Methods for testing voltage drop

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blinkous

Member
As an electrical engineer, I have recently encountered a situation in inspecting a project where the voltage drop measurements in standard 120V-20A circuits were in the 15%-20% range (as measured by the SureTest #61-164 Circuit Analyzer). Is there another 'industry standard' method for testing voltage drop, or a recommended method other than using the circuit analyzer? Thanks.
 

brian john

Senior Member
Location
Leesburg, VA
Are theses instantaneous voltage drops due to associated loads? Or is this a steady state condition.

Is the VD only in the branch circuits, or is this from the Main Service to the utilization outlet? IE You measured voltage at the Main Service and there is a 15% drop to the furthest outlet?

Is the VD % you mentioned from a maximum (typical off hour measurement) to a minimum peak operating hours. Or is this a calculation you made using nominal 120 VAC.
 

blinkous

Member
Are theses instantaneous voltage drops due to associated loads? Or is this a steady state condition. Re: This is an instantaneous measurement performed by the meter on a corridor receptacle circuit in a school. No other loads are present on the circuit.

Is the VD only in the branch circuits, or is this from the Main Service to the utilization outlet? IE You measured voltage at the Main Service and there is a 15% drop to the furthest outlet? Re: From my understanding, the meter measures the no load voltage and then drops and instantaneous load onto the circuit and measures that voltage. It then calculates the delta V based on those values and divides that number by the no load voltage at the outlet. So, it is relative to the branch circuit voltage drop only.

Is the VD % you mentioned from a maximum (typical off hour measurement) to a minimum peak operating hours. Or is this a calculation you made using nominal 120 VAC. Re: This test/measurement was performed on a non-loaded 120V receptacle circuit.
 

catchtwentytwo

Senior Member
blinkous said:
As an electrical engineer, I have recently encountered a situation in inspecting a project where the voltage drop measurements in standard 120V-20A circuits were in the 15%-20% range (as measured by the SureTest #61-164 Circuit Analyzer). Is there another 'industry standard' method for testing voltage drop, or a recommended method other than using the circuit analyzer? Thanks.
  1. Was there a problem that prompted you to use the SureTest? Or was using it the first indication something might be "wrong"?
  2. You could put a decent DVM set to min/max at the branch circuit breaker(s) and put a known load (like a heater) down-stream to see what happens. If these are receptacle circuits, you can experiment by placing the load and/or DVM at various places in the circuit.
 

brian john

Senior Member
Location
Leesburg, VA
When reading your post some how I missed the reference to the Sure Test.


One I would test the Sure Test.. I do not believe this is a calabrated device..

I always test my test equipment when I have questionable readings before raising flags...Crow is not my favorite meat.

Verify the wire size, verify the length and verify all connections.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Another issue is that the VD being measured in this case is not just the branch circuit, the Sure Test is measuring all the way back to the source including all feeders (impedances) in between.


The tester is also simulating a 100% or higher load that is highly unlikely to occure

Roger
 

iwire

Moderator
Staff member
Location
Massachusetts
I don't have a sure test but if I recall it takes a no load base mesurment, then a loaded mesurment so that it can give you the actull voltage drop even if the utility is running low (or high) already.

I do agree, either way the mesured voltage drop includes the utility condutors. But in most cases this should not mean much to a 15 or 20 amp test.
 

Smart $

Esteemed Member
Location
Ohio
blinkous said:
Are theses instantaneous voltage drops due to associated loads? Or is this a steady state condition. Re: This is an instantaneous measurement performed by the meter on a corridor receptacle circuit in a school. No other loads are present on the circuit.

Is the VD only in the branch circuits, or is this from the Main Service to the utilization outlet? IE You measured voltage at the Main Service and there is a 15% drop to the furthest outlet? Re: From my understanding, the meter measures the no load voltage and then drops and instantaneous load onto the circuit and measures that voltage. It then calculates the delta V based on those values and divides that number by the no load voltage at the outlet. So, it is relative to the branch circuit voltage drop only.

Is the VD % you mentioned from a maximum (typical off hour measurement) to a minimum peak operating hours. Or is this a calculation you made using nominal 120 VAC. Re: This test/measurement was performed on a non-loaded 120V receptacle circuit.
If the result is the % of instantaneous voltage drop, i.e. ΔV (no-load voltage minus minimum instananeous voltage) ? Vmax ? 100, what you are measuring is not your typical voltage drop... the kind where the conductors are upsized if the drop exceeds an acceptable value. What you are measuring system voltage "sag". This is the amount of instantaneous voltage drop on current inrush, prior to system, conductor, and load stabilization.
 
