Methods for testing voltage drop

[blinkous]

Good point.

In measuring the voltage drop on these corridor circuits using the Ideal tester, the numbers were 10%-14% voltage drop at 12A load. A space heater was then plugged into the receptacle tested (nameplate of 12.5 amps) and voltage measured using a Fluke meter. Again, numbers in the 10%-14% voltage drop range...of course with this voltage drop, the space heater is not going to be pulling its nameplate amperage, so these voltage drop numbers were slightly lower than seen with the Ideal tester.

 

[haskindm]

You are still fixated on voltage drop. Voltage drop does not trip breakers, amperage trips breakers. I realize that voltage drop may effect amperage, especially where motors are concerned. What is the amp load on the circuit when the breaker trips? You may have a 20-amp breaker that is tripping prematurely. A 20-amp breaker should hold 20-amps indefinitely, if it is tripping with a 12.5 amp load, you have a bad breaker; period. Use a good old fashioned amprobe on the circuit at the breaker and read the amperage when the breaker trips, then you will then know why the breaker is tripping. If the voltage drop is so severe that your 12.5 amp buffer is trying to draw 30-amps, then the breaker has good reason to trip, but you will not know what is going on until you read the amperage on the circuit when the buffer trips. It is also possible that the buffer has an intermittent short. In that case you will see the amperage spike to perhaps 100-amps or more just prior to the breaker tripping.

 

[RayS]

I think the voltage drop may be part of the problem, because the buffer is gonna try to deliver the required power regardless of voltage- at reduced voltage, this means more amps.

Sounds like the branch wiring may be a little too light for the intended load, because of a long branch run.

Question- does NEC require 5% or less drop at the farthest outlet under maximum load? or intended load? or calculated load?


(edit to rephrase question)

 

[blinkous]

NEC does not stipulate under which load the voltage drop is to be calculated, though there may have been a formal interpretation at some point.

Point well taken in regards to using an amp probe. However, doesn't a voltage drop of 14% sound quite excessive when using a simple space heater? There may be multiple issues inolved.

I don't believe it is a bad breaker, because this is happening in corridors throughout the school, which is approximately 300,000 square feet.

A 20 amp breaker will only hold 16 amps indefinitely (80% of rating).

 

[coulter]

... However, doesn't a voltage drop of 14% sound quite excessive when using a simple space heater? ...

Yes. Make up a sketch showing the wire sizes and lengths. This will tell you if the circuit is poorly designed

...I don't believe it is a bad breaker, because this is happening in corridors throughout the school, which is approximately 300,000 square feet. ... A 20 amp breaker will only hold 16 amps indefinitely...

I didn't see in the posts where you measured, at the CB, the running current of the buffer. As haskindm said, this will also tell you if there are other loads on the circuit or if the buffer has an unexpectedly high current draw.

The buffer could well have a universal type motor - if so, a high Vd could well shove the current way up.

carl

 

[haskindm]

A 20 amp breaker will hold 20 amps indefinitely. In fact it will hold a good bit more than 20 amps for fairly long periods. I believe the UL standard is that it MUST hold a 125% overload for a minimum of 300 seconds (five minutes) at the rated temperature (usually 40 degrees C). At cooler temperatures it will hold even longer. The 80% is for CONTINUOUS LOADS only, which are three hours or more. I doubt that a floor buffer will run CONTINUOUSLY (never releasing the trigger) for three hours. This theory that a breaker can never be loaded more than 80% is just plain wrong. The 80% only applies to a continuous load.
Also voltage drop is not a code requirement except on fire pumps. It is only mentioned in a fine print note, which are not enforceable. So in no way are we discussing an NEC violation in this case.
At 12.5 amps a #12 conductor would have to be 340 feet long (170 feet from the power source) to drop 16.8 volts (14% of 120 volts). Is it possible for your circuit to be that long? If not, there is another problem somewhere.

 

[coulter]

... I believe the UL standard is that it MUST hold a 125% overload for a minimum of 300 seconds (five minutes) at the rated temperature (usually 40 degrees C). ...

