240V 3 prong vs 4 prong

I am currently building an electric rig. It will have a 240V feed for the element as well as two pumps (120V) and the PID controller.

I have a 3 prong outlet in the house already. Is there a real advantage to either? Is it wrong to use either as long as there is GFCI protection on the line?

I am wanting to understand this more.

With a three prong, you will be missing the neutral for the 120VAC stuff

I thought the ground and neutral were tied together at the box and so functioned as either.

If you want to run 120 loads you'll need the 4 prong. I'm running separate 110v & 220v in to my panel so that I can use it where 220v is unavailable, but that may not be a concern for you.

jfkriege said:

I thought the ground and neutral were tied together at the box and so functioned as either.

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Ground and neutral should not be tied together at the box. If you're using GFCI, and you should, then that would present a big problem designing current to go to ground... since that is what causes gfci breakers to trip.

EDIT: This might not be correct, and I'm not an electrician

jfkriege said:

I thought the ground and neutral were tied together at the box and so functioned as either.

Click to expand...

You are right, ground and neutrals are common in the panel, they are on the same bars... but


You cannot run a ground wire as a neutral, and a ground in your system. Otherwise, there would be no neutrals in your home. The electic guys are going to be all over this, any bet on how long?

I hope the electric guys are not all over this. I asked because I wanted to know before hand and not do something stupid. I understand more now.

I will wait until I can get a 4 prong circuit installed before I do anything, or I will run a second 120V line into the box in order to power the pumps and the PID.

Somewhat related: If I have a 4 wire 30A circuit, what is the amp rating on the 120V lines if they are isolated and not used for 240V (e.g. if I wanted to run 2 smaller 120V heating elements)?

30A?

I mean your breaker and your wiring will handle 30A on a single leg...

I dont see why it wouldnt

Just make sure that your total draw on BOTH legs doenst exceed 30A too

The question in my mind is about the neutral.

If I have 15A running through one 120-Neutral and 15A running through the other 120-Neutral, am I running a total of 30A down the neutral line and therefore at my limit for a 30A circuit? Does that even make sense?

I am officially lost.

As long as it is wired properly, what matters is how much load you have on the circuit, not what lines the load is on.

It is quite possible that I am the one that is totally lost.

jfkriege said:

The question in my mind is about the neutral.

If I have 15A running through one 120-Neutral and 15A running through the other 120-Neutral, am I running a total of 30A down the neutral line and therefore at my limit for a 30A circuit? Does that even make sense?

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The two hot legs are out of phase so the current returning on the neutral does not add exactly. You would not be be able to overload the neutral even drawing 30A on both legs.

I use a 3 wire feed for my element and a seperate 120v circuit for my controls. IMO this is the best way to go.

Don't compare neutral lines, just consider the power demand of what you are running. If you are running something (heating element for example) that is pulling 15 amps then you are fine. If you are running two elements on the same 30 amp circuit, both of which pulling 15 amps, than you are pushing it. To be safe, use 80% as a rule...if it's a 30 amp circuit, anything over 24 amps is pushing it.

Also, on a 110/120v circuit the hot wire acts as 'in' and the neutral acts as 'out' (if that makes any sense), there shouldn't be any difference between them. A GFCI compares these two and if they don't match up the GFCI will trip sensing that power is being lost from the hot to somewhere other than the neutral (could be through a properly installed ground or it could be through your body [hopefully not]). This isn't the 'scientific' explanation but helps to basically understand it. A 220v/240v circuit doesn't have a neutral, each hot is out of phase (meaning that they are transmitting electrons in opposite directions multiple times per second, acting as in/out by being out of phase).

I'm not an electrician or an engineer, but this is how I understand it in lay man's terms. I may get bashed for not referencing appropriate terminology or processes, but in a quick minute it helped me to understand it.

I forgot about them being out of phase. So, to clarify, I can run two 15A elements on the two 120V legs and still have enough left over to run pumps and PID if I want without exceeding the 80% rule as long as I have a 4 wire circuit (if I have a 3 wire circuit it wont work)?

If I were to do a 3 prong 240V for the elements and 120V circuit for the pumps and PID, what would be the best way to implement GFCI as a part of the control box or in the cord? I would like to be able to travel with the system and cannot rely on GFCI as a part of the circuit I am plugging into.

jfkriege said:

I forgot about them being out of phase. So, to clarify, I can run two 15A elements on the two 120V legs and still have enough left over to run pumps and PID if I want without exceeding the 80% rule as long as I have a 4 wire circuit (if I have a 3 wire circuit it wont work)?

If I were to do a 3 prong 240V for the elements and 120V circuit for the pumps and PID, what would be the best way to implement GFCI as a part of the control box or in the cord? I would like to be able to travel with the system and cannot rely on GFCI as a part of the circuit I am plugging into.

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No, you are drawing 30A off the circuit already, right? Your breaker still has a 30A draw and those 120VAC lines are running at 30A which is probably their max too, right?

