I have a hot tub requiring a 240v circuit but it has a factory 3 wire plug on it. I put in a 2 pole gfi breaker and ran a 10-3 to the tub. Problem is there isn't any where for the neutral on a 3 wire 240v receptacle. It's my understanding that you must have a neutral for the gfi to operate properly but 240v circuits don't need a neutral to work so how does it measure the returning current to trip if there's a leak. If it measures between the 2 hot legs what's to say they carry equal amounts of current so it will not trip? I'm not sure how to properly gfi protect this tub but must need a neutral to work I think?? :confused: So why would the manufactorer put a 3 wire plug on the tub. Oh yea the motor is 120v but the heater is 240v. It actually required 2 separate circuits.

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Originally Posted by cbarrow

I have a hot tub requiring a 240v circuit but it has a factory 3 wire plug on it. I put in a 2 pole gfi breaker and ran a 10-3 to the tub. Problem is there isn't any where for the neutral on a 3 wire 240v receptacle. It's my understanding that you must have a neutral for the gfi to operate properly but 240v circuits don't need a neutral to work so how does it measure the returning current to trip if there's a leak. If it measures between the 2 hot legs whats to say they carry equal amounts of current so it will not trip? I'm not sure how to properly gfi protect this tub but must need a neutral to work I think????:confused: So why would the manufactorer put a 3 wire plug on the tub. Oh yea the motor is 120v but the heater is 240v. It actually required 2 seperate circuits.

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I know a GFI (ground fault interrupter) uses the ground to assist in protecting around water.

A 240 circuit requires 3 wires plus a ground. 2 of the wires carry 120V each (Black and Red) The white is the neutral and it is common to both sides of the 120V supply. Just like in your circuit panel.

One pole on the breaker is Red and the other is for Black. Black is always on top Red is on the bottom. The neutral does not get wired in to the breaker. I simply makes contact when you lock the breaker into the panel.

Hope that helps.

The neutral does not get wired in to the breaker. I simply makes contact when you lock the breaker into the panel.

Hope that helps.[/QUOTE]

On Gfi breaker the neutral actually does land on the breaker and there is a wire on the breaker that lands on the neutral bus

<There is a pigtail on the GFCI that goes to panel neutral and a terminal that goes to load neutral of the protected device - KISS>

Donnie, can you provide backup to the statement:

"a GFI (ground fault interrupter) uses the ground to assist in protecting around water."


Not all 240 volt circuits require 3 wires plus ground. Many use no neutral only two hot legs and ground. How does a GFI 2 pole breaker works then?

I am an electrician and I've asked every other electrician I know and this seems to have stumped everyone. I guess I'll try calling the manufactorer seeing as how they specifically call for a ground faulted circuit maybe they can tell me how to do it? I also found a post in another forum claiming that it measures current on the hot legs but what if they don't carry the load equally and why does the neutral on a 2 pole still land on the breaker and not on the neutral bus like everything else? If anyone knows anyone that has an answer that makes a little sense please ask them. If you aren't an electrician or engineer or some way in an electrical field please don't make up answers. If this was a simple question I would hope that some of the people I have already asked would've known the answer. Thanks for your time

There are two types of two pole GFI breakers, straight 240 volt, with no neutral lug, and 120/240 volt with a neutral lug. Not all manufacturers make both, so this is something to consider before installing a panelboard. The 120/240 volt works for both applications.

A 240 volt load, such as a heater element, needs no neutral, but must have a n equipment ground. A straight 240 volt works simply. It will monitor the current flow in both hot legs. If the current flow differs between both by 5 milliamps or more, the difference is assumed to be leaking to ground, and the CB will trip.

A unit that needs 120 and 240 volt needs a neutral. For example, the heater needs 240 volts, no neutral, and the current in both hot legs will be the same, let's say 10 amps. If only the heater is running, there is no current in the neutral, only 10 amps on each hot leg.

Now add a 120 volt pump that draws 5 amps. It only connects to one hot leg, so it impose 5 amps on one hot leg and you can measure 5 amps on the neutral.

The one hot leg the pump is connect to sees the 10 amps of the heater plus the 5 amps for the pump, while the remaining hot leg stills only sees the 10 amps of the heater.

I believe it is this type of scenario that is concerning you.

They contain intricate and intuitive electronic measuring circuits that must monitor both hot legs and the neutral, and do the all the comparisons, by knowing that one hot leg, while has more load on one due to the pump, adds it, compares to the neutral, and as long as it sees the proper current , no tripping occurs.

Once the current flow differs 5 ma or more in any hot leg, and does not see the current flow in the return, whether the other hot leg or the neutral, it then assumes a leak to ground and trips.

This is why these breakers are so costly, in the range of $130.00 each.

Contacting the manufacturer is a great idea. I know, as I have done this many times before for various reasons, due to proprietary information and industrial secrets, you probably will not get a much better answer.

Plus, on your initial call ,I guarantee you will get some layperson in a call center reading a generic script for answers, and you will hang up not knowing any more than your when you first called.

Be patient and persistent to speak with a product engineer. He/she will do their best to explain the operation without giving away trade secrets.

I find that GE and Square D are fairly useless getting tech info. Siemens and Eaton ( Westinghouse and Cutler Hammer) are much better.

