Also, when a motor operates at the higher voltage is has more umphhhhhhhh, so it doesn't have to work as hard to do the same job, (again I reference Industrial Equipment using 3 phase at much higher voltages).

Yes trying to understand how and why they refer to single phase in such different terms is very confusing, but if you really want to get confused, compare our 60 HZ to the rest of the world that uses 50 HZ, lol.

@Bama,

Exactly, inrush current, can go pretty high, generally a motor will spike at 25% over it's rated amp draw on startup, mostly depending on how it is made, these days they do make what is called "Soft Start" motors, and if everything should start all at once, pump, heater, lights, acc, ect., the draw could be really high.

 

[JasonLion]

Also, when a motor operates at the higher voltage is has more umphhhhhhhh, so it doesn't have to work as hard to do the same job

That is not really true. For a typical pool pump that is hooked up to an appropriately wired circuit it really doesn't any difference at all.

There are some differences, but they don't have anything to do with "umphhhhhhhh". To get the same total amount of power out of a lower voltage line you need to use a thicker wire gauge. So higher voltage wiring ends up being less expensive (because it is thinner). At some point as the HP of the motor increases using a heavy enough wire gauge becomes impractical, so larger motors are only available at higher voltages.

Another aspect of all this comes up when you have already run the wire, so the wire gauge will be the same regardless of what voltage you use for the motor. At 120 volts you will be drawing more amps (to get the same total amount of power) so there will be more loss in the wire and the overall system will be less efficient than if you ran the same wire/motor at 240 volts.