You have nothing left for pumps etc.

I still don't understand why I'm not supposed to get 120v from a 3 prong 240v by using one hot leg and the ground...

Can anyone explain why not in layman's terms?

klyph said:

I still don't understand why I'm not supposed to get 120v from a 3 prong 240v by using one hot leg and the ground...

Can anyone explain why not in layman's terms?

Click to expand...

Ask an electrician why they dont do residential wiring this way, there must be a compelling reason.

willynilly said:

No, you are drawing 30A off the circuit already, right? Your breaker still has a 30A draw and those 120VAC lines are running at 30A which is probably their max too, right?

You have nothing left for pumps etc.

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This is where I am getting hung up. I see how that adds up, but at the same time, I am only drawing 3600W from the circuit (120Vx15Ax2 elements). I should be able to get 7200W at 100% capacity.

I am confused.

I see what you are saying now...

So I can have (4) 1500W 120VAC elements on a single 30A circuit?

Id have never seen that coming.

jfkriege said:

I forgot about them being out of phase. So, to clarify, I can run two 15A elements on the two 120V legs and still have enough left over to run pumps and PID if I want without exceeding the 80% rule as long as I have a 4 wire circuit (if I have a 3 wire circuit it wont work)?

If I were to do a 3 prong 240V for the elements and 120V circuit for the pumps and PID, what would be the best way to implement GFCI as a part of the control box or in the cord? I would like to be able to travel with the system and cannot rely on GFCI as a part of the circuit I am plugging into.

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Yes, you would only be using half the capacity of the 30A 240V circuit so you would have room for other things. This will not work with a 3-wire circuit as you will not have a neutral to use 120V elements.

I would suggest a small breaker panel built into your rig. You could feed this with a 4-wire cord and put gfi breakers in the panel. This would be a good portable option. There is also an angle plug end that can quickly be converted from 30 to 50A configuration depending what is available.

klyph said:

I still don't understand why I'm not supposed to get 120v from a 3 prong 240v by using one hot leg and the ground...

Can anyone explain why not in layman's terms?

Click to expand...

A 3-wire 240V circuit will have 2 hots and a ground. A ground wire is not intended to carry current. It is there to trip the breaker if a hot wire should contact metal parts. I have seen people use conduit as a neutral - it carries the neutral current but is very unsafe.

A 4-wire circuit will also have a neutral. This wire is intended to carry current and is necessary if you want to use a GFI device.

What would be the easiest way to provide GFCI to the system in a way that would travel with the system?

Either a GFCI cord, or a spa panel mounted to it maybe

I know where to get the GFCI cord, has anyone found a good source for the spa panels? Is there an advantage to either one?

Quick point...I *think* the reason for not switching the neutral and ground is that a situation could occur where power could 'backfeed' through the neutral/ground and present substantial safety issues.

So if I use the ground as a neutral, then while my rig is running, I should avoid licking my oven door while standing in a pool of water?

Just my $.02's worth. You need a neutral if you are going have an electric brewery. If you are not sure about this then yes it's great to ask here on this forum. I don't know if you've seen this thread or not: https://www.homebrewtalk.com/f51/electrical-primer-brewers-145019/

But, it has been very helpful to alot of us that are getting into the electric build part of our breweries. Personally I'd make sure I had a neutral with a GFCI breaker in my panel before I started anything. Think about it; water and electricity = GFCI. You WANT a neutral in your panel.

klyph said:

I still don't understand why I'm not supposed to get 120v from a 3 prong 240v by using one hot leg and the ground...

Can anyone explain why not in layman's terms?

Click to expand...

Take a look at this post and see if it helps.

jfkriege said:

This is where I am getting hung up. I see how that adds up, but at the same time, I am only drawing 3600W from the circuit (120Vx15Ax2 elements). I should be able to get 7200W at 100% capacity.

I am confused.

Click to expand...

First off, if you intend on loading the circuit beaker up to 100%, make sure to by a breaker that is rated for continuous duty. Most breakers are only rated for 80% at continuous loads.

If you're having trouble envisioning the load, make a little panel schedule

phase A phase B
1800 1800 VA

15 15 A (VA/120V)

Total 3600VA
15A (3600VA/240V)

Since the load is even on both phases, the amperage per phase and total amperage is the same. It's a little difficult to see (and demonstrate) in the confines of a text box like this. I have a panel schedule in excel I can e-mail if anyone is interested. (p/m me please)

As for the neutral/ground issue, yes, they are bonded at the main panel, but are differentiated from there. The neutral is most often the same wire size as the phase conductors while the ground is fairly smaller. Just to confuse things a little more, for a 30A circuit, the code requires the phase conductors to be #10 awg, a full rated neutral would be 10 awg, and the ground is also 10 awg. The grounds get smaller as the current goes up. It is a violation of the code to use the ground as a neutral.