Voltage drop when water heater turned on?

[twinboat]

You can try larger gauge extensions but if your plugging into a 15 amp outlet, that's typically wired with 14 gauge cable.

Then you need to figure in the length of that 14 gauge cable between the outlet and breaker box and what else is running on that circuit.

The fix is a homerun to a dedicated outlet with the proper size wiring.

[BCam]

Originally Posted by TandW

About a 3% voltage drop can be expected for most 100 foot wiring runs.

That's not a constant, it's a variable used to calculate required wire size as in: "What wire size is necessary for a 100' run at 15A, 120V AC in order to only have a 3% voltage drop?" For building/house wiring, the National Electrical Code specifies a maximum voltage drop of 5%, combined "feeder and branch" circuit with a 3% max for either one. If the wire gauge is to small, the voltage drop will increase accordingly.

https://www.protoolreviews.com/exten...re-gauge-amps/

[TandW]

Originally Posted by BCam

That's not a constant, it's a variable used to calculate required wire size as in: "What wire size is necessary for a 100' run at 15A, 120V AC in order to only have a 3% voltage drop?" For building/house wiring, the National Electrical Code specifies a maximum voltage drop of 5%, combined "feeder and branch" circuit with a 3% max for either one. If the wire gauge is to small, the voltage drop will increase accordingly.

https://www.protoolreviews.com/exten...re-gauge-amps/

Thank you for providing the nitty gritty details!
The OP's greatest problem is an inadequate for the purpose 15 amp house circuit that may have as much as a 50 foot run of it's own. Add in the 100' 12ga extension and 25 feet of RV cord and half of the drop he is seeing is because of total circuit length alone. If his 15 amp circuit has other draws on it that also is detrimental. If his 15 amp outlet was stabbed with the wires rather than looping the wires around the screws that also is detrimental. Each plug, male and female, are potential sources of resistance which can be found out with a temperature gun.
I think he'll get it sorted out.