step down transformer mystery

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Old 05-14-12, 08:30 PM
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step down transformer mystery

I have a Stacor auto step down transformer 240V - 120V that comes prewired with a NEMA 6-15 on the input end and a standard NEMA 5-15 on the output side. I replaced the 240V plug with an IEC C14 so that I can use the transformer with an IEC power strip.

Here is the mystery: when I test the 120V output hot and neutral are reversed. Leaving the green ground connection alone, I rewired H & N (i.e. swapped them) and I still get 120V between neutral and ground. No matter what I do the measurement I get between neutral and ground is 120V. My measurements are as follows:

N (tall blade) and Ground = 123.7V
N and H (short blade) = 126.16V
H and Ground = 4.16V

Just so you know, I checked my multimeter and methodology with different outlets and everything checked out okay.

I should be measuring near zero volts AC between the neutral and ground, line voltage between the hot and neutral, and line voltage between hot and ground.

Any ideas? Thank you.
 
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Old 05-15-12, 06:10 AM
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Is there a reason you cannot just use the 120 volts from a regular receptacle to feed the strip?
 
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Old 05-15-12, 08:27 AM
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Yes, the room and all the gear operate off of a dedicated 240V line but there are a couple of pieces of equipment that cannot operate at 240V. Thanks.
 
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Old 05-15-12, 08:58 AM
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Yes, the room and all the gear operate off of a dedicated 240V line
Seems adding 120v to the room would be the better long term solution. In fact I'm wondering what commercial code says about 120v receptacle requirements.
 
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Old 05-15-12, 09:14 AM
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The NEC does not specify convenience receptacle locations like they do for a residence. In commercial there is no 6/12 rule.
 
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Old 05-15-12, 11:45 AM
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"I have a Stacor auto step down transformer..."
No, what you have is a Stacor step down autotransformer.

An autotransformer is a single winding transformer with one or more "taps" along the single winding to allow for different voltage inputs or outputs. Since the 240 volt input does not have a grounded conductor it also has no ground reference in the 120 volt output. You need either a 120 volt line to your room OR an insulating (two-winding) transformer with one lead of the 120 volt secondary grounded in order to get the proper ground reference that you desire or need.
 
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