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This is the receptacle I'm trying to wire. The ground says green next to it, but the other two wire inputs don't say anything. I need to know which is positive and negative. there is no + or - the screws are not different colors. No indicators at all. I would appreciate any help.

enter image description here

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AC circuits don't have + or -, they have hot or neutral. And I believe for your receptacle in the US, both sides are hot, so it wouldn't matter which hot wire goes to which side. But I am not an electrician. –  Henry Jackson Sep 7 '13 at 16:06
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up vote 5 down vote accepted

Assuming you mean this 6-30R:

enter image description here

After you wire the green (ground, not neutral), it doesn't matter which of the two ungrounded (hot) wires are placed on the 2 'flat' blades. 240VAC (in the North American split single phase) is not polarized with respect to ground, because the neutral is not being used.

This connector can also be used with 2 hot connectors in a 3 phase system, which would then provide 208 VAC. This is not common for residential use.

A Wikipedia article on NEMA connectors in general and NEMA 6 in particular may be helpful.

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thank you very much HerrBag! –  Kevin Sep 7 '13 at 15:19
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It's important to note that the two hots are equivalent in this case, but also in the case with a neutral! Both hots are at 120 V with respect to ground, and they are 180 degrees out of phase. This is true with or without a neutral. One can swap the two hots and no one will be able to measure a difference (except with some external time source). You could think of each hot as being a pendulum, where when one pendulum swings to the right, the other swings to the left. One could not say which is "before" the other, or somehow order them, since they are 180 degrees out of phase. –  Pigrew Sep 7 '13 at 17:00
    
@Pigrew I think we agree on how to wire this connector and why. I only made the distinction about the neutral to distinguish from the USE of ground, neutral and 2 ungrounded in the same connector (like the 14-30) where having the neutral in the correct spot is critical. I agree that the placement of the ungrounded connectors is not a polarizing situation. I will remove the offending statement. –  HerrBag Sep 7 '13 at 18:01
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