My understanding is this is usually tested in free air. Doesn't count if CB is installed in a panel.

edited to add: Read this as a queation, not a statement.

carl

 

[haskindm]

I am not sure about the free air, but the point is that a 20 amp breaker will hold 20 amps for as long as anyone cares to time it. Of course some 20 amp breakers (I will not mention the brand name) will hold 100 amps indefinitely!

 

[blinkous]

I was assuming 'indefinite' was referring to a continuous load. I agree with your statement. Thermal magnetic devices should operate accordingly.

It sounds as though the only way to discern the source of the problem is to measure the current when the device is plugged into a receptacle directly adjacent to a panelboard, and then again at a receptacle some distance away.

 

[khixxx]

I didn't see in the posts where you measured, at the CB, the running current of the buffer. As haskindm said, this will also tell you if there are other loads on the circuit or if the buffer has an unexpectedly high current draw.

The buffer could well have a universal type motor - if so, a high Vd could well shove the current way up.

carl

Maybe the EC that wired the school or joe maintenance guy added another load circuit post construction to those circuits?

Does a buffer have a capacitor that could be bad? If there are multi buffers and one is flacking out it would be hard to pinpoint an exact location of the problem.

I have seen breakers trip because of VD. HO called me asking why his pool pump keeps tripping. I guessed that 300 feet of 16ga extension cord had something to do with it. Not sure of the outcome.

 

[LWFLASH]

Buffer Motor

Buffer Motor

How old is the buffer? Is the motor hard to start or dragging armature? Is the Buffer drawing more than the label indicates due to age, wear and tear, etc. Also is the person doing the buffing of the floors wipping the cord to disconnect it and then plug it in again only closer? There are a lot of variables that I would want to watch and check before I said there was VD on the Circuit. Breakers could be warn although all coridoors outlets and breakers at the same time is a bit rare. Is the Buffer Ground good or is there a leak and trip situation in the buffer causing the breaker to trip? Have you pulled the end off the buffer to see if it has strands loose that could be touching across when moved but not when plugged in? Check the equipment first then look at the circuits and breakers next.
Just my 2 cents worth.
Scott

 

[quogueelectric]

motor loads voltage dip

motor loads voltage dip

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.

This statement is not true I am sure unintentional of course and no disrespect intended. However that all being said about 20 yrs ago I wired a small refinery and also a backup generator. This generator had a voltage adjustment dial on it so I wanted to see for myself how much the load in amps increased as I turned down the voltage. And being that most motors are not pushed to thier max there is some play in the design . I turned down the voltage on a generator test and measured the amperage which also reduced proportionately. I was completely shocked as this went against everything that I was taught . This does not hold true when a motor is pushed past its point of diminishing return of course but to me this statement should be put on mythbusters for all to see.

 

[winnie]

I turned down the voltage on a generator test and measured the amperage which also reduced proportionately. I was completely shocked as this went against everything that I was taught . This does not hold true when a motor is pushed past its point of diminishing return of course but to me this statement should be put on mythbusters for all to see.

It is more fair to say that the statement (that current will increase as voltage drops for motor loads) is an (over)simplification.

As I said in my first post in this thread, for any given mechanical output (combination of shaft torque and shaft speed) a given motor will have an optimal operating voltage (and as appropriate, frequency). Operation _above_ or below this voltage will mean that the motor is less efficient. Going below this optimal voltage will _increase_ current consumption.

For common induction motors operated directly off line voltage, the optimal operating point will probably have been designed to be be somewhere in the middle of the allowed supply voltage range when the motor is operating at rated speed and 100% torque. At lower torque, the optimal voltage will be lower than this. This means that a lightly loaded motor, operated directly from line voltage, is almost certainly being supplied at greater than the optimal voltage.

Reducing the voltage to a lightly loaded motor will improve its efficiency, improve its power factor, and quite possibly _reduce_ the current that it consumes. Reduce the voltage more, and eventually the current consumption will start to go up.

-Jon

 

[ramsy]

Reducing the voltage to a lightly loaded motor will improve its efficiency, improve its power factor, and quite possibly _reduce_ the current that it consumes.

Inductive-motor circuit power factors are related to current lagging voltage by 90? (1), and corrected by adding capacitance or by removing motor load. I would need a reference to see where voltage potential affects motor power factors.