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blinkous

Member
2nd Test Conducted

2nd Test Conducted

On a subsequent visit to the site, I used a combination of a Fluke multimeter, the Ideal tester, and a 1500 watt space heater. In testing the circuits at the receptacles, I found that the Ideal tester and the space heater/Fluke measurement method were within 2% accuracy with one another. Also, though not officially calibrated, I found that the Fluke multimeter and the Ideal tester measured exactly the same voltage in every location tested under a 'no load' voltage measurement taken at the receptacle.

It seems as though there is not a set, pre-defined method of measuring voltage drop, only each individuals own rendering, given that their interpretation has accurate methodology.
 

haskindm

Senior Member
Location
Maryland
Voltage drop is relevant to the amp load on the circuit. Who cares if a circuit would have excessive voltage drop IF it was fully loaded unless it is fully loaded? Also some loads are not affected terribly by voltage drop. For example incandescent light bulbs will last much longer and put out a little less light with "excessive" voltage drop. With lower voltage, resistance heaters put out a little less heat. Motors, on the other hand are more sensitive to reduced voltage. Are you trying to fix a problem or looking for a problem where perhaps none exists? A well designed circuit should not be continuously loaded to its maximum ampacity which is what the Sure Test simulates. What you need to be checking is the voltage drop under the actual working amperage of the circuit. If that is not excessive, there is no problem.
 

blinkous

Member
Ah, but there is a problem...

Ah, but there is a problem...

The issue is that a single floor buffer, 12.5 amp nameplate, is causing the corridor circuit to trip. This is what aroused the investigation. I would agree with your statement regarding the load under which the circuit is to be tested for voltage drop, except that for corridor circuits, where 'convenience use' receptacles are located as defined by the NEC, the assigened load is to be 180VA per device. This makes it quite difficult to acheive the trip condition being experienced by using these smaller numbers.
 
blinkous said:
The issue is that a single floor buffer, 12.5 amp nameplate, is causing the corridor circuit to trip. ...

High resistance (causing VD) in the circuit would not be contibuting to the buffer's tripping the breaker.

The buffer in a good circuit (with no VD) would use more current than the same buffer in a bad circuit (with VD).
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
David Rinaldi Coleman said:
The buffer in a good circuit (with no VD) would use more current than the same buffer in a bad circuit (with VD).

While this may be true for resistive loads, it is _not_ true for motor loads.

The current drawn by the motor depends upon the applied voltage and the mechanical load. For any given mechanical load, there will be an optimal voltage to run the motor. At higher or lower voltage, the motor will draw more current.

Additionally, if the supply voltage gets too low, the motor may stall, drawing still more current. Once the motor is drawing too much current, it becomes a race between the circuit breaker and the overload protection built into the motor.

Though truth be told, a 12.5A motor tripping a 20A breaker is a bit fishy...Another possible issue: if the voltage drop is caused by a loose connection at the breaker, or bad contacts in the breaker, then this will directly heat the breaker, causing thermal tripping.

-Jon
 

haskindm

Senior Member
Location
Maryland
A buffer drawing 12.5 amps is a pretty hefty load all by itself. It would not take much other equipment on the circuit for the circuit to be overloaded. Of course that much load would also exacerbate any voltage drop problem; and since it is a motor load, as the voltage drops the amperage raises, and the voltage drop increases, which causes the amperage to increase and the cycle continues. The only sure remedy may be to install a new circuit to be used only for buffer receptacles. All you would need is for someone to connect a coffee maker to the "general use" receptacles in the hallway and there is no way that the circuit would hold. I believe you probably have a LOAD problem more so than a voltage drop problem. What is the normal load on the circuit without the buffer being used?
 

blinkous

Member
This floor buffer is being used during the night shifts at the school. It is the only item that is operating on its circuit. The nuisance tripping is a common problem in most main corridors throughout this school, not in just one location.

The interesting thing is that there are several other 'identical' high schools in this district that operate the same way and use similar equipment, but this is the only one that has nuisance tripping problems.
 

haskindm

Senior Member
Location
Maryland
What is the actual load on the circuit when the breaker trips? Do you KNOW that there is no other load on the circuit? It could be that your buffer is going bad and drawing much more than the rated 12.5 amps. It is also sometimes surprising what other loads appear on circuits that supposedly have no loads. I was on a service call once at a building that did testing of electronic equipment. They were experiencing "nuisance tripping" on the circuit for the equipment under test. The engineer was convinced that the circuit breaker was defective because he knew that his equipment only drew 10-amps. We found that the coffee maker in the adjoining office was on the same circuit. I believe the coffee maker (commercial unit) drew 1500 watts. When this was added to the draw of his equipment it was no wonder that the breaker occasionally tripped!
 
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