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Old 03-15-2010, 05:01 PM   #1
jfkriege
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Default 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.


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Old 03-15-2010, 05:03 PM   #2
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With a three prong, you will be missing the neutral for the 120VAC stuff


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Old 03-15-2010, 05:04 PM   #3
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I thought the ground and neutral were tied together at the box and so functioned as either.
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Old 03-15-2010, 05:04 PM   #4
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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.
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Old 03-15-2010, 05:06 PM   #5
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Quote:
Originally Posted by jfkriege View Post
I thought the ground and neutral were tied together at the box and so functioned as either.
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
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Reason: Not sure if my statement is correct
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Old 03-15-2010, 05:07 PM   #6
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Quote:
Originally Posted by jfkriege View Post
I thought the ground and neutral were tied together at the box and so functioned as either.
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?

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Old 03-15-2010, 05:22 PM   #7
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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)?
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Old 03-15-2010, 05:27 PM   #8
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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
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Old 03-15-2010, 05:32 PM   #9
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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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Old 03-15-2010, 05:35 PM   #10
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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.


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