(newb) Why no white (-) leads on power trafo - diyAudio
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Old 6th April 2008, 03:58 PM   #1
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Default (newb) Why no white (-) leads on power trafo

Can someone explain why power trafo lead pairs do not indicate (+) and (-) given the facts of AC mains distribution and that grounded power cords use black/white/green color coding to indicate same?
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Old 6th April 2008, 04:06 PM   #2
DHT112A is offline DHT112A  United States
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AC does not have a negative(-). The white wire merely designates which wire is common to ground. Since transformers don't really care which wire is grounded the manufacturer probably does not see a need to make the distinction.
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Old 6th April 2008, 05:39 PM   #3
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Think of them as 'live' and 'neutral' of the mains
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Old 7th April 2008, 02:13 AM   #4
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OK - "live" and "neutral". Same question though: since the trafo primary will get "live" on one lead and "neutral" on the other, and the secondary pairs will output "live" and "neutral", why not indicate the distinctions?

As a novice, the lack of distinction is confusing and I thought maybe there was a concept I was misunderstanding.

But perhaps it's just a legal issue - i.e. by requiring users to confirm "live" and "neutral" with their own test equipment, the manufacturer avoids liability if, say, your trafo leads were labeled backwards, causing you to wire your circuit backwards, leading to damage or injury.
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Old 7th April 2008, 02:47 AM   #5
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I think you're missing the point. The colour white indicates neutral. White and neutral both have very specific guidance to their use in the NEC and CEC. White wires must only be used when there is an uninterrupted connection from that wire to neutral(and thus to ground at the breaker box). However, the transformer primaries can easily be wired in a manner that that does not have an uninterrupted connection to neutral, such as use of a 2 pole power switch. If using such a switch to switch both AC leads, the colour black would be correct for both wires leading to the transformer.
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Old 7th April 2008, 01:52 PM   #6
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Thanks leadbelly - that makes a lot of sense. I guess two black leads on the primary indicate that it is possible to wire a hot lead to either side.
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