I Melted an Extension Cord in My Garage — Here's the Wire Gauge Math I Should Have Used
I ran a $12 extension cord from an outlet on the far wall of my garage to a 1,500-watt space heater one January night. Twenty minutes in, the cord was warm enough to notice through my sock when I stepped on it, and there was a faint plastic smell near the plug end. I unplugged it, felt the insulation — genuinely hot, not "warm from being coiled" hot — and only then thought to check what gauge the cord actually was. It was 16 AWG, rated for about 13 amps at short lengths. My run was 100 feet, and a 1,500-watt heater on a 120V circuit draws 12.5 amps. On paper it looked fine. In practice, the extra 100 feet of thin copper was acting like a resistor, and resistance turns into heat.
⚠️ Disclaimer
This article is for informational and educational