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Steve G., Electrical Engineer
Category: Electrical
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Experience:  Spent 20 years as an Electrical Engineer, designing electrical plans and specifications.
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# I have a Square D breaker panel, and I need to add a 2 phase,

I have a Square D breaker panel, and I need to add a 2 phase, 220 20 amp breaker, to power a small table mill. I know that since it is 2 phase, power is needed on 2 of the 3 poles, and the 3rd is grounded. I see inside the breaker panel, There are 3 large gauge cables coming in, terminated to 3 different buses, one left, one middle, one right. My question is, do I take positions 4 & 5 on the left side, for the breaker, which puts one pole on the breaker to the left side power bus and the other pole of the breaker to the middle power bus?
Or, do I install the breaker on the right side, positions 3 & 4, which puts one pole on the breaker to the power bus on the left, and the other pole of the breaker to the power bus on the right?

Hello there and welcome to Just Answer. My name is XXXXX XXXXX I will be assisting you today. I am an engineer with over 30 years of electrical and electronic training, repair and installation experience. I will try and answer your question accurately and precisely so that we can get you on your way.

You basically have it correct but let's make sure.

You have a three phase panel there. Do you know if it is 120/208V or 277/480V?

If you are not sure, measure the voltage from any single breaker to ground please.

Please let me know so that we can continue.

Regards

Steve

Customer: replied 4 years ago.

Stand by. Let me try to get a voltmeter.

Customer: replied 4 years ago.

From a single pole to ground is 119 v AC

Ok, so what you have there is a three phase 120/208V panel.

The voltage from any single pole is 120V, voltage between any two poles is 208V single phase and if you need three phase 208V, a 3 pole breaker is used.

So, your mill should work on 208V. You will need a 2 pole breaker. That breaker can go anywhere where you have two adjacent breaker spaces. This gives you 208V single phase.

If there is a neutral coming from the mill, that goes to the neutral bar in the panel. Similarly, the ground goes to the ground bar.

Assuming that your run from panel to mill is not excessive, you can use #12 conductors with that 20A breaker.

If you have questions or comments or need further help, please let me know. I'll be here.

Steve

Customer: replied 4 years ago.

So, I installed the breaker and measured to voltage at the outlet, and I am getting 119v AC from the right side pole to ground. But, 218 from the left side pole to ground.

You have the outlet wired incorrectly.

Can you measure the voltage across the the two poles of the breaker? You should have approx 208V.

Please let me know so that we can continue.

Regards

Steve

Customer: replied 4 years ago.

So, I measured voltage across the 3 cables into the breaker box.

Left side and right side show 118V AC. middle shows 218v AC.

The breaker is spanning the left side bus (118v AC) and the middle cable (218v AC).
I am thinking that the breaker needs to span the power bus from the left and right, both being 118v AC, correct?

Ok, I need to know exactly how you measure these voltages.

Let's call the three cables 1, 2 and 3 along with the ground G.

Please measure 1-2, 1-3, 2-3 then measure 1-G, 2-G, 3-G and let me know what you have there.

Thanks

Steve

Customer: replied 4 years ago.

I found the issue.

Cable 1, far left measures at 118v AC from cable to ground

Cable 2, middle, measures at 218v AC from cable to ground

Cable 3, far right, measures at 118v AC from cable to ground

I moved the breaker to span cable 1 and cable 3, now the outlet is measuring correctly. 118v AC from top left to bottom ground, and 118v AC from top right to bottom ground.

So you have 236 between the two?
Customer: replied 4 years ago.

I sure do.

Ok, so connect those two wires to L1 and L2 on your 240V outlet.

That will give you 240 (236V).

Steve