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480-4wire service

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User 1.nospam

Jan 1, 1970
0
I saw a new one (to me) yesterday. Trouble shooting an aircompressor
control panel I got strange readings on the incoming line to ground. I had
thought it was a standard 480Y277 service. On one leg to ground I got
242volts.

I thought I was familiar with all the different configs available in the US,
so imagine my surprise when I examined the three xfmrs on the pole and
discovered that they were connected 480 delta and that one of them had a
center tap, which created a neutral, hence the 242 volts. The connections
could have easily been for 240, 4 wire, which is fairly common here, but it
is definitely 480-4wire. The installation is about 10 years old.

I have done electrical work in this area (Kentucky) for 30+ years and this
is a first. So I ask, anyone else ever seen this? What applications does
it have? Is it common anywhere?

Maybe the power company ordered the wrong xfmrs, or got a real deal on a
closeout!
 
J

John Gilmer

Jan 1, 1970
0
s falke said:
User 1.nospam said:
I saw a new one (to me) yesterday. Trouble shooting an aircompressor
control panel I got strange readings on the incoming line to ground. I had
thought it was a standard 480Y277 service. On one leg to ground I got
242volts.

I thought I was familiar with all the different configs available in the US,
so imagine my surprise when I examined the three xfmrs on the pole and
discovered that they were connected 480 delta and that one of them had a
center tap, which created a neutral, hence the 242 volts. The connections
could have easily been for 240, 4 wire, which is fairly common here, but it
is definitely 480-4wire. The installation is about 10 years old.

I have done electrical work in this area (Kentucky) for 30+ years and this
is a first. So I ask, anyone else ever seen this? What applications does
it have? Is it common anywhere?

Maybe the power company ordered the wrong xfmrs, or got a real deal on a
closeout!


"480/240V" 4-wire systems turn out to be weird mutations of 480V wye-grounded
and 480V corner-grounded systems. There are hundreds (if not thousands) of
electric utilities on the North American continent, and mid-coil grounding of
utility-domain transformer banks is just one undesirable product of some
in-house construction standards.

Unlike with 480V wye-grounded systems, common 'slash-rated' circuit breakers
and 300-volt fuses cannot safely be applied to such systems, and the
single-pole interrupting rating of non-slash-rated circuit breakers is
relatively low [usually 8660 amperes] where their "advertised' symmetrical
interrupting rating may be significantly higher, like 25,000-65,000
amperes.

Yes, but ...

Are not these arrangements accomplished with just TWO transformers in a "dog
leg" configuration?

The "special" problem is the 3rd phase which is supplied by the smaller
transformer. A significantly lower interrupting capacity when used in a
"supply" that can't supply much current, isn't much of a risk, is it?

EMWTK
 
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