Our Staduim is a 208/120 Y system. The other day at the football game a video trailor plugged into a dedicated 20amp 120volt circuit. They could not use the plug because the voltage would drop to 105 volts and it was too low for them. I went over there today to test it. I tested the voltage and it was 118.5 volts without a load. I put a load of 16amps on it and it dropped to 108volts. Is this normal? If not, any thoughts on what the problem could be and how to fix it? Thank you
Voltage Drop
- Thread starter JdoubleU
- Start date
- Status
- Occupation
- Licensed Electrician
105V is not expected but that does not make it not normal. Can you give some more details?
The voltage only drops at that dedicated plug. It's a number 12 wire. Maybe the distance is an issue but not any further than I have seen before. I will run this test on another circuit in another building. If it does it in another building then what are your thoughts.
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meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
If you have access to the three phase source (as close to the load as possible), put a meter on and see if either of the other two phases increase in voltage when the loaded phase drops. If so, you have a neutral problem. If not then you have a problem with the phase wiring. 118.5 is pretty normal for a loaded 120/208 service with a fairly long secondary. You could ask the utility to test their incoming voltages. Should be free.
The circuit is part of a multiwire circuit. I will test the other phase to neutral with the load on and see what happens. The circuit comes off a 3 phase sub-panel that is only 40 feet for the plug. The service main is probable 100 feet. One thing I did notice though is at the plug I have 3 volts between neutral and ground. Could I have a bad connection at the main bonding jumper?
With out load on that circuit at the sevice, sub-panel, and plug we have A-N=119, B-N=118.5 and C-N=120. This all seems normal to me. When I Apply a load of 17amps to B phase at the plug it reads at the service the following. (A-N=120.1, B-N=117, and C-N=121.5). At the sub-panel the following (A-N=120.1, B-N=117, and C-N=121.5). At a box 5 feet before the outlet it reads the following. (A-N=120.2, B-N=113.5, and C-N=122.1) and at the outlet the B phase reads 107 to the Neutral. ANY IDEAS
Last edited:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If the wire sizes are equal all along the way back to the panel, you would expect the voltage drop on the neutral and hot to be the same, each accounting for 1/2 of the voltage drop across the load terminals. There is no corresponding voltage drop on the EGC because it carries no current.
If the no load voltage is 115V and the voltage under load is 108V, I would expect that 7 volt drop to split up as 3-1/2 volts each on hot and neutral.
If the no load voltage was 120V, then I am a little surprised that the neutral to ground voltage difference was only 3 volts.
A bad connection at the main bonding jumper should only show up during a fault condition since only then would it be carrying current.
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kwired
Electron manager
- Location
- NE Nebraska
If you are dropping ~8 volts @ 16 amps, then you are losing 128 VA somewhere. Some of it is line loss but you seem to have relatively short lines so not all of it is likely to be line loss. It would seem to me if you have a bad connection somewhere the easy way to find it may be to leave a heavy load on it and that bad connection will likely get hot fairly quickly, hopefully making it easier to find. Leave it loaded long enough and you likely have an open circuit eventually at the weak point.
Here is what I have. 120/208 volt service panel at the north end of the stadium feeds a 120/208 volt sub-panel at south end of the building. The load is placed on a 120 volt receptical comming off the sub-panel. The load is 20amps and at that receptical we now read 110volts. The voltage at the subpanel without this load is around 119 on each phase. The voltage at the subpanel with this load is as follows. (118, 116, 122). These numbers are again rough I am sorry, but the thing I see here is that when one goes down the others raise indication a possible neutral issue. Finding it is my next step if you would all agree. I then went to the service and measured the voltage on the Supply side of the building main overcurrent device. It reads the same as the sub-panel. If I go to the transformer and measured the voltage and it reads a better voltage balance would this indicate that the neutral issue has to be the neutral feeding the sub-panel?
kwired
Electron manager
- Location
- NE Nebraska
even if your neutral is in good condition the voltage will drop on the loaded line and rise on the non loaded lines, but the amount of voltage drop is the key to how much. Long runs can fool you into thinking you have a bad neutral when all you really are seeing is the voltage drop of an otherwise good line. How long is the run to the main panel - run to the transformer, also need to consider other loads and their contribution to readings you are seeing. IIRC you said your branch circuit was only like 40 feet long - I wouldn't expect too much VD in that distance if connections are good, but in a stadium (with no other details) the feeder to the panel your circuit originates in could be fairly long and conductor size and load level do become much bigger factors in voltage drop if the run is long.
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