I suspect turning down the generator voltage is more likely to reduce that generator frequency / RPM, lowing Hz, motor speeds, and motor-power consumption.

1) HERMAN, DELMAR'S STD Textbook of Electricity, 1999 p445.

 

[winnie]

I would need a reference to see where voltage potential affects motor power factors.

Pick up any text on control of induction motors via adjustable speed drives. The one that I have at hand (not the best for this discussion, but convenient) is Motor Control Electronics Handbook, Richard Valentine, McGraw-Hill. For this discussion, the section on slip optimization of induction motors (8.3.4) is most relevant.

The output of the motor can be called its mechanical operating state, a particular combination of torque and speed. A given mechanical operating state may be produced by a large number of different combinations of voltage and drive frequency. You could use relatively high drive voltage, combined with low slip, or you could use lower drive voltage combined with greater slip, or anything in between. The lowest possible drive voltage for a given mechanical state is set by the break-away torque of the motor at the particular drive voltage and frequency. The highest possible drive voltage is limited by saturation current flow and excessive heating.

As the terminal voltage is increased, the magnetizing current flow increases, the air gap magnetic flux increases, and the 'torque producing' current flow decreases. At low voltage, lots of current has to flow in order to produce torque in the weak magnetic field, and you get standard I^2R heating of the coils. At high voltage, lots of current has to flow to produce the magnetic field, and again you see I^2R heating, in addition to lots of magnetic losses in the iron.

Between these two limit points, there is a particular combination of voltage and frequency which will cause the motor to operate most efficiently. This is the operating point where the sum of all losses in the motor is minimized.

As the voltage goes down from the most efficient operating point, you will have less magnetizing current flow, more torque producing current flow, and power factor will _go up_ even though efficiency is going down.

As the voltage goes up from the most efficient operating point, I would expect magnetizing current flow to go up, but losses are also going up, so I don't know which way power factor would go.

-Jon

 

[ramsy]

..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

The instruction booklet for that ShureTest unit has a troubleshooting guide that describes how to find a high-resistance point along a branch circuit by checking one outlet at a time, perhaps up to the panel with the banana clip accessory, if the qualified person is OSHA competent for hot work and proper use of PPE . 29 CFR 1910.333(c)2, NFPA 70E Part 2, 1-5.4

A high enough upstream / service impedance can raise inductive motor currents also, however, the ShureTest unit can't check feeder problems upstream from the panel since voltage over 150VAC disables the load test, impedance, and VD functions.

As you already know the _resistance_ function of the ShureTest checks impedance at receptacles by putting a load across (H-N) and (H-G). And, any cl/amp & meter gets the same by metering a constant load for Z=VD/I, where VD=(Et-Er) or (Nominal volts - load volts).

Branch EGC & Neutral impedances are rarely identical, but neither should be above 0.25 Ohm, according to ?6.4.1.1.4 of IEEE 1100. Newer Shuretest units show impedance tests above 1 Ohm as "FAILED", my older unit is designed to show "FAILED" above 2 Ohms.

A qualified person should stick with impedance values to compare branch outlet readings to upstream measurements. When qualified persons find building panels are OK, but feeder or service impedances are unusually high, its most commonly the service neutral.

 

[ramsy]

..Motor Control Electronics Handbook, Richard Valentine, McGraw-Hill. For this discussion, the section on slip optimization of induction motors (8.3.4) is most relevant.

Jon, you blow me away once again. I never thought DC drives used anything more than frequency to control motor speed. If you had a dime for each amazing effort you put into this forum, you might be a very wealthy man by now.

 

[winnie]

Jon, you blow me away once again. I never thought DC drives used anything more than frequency to control motor speed. If you had a dime for each amazing effort you put into this forum, you might be a very wealthy man by now.

Yea, but ya have to deduct every time I get something wrong and then I need to hide from the collection agency

-Jon

 

[Pierre C Belarge]

Yea, but ya have to deduct every time I get something wrong and then I need to hide from the collection agency

-Jon

Jon
I believe we are all in the same boat here...

 

[Pierre C Belarge]

I would like to add that this thread has been interesting reading, especially for one who has not had extensive training in electric motors.