In a strictly 240 volt system like your hot tub, there is no neutral. The GFCI breaker looks at the difference between the two hot legs, which should equal the current in the neutral. If the difference between the two hot legs doesn't equal the neutral current, the breaker will trip (because the missing amount of current not returning on the neutral is assumed to be shorting to ground at the tub). But , since your hot tub uses 240 volts for all components and circuit boards, the current in each of the hot wires should always be equal, which means the GFCI will see a 0 amp difference between the two hot legs, and compare that to the constant 0 amp neutral current. If there is any difference in the two hot leg currents, the GFCI will compare it to the zero neutral current and trip. So just hook your GFCI up as though you had a connected neutral at the tub and rest assured the GFCI will do its job.

Huh? The circuit certainly does require a neutral, and a ground. Without a neutral you can't run the 120 volt pump motor.

Remember, we're talking about a hot tub that doesn't use any 120V components. The pump motor, heaters and circuit boards all use straight 240, with no neutral. And yes, it goes without saying that the ground wire must always be used.

OK, the pump motor in this case used 120V it turns out, but required an entirely separate circuit with a neutral. Missed that last bit in the original post. Still, the 240V components were wired to a cord on a separate circuit requiring no neutral to/from the heater GFCI. ZThanks for pointing out my oversight, MB!

You could have saved the trouble of the extra circuit by using a spa panel. Wiring a Hot Tub - Electrical Installation and Wiring Diagrams

To answer your question on how a GFCI works, lets take a look at one for 120 Volts first. By wrapping a coil of wire on a transformer (what might be called a current transformer) and connecting it to a load, you can measure current. It's not the only way, but it's one way and the easiest.

By sending the neutral current in the opposite direction, it directly cancels the current flowing in the hot. That difference current is what causes the trip.

Now there is some other circuitry that can detect grounded neutrals downstream, but I'll leave that be for now.

Now for the 240 breaker, panel neutral can be used to power the circuit itself. With no neutral current, the 2 pole breaker acts just like the single pole 120 V breaker.

If you take that same two wire scenereo and add a third wire in the correct direction, the neutral current will also add or subtract.

The key to understanding this is that the neutral current has direction.

I'll take this further if you want. Both the grounded neutral circuitry and the 2 pole scenereo with a neutral if need be.

Adding a bit more:

We know if we take a torroid core and put cuurent in one direction with one wire on in the other direction with another, they cancel. Hence the torroid core and it's geometry of current flow is the basis for a simple 120 V GFCI.

Ok, now suppose to make the math a little simpler, hopefully lets define the L1 current to be the opposite of the L2 current.

so IL1+IL2 = 0; or the line 2 current is the same as the line 1 current with opposite sign and this makes sense.

No suppose we define the difference current as In, so now

IL1+IL2=In which is the same as IL1+IL2-In=0

Now suppose we arbitrarily define positive as flowing toward ground or neutral for that matter we make IL1 and IL2's wires flow in the direction toward neutral. Wed make the neutral current flow away from ground.

Now the relationship of IL1+IL2=In makes sense because I did define IL2 to have the Opposite sign of IL1 in the Balanced load condition.

I could have easily said IL1+IL2+In = 0 and made everything come out in the wash.

In any event it's simple addition/subtraction done without any real fancy electronics and measuring the values independently etc.

You can also see that ground isn't even in the equations. Just the fact that the currents are unbalanced.

An extra winding is placed on the torroid to measure the amount of flux in the core and thus the difference current. The measured current is compared to a threashold and the GFCI trips.

Neutral, MAY be required as an input to a GFCI, but only so you can derive power for the circuit from a lower voltage.

The harder circuit to understand is how a single pole GFCI can detect a grounded neutral.

That's somewhat simple too. The torroid and neutral and hot currents are set to subtract as before, but instead of the sense winding being used to monitor current, it's used to modulate the voltage on both the neutral and ground with a high frequency.

With a neutral not connected to ground at the output of the GFCI, the modulation will cancel, since it's able to modulate the neutral. With neutral connected to ground downstream, the circuit is unable to modulate the neutral leg, thus the modulation frequency can be detected. This then allows detection of a grounded neutral.

So how does a GFCI work with a multiwire branch circuit? Kitchen receptacles were wired with 12/3 so with two duplex's in a box, one was on one circuit and the other on another circuit, the neutral being shared.

A single pole GFI breaker will not work with a mulit-circuit feeder with a shared neutral.

A 120/240 Volt 2 pole GFI breaker will work with a shared neutral.

Not sure if it is practical to use a 2 pole GFI breaker, as the cost for one is $100- $200.

Normally GFI receptacles are used at the point the shared neutral splits to two circuits, so the GFI devices are downstream.

I now see I went over all of this in detail in Post #6

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Originally Posted by tkrussell

I now see I went over all of this in detail in Post #6

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Ummmm, yes you did and it was dealing with a 240 volt use along with a 120 volt use. My installation was done more than 25 years ago and it was for a bunch of 120 V receptacles at the kitchen counter. They are all connected in parallel with 12/3 Bx. Now it has to be upgraded with GFCIs. If I install 2 GFCIs at the first box (2 duplexes each), I could certainly put in one for each circuit but feeding out the load sides of the devices, the neutral would still be shared and I am not sure that will work. I am not in a position to run new 12/2 feed lines to the protected devices down the line.

In that situation, you'd have to install a GFCI device at every point that you still have a shared neutral.
Bummer...

Most shared neutrals split at one box, one circuit in one direction the other in another.

There can be some situations that two circuits are in several boxes with two circuits on one device,and in that case, yes, each device would need to be a GFI device.

Yeah, it seems in the older days (pre GFCI requirements) people saw no need to terminate that multiwire circuit, and then split it off. Many times they just carried that three (or four) wire cable around to each drop, using the circuit they wanted at each. Not anymore though, for the most part.