step down transformer mystery
#1
05-14-12, 08:30 PM
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.
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.
#2
05-15-12, 06:10 AM
Forum Topic Moderator
Is there a reason you cannot just use the 120 volts from a regular receptacle to feed the strip?
#4
05-15-12, 08:58 AM
Yes, the room and all the gear operate off of a dedicated 240V line
#5
05-15-12, 09:14 AM
Forum Topic Moderator
The NEC does not specify convenience receptacle locations like they do for a residence. In commercial there is no 6/12 rule.
#6
05-15-12, 11:45 AM
Join Date: Mar 2006
Location: Wet side of Washington state.
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"I have a Stacor auto step down transformer..."
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.
