Incremental voltage drop

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1360

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Hello, I'm new to the site....

I'm dealing w/ incremental voltage drops on a troubled pre-existing subpanel (inside the same building) w/ 208/120 single phase fed w/ a 50A 2-pole breaker #8 3-wire THHN over 600'......

The panel circuit's w/ drop every minute or so from 120V to 105V, driving the UPS crazy.

I know the 3-wire set-up is a problem, as is the #8 on the 50A circuit.....this is coming out.

I have the ability to go 480V 3-phase on the feed (same distance) a set a small transformer......

I just want the finished product to provide a stable system for the office sub-panel.

Any sense that can be made of this post & problem is appreciated.

Thanks!
1360
 
1793

1793

Senior Member
Location
Louisville, Kentucky
Occupation
Inspector
You might want to check the breaker in the main panel feeding the "sub" for any voltage fluctuation. I had this issue come up just last week. I replaced the breaker and all is well.

I think it would be helpful if we knew more about the load on the circuits.

Others may have more to offer but this is a start.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
At the subpanel you need to measure the incoming voltage and current during this 1 minute or so period. The 15 V drop you are referencing would correspond to a current change of about 15/0.5 = 30 A. If the current does not change this much during this interval, then the problem is more than just the wire. So you may be looking at multiple problems.

If you put a 480 to 120 transformer at the subpanel, then assuming a 50 A 120 V load is maximum the input current over the 600 ft distaance would be 12.5 A. With the same 0.5 ohm cable resistance the input drop is 6.25 V and as seen at the 120 side this is 1.56 V. Rounding off the wire resistance to 0.5 ohm gave me a 25 V drop at 50 A.

(edit) When I referenced the 1.56 V drop as a resultant of the feed lines I was only showing the effect of that feeder resistance. Your actual load variation out of the transformer has to add a component from the transformer internal impedance. If the transformer had a 3% impedance and was rated 50 A, then you add 3.6 V to the 1.56 . (end edit)

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jerm

jerm

Senior Member
Location
Tulsa, Ok
The better UPS(es) have a sensitivity setting. You might need a USB or serial cable to change it. That may make them happy in the short term without any electrical work. But yeah, your wire is too small for that length.
 
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1360

Member
Looking to install copper #4 to feed a 480 3-ph./208Y120 30kva transformer this week.....it should help out substantially.

thanks guys!
 
wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
ptonsparky said:
What do you have cycling every minute or so that draws 30 amps? If that is what it is supposed to do, the transformer is your answer.
Click to expand...

My guess is it is a laser printer. If it is, then another option is to put the printer on a dedicated circuit from the subpanel. If the UPS has a sensitivity setting set to high, I would definately switch it to medium or even low ASAP, as keeping it on high will result in it beeping constantly and draining the battery, killing the UPS quickly.
 
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