Relevant code sections for voltage accuracy?

[jdm95003]

Hi All,

I work in an office building that has each floor
powered by a 45 kVA 440V delta / 208 wye
transformer with our office using 120 V phase to
neutral connections. For the past day I have
been monitoring a line voltage continuously
by setting an oscilloscope on infinite
persistence. During this time a bit over
a million acquisitions were made and superimposed
over each other. The RMS voltage has ranged from
115.8 V to 149.9 V. I have previously noticed
voltages around 130 V using a voltmeter, and I
have asked the building supervisor to correct
this a couple of times. To date nothing has
been done, so I would like to know what this
violates.

Also, I have measured over 3 V from neutral to
ground which seems quite high. Assuming equal
voltage drop on each wire, this would be a total
of about 7 V voltage drop in the circuit (about
6%), and that was not even with much known load
applied. Is this allowable? Incidentally, this
is a fairly new building. Thanks for any
information.

Jim Monte

 

[George Stolz]

Re: Relevant code sections for voltage accuracy?

Originally posted by jdm95003:
The RMS voltage has ranged from
115.8 V to 149.9 V.... To date nothing has
been done, so I would like to know what this
violates.

I don't believe there is a code for this. This is a power quality issue, not a safety issue.

Also, I have measured over 3 V from neutral to ground which seems quite high....Is this allowable?

Yes.

Others may have better recommendations (this is outside my normal scope) but I would imagine installing quality surge protectors with battery backup would counter any problems that arise from using the system.

The fluctuations could be from the utility, which would mean that your premises wiring is fine. You might try to find an electrician in your area with appropriate recording equipment to monitor your wiring and diagnose (if not repair) the problem.

 

[jim k]

Re: Relevant code sections for voltage accuracy?

It sounds like a harmonics problem to me. Does the building serve a lot of non-linear loads?

Mind you, I don't know squat about harmonics, but I'm hoping that someone who does will take over from here and explain it.

- Jim K

 

[petersonra]

Re: Relevant code sections for voltage accuracy?

you can easily get a few volts from N-G just from the I^2R losses in the neutral, depedning on where you do your measurement.

The 149 V bothers me though. I would be inclined to contact the power company and see if they can help out.

 

[jwelectric]

Re: Relevant code sections for voltage accuracy?

Originally posted by jim k:
It sounds like a harmonics problem to me. Does the building serve a lot of non-linear loads?

Mind you, I don't know squat about harmonics, but I'm hoping that someone who does will take over from here and explain it.

- Jim K

I am not sure about this but I think that Willie Nelson can blow into one of them things and make some pretty good sounds.

 

[jdm95003]

Re: Relevant code sections for voltage accuracy?

Thanks for all of the replies. Like most office
buildings, this one is filled with computers,
and those have switching power supplies. Cheap
switching power supplies will draw a lot of
current for brief instants, which could distort
the line voltage. But having said that, I have
been looking at the actual line voltage, and
it looks like a pretty clean sine wave. So
Willie is not the guy to talk to...

I had thought that wiring the outlets that
are probably rated at 120 V to a 150 V source
would have to violate something. I had also
recalled that there was something special about
a 5% voltage drop. I am seeing 6%, and that
is not with many known loads running on the
circuit. However, this circuit may be running
other things and be closer to its limit than I
think.

A good UPS or line conditioner would work
around these problems, but I do not think it is
a utility issue since the voltage varies a lot
from phase to phase. For example, a typical set
of values would be 115 V, 120 V, and 125 V.

Jim

 

[George Stolz]

Re: Relevant code sections for voltage accuracy?

Originally posted by jdm95003:
I had thought that wiring the outlets that are probably rated at 120 V to a 150 V source would have to violate something.

The nominal voltage never changed.

For example, a typical set of values would be 115 V, 120 V, and 125 V.

This is from left field, but I wonder about the neutral connections at the local transformer.

 

[ron]

Re: Relevant code sections for voltage accuracy?

Utilities are generally regulated to within plus or minus 10% of nominal (120V).
Some leases specify voltage tolerances, especially if there are lots of computers.

 

[paul]

Re: Relevant code sections for voltage accuracy?

What are the readings on the line side of the transformer? Do they also fluxuate up and down?

I'm thinking along the same line as George. Have a qualified 'lektrishun check the neutral connection and bonding at the transformer. Might be a good time to power it down and do a once-over on it by checking and re-torquing all the connections.

 

[bob]

Re: Relevant code sections for voltage accuracy?

JDM

The RMS voltage has ranged from
115.8 V to 149.9 V. I have previously noticed
voltages around 130 V using a voltmeter

How low did these levels last? The voltage should be about 120 volts and may need to just have the taps on the transformer changed.
I've had it happen before. I would be more concerned about the 140 volts that the 115 volts.

 

[jdm95003]

Re: Relevant code sections for voltage accuracy?

What are the readings on the line side of the transformer? Do they also fluxuate up and down?

I would assume so, but measuring them is
something that I probably should not do.
Walking into unlocked rooms that also have
janitorial supplies and looking around
is one thing. Removing panels to measure
voltages is another.

How low did these levels last? The voltage should be about 120 volts and may need to just have the taps on the transformer changed

They tend to rise at night when large loads
like air conditioning are turned off. On
another phase where there is a UPS that measures
line voltage, I have been recording values at
5 minute intervals for the past week. During
that time, it has varied from 115 V to 121 V,
which is not bad at all. The 116 V to 150 V
on the phase that I initially reported was
continuous monitoring during the first two days
of that period. Another interesting thing is
that the voltage on the "bad" phase sometimes
fluctuates a lot even between readings. I have
seen a variation of a couple of volts in
readings taken a second apart. And that was
not meter settling time either.

I would be more concerned about the 140 volts that the 115 volts.

I am concerned about the 150 V. Something
interesting to try is measuring the input power
of a typical induction motor vs. voltage. I
tried it on an air conditioner compressor and
saw a doubling when the voltage increased
from 115 V to 140 V. Since the motor was turning
at the almost exactly the same speed, it was
doing the same amount of work. So all of
the additional input power was just heating up
the motor.

Incidentally, I realize that the discussion is
drifting from code violations. That is OK
with me, but I did not post to get a diagnosis.
I was just hoping for a potentially bigger stick
if the building management tells me again that
they sent an electrician up an he did not see
anything wrong.

Jim

 

[wirenut1980]

Re: Relevant code sections for voltage accuracy?

If the voltage is only high during off peak hours and it is at an acceptable level during higher loading hours, like when the AC is going and computers are on, etc, then I think instead of properly sizing wires for voltage drop, maybe someone adjusted the transformer tap to compensate. I am wondering if the tap is adjusted to give an acceptable voltage reading at off peak hours, then will the voltage drop down to an unreasonable level during the high peak loading times? As has been said, this does not sound like any code violations, just design issues.