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I finally checked the math on voltage drop in a 200 foot run | Electricians - Guest Forum

I finally checked the math on voltage drop in a 200 foot run

Got a call last week from a homeowner who had lights dimming at the end of their driveway. They ran 12 gauge wire about 200 feet to a shed and were pulling 15 amps. I used a voltage drop calculator online and realized they were losing almost 12 volts. That's way over the 3% recommendation for a branch circuit. Ended up telling them they needed 10 gauge at minimum or even 8 gauge if they wanted to run power tools out there. Has anyone else had homeowners argue with you about the wire size after you show them the numbers?

2 comments

2 Comments

roses693mo ago

Ampacity charts aren't really the tool for voltage drop, they're for how much current a wire can handle before it melts. The 12 gauge wire at that shed is fine for 15 amps ampacity wise, it just can't carry that much load 200 feet without dropping voltage. I had a guy pull out the Southwire voltage drop app on his phone and still argue that his grandfather ran 14 gauge a quarter mile and never had issues. Grandpa probably wasn't pulling 15 amps steady either, just had a single light bulb and a radio. If you show them the math and they still want to argue, I just shrug and move on. Keep the receipt.

jenny_fox3mo ago

Gotta love that look they give you when the calculator proves you right but they still think you're upselling them. I've had people tell me "but the wire is new, it should be fine" like copper degrades in a year or something. The 3% rule is no joke especially on long runs like that, I've seen dimming lights and even motors that just hum instead of start. Your mileage may vary but I usually show them the ampacity chart from the manufacturer and let them do the math themselves. Sometimes they still argue and I just say "your call, but I won't warranty it for power tools" and walk away.