Causes of Incorrect Electrical Voltage
Why is the Voltage wrong at the Electric Service Panel? How to Troubleshoot Incorrect Voltage Readings.
Troubleshooting Incorrect Voltage Problems
Electrical Question: Why is the voltage wrong at the electric service panel?
This is a residential service in a mobile home park, 120/240 volt, grounded, however this could also happen at any residential or home electrical panel.
- Line1 to Neutral and Ground = 113 volts.
- Line2 to Neutral and Ground = 137 volts.
- Line1 to Line2 = 009 volts, not a typo.
- Neutral to Ground = 337 volts, not a typo.
- What do I have happening here?
- I have been a licensed electrician for 40 years and never run into this.
This electrical troubleshooting question came from: Roy, a Contractor from Payson, Arizona.
Thanks for your electrical troubleshooting question Roy.
How to Troubleshoot Incorrect Voltage Readings
Skill Level: Advanced – a Licensed Electrical Contractor or Certified Electrician should be consulted.
Tools Required: Basic Electricians Pouch Hand Tools, Ammeter and Voltage Tester.
Estimated Time: Depends on age of the home, the condition of the electrical system and available access to electrical components.
Precaution: Electrical troubleshooting of this level is best performed by a qualified electrical contractor.
The Causes of Incorrect Voltage Readings at an Electrical Service Panel
- Roy – you have two possibilities that is causing these voltage readings:
- One of the main incoming lines has a loose or burnt connection (my guess is L2 because L1 to N reading is correct) or you have an Open Neutral, which could be a loose or burnt connection as well.
- Roy, your getting a voltage feed-back due to one line that has been lost.
- Also, keep in mind that in Mobile Home Parks they have a 4-wire system, where the ground and the neutral are Isolated and this will explain why the high reading on the neutral to the ground, which is again due to the loss of one of the incoming main lines.
- Thoroughly inspect the main panel, especially the incoming service wires.
Check the cables at the main lugs and at any main breakers.
Don’t rule out that a connection of a breaker to the panel buss may be bad due to corrosion or arcing.
- If the problem is not found at the the home panel location then go upstream to where the power source. Many mobile home parks are fed from a large main distribution panel at another location. Be careful and keep yourself well insulated. The voltage on the neutral can be lethal as you know.
Continued Discussion, Electrician to Electrician, Roy’s Reply:
Hi Dave, Thanks for the reply,
I have been a licensed electrician since 1967 and never run into this before. I have had back feeds for one reason or another but not to this extent. The open has to be upstream because this condition exists at the power pedestal meter. I am surprised that none of the neighbors have complained about a similar problem. I had forgotten or never realized the mobile home parks were a true 4 wire system, thanks for that info. I will let you know what we find as soon as I get somewhere with this problem.
Dave’s Follow Up:
Thanks Roy, Yes, these feed backs can be a little interesting to say the least.
There is a possibility that other homes may be affected, it just depends on where the problem is. If the entire service checks out OK and the whole main panel has the same voltage readings then the problem may be with the utility company. Every once and a while one of their transformers may go bad and can produce a feedback like this too. I have seen this happen at the meter socket as well. Be Careful.
Dave’s Note: Problems like this require a Professional Licensed Electrical Contractor.
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Bathroom circut outlet has a gfi. Worked for a long time. Now gfi trips everytime. I disconnect gfi and found 0.26 acv from neutral to ground. Went to the panel and disconnected the that line and found the same voltage on the nuetral line that was disconnected. I first thought the gfi was bad until I found the voltage. There are other loads down stream from the gfi such as shower lights, exhoust fan, and stove hood.
I think I know what you are going to tell me. Trace the wire from panel to outlet amd make sure all connections are good. The problem is the panel is in the basement, all wire runs through drop cieling into attic and everything is buried uner a blamket of insulation.
Any help would be appreciated.
Hi Jim, yes checking your connections are a good idea, however I would mainly suspect connections between the ground system and the neutral. Apart from the main electrical panel, the neutral and ground system should be separated. There is a possibility that the neutral and ground are not separated at some point. I would also check the main ground system connections making sure that they are tight and that the ground system is bonded throughout. you may consider checking the polarity of all the outlets using a Receptacle Tester and Circuit Analyzer These testers are inexpensive and easy to use and can be very handy to identify devices that are not wired correctly.
My Name is John I installed electric hydronic baseboard heaters in my mobile home. They are 240 volts with Honeywell 240 volt 4 wire non programmable wall thermostats in 8 rooms. I have a dedicated 20 amp 2 pole breaker for each room in a 100 amp electric panel. Only 4 of them work, the other 4 heaters that do not work have low voltage readings coming off the breakers. Could the panel be the problem because I replaced the 4 breakers that had low voltage and they still do not work. Thank You, John.
The electrical panel would be the place to begin troubleshooting. Voltage readings should be taken starting at each individual 2-pole circuit breaker for the hydronic heaters that are not working. If you find that 240 volts is not being produced, and the circuit breakers have already been replaced, then there could be a problem within the panel bus where arcing may have caused damaged. The panel should be turned off before making any inspections of this kind, and obviously this is best performed by a qualified electrician.
There are several unknown factors which could contribute to this type of problem, especially the main electrical panel, such as the age, brand and type. There have been a history of know problems with the older Zinsco and Federal Pacific FPE, and Challenger panels and circuit breakers. Also keep in mind that the panel type, the bus configuration, the type of circuit breakers, and the way the circuit breakers have been installed, and the configuration of the circuit wires to the circuit breaker terminals, should be carefully examined as well.
I hope this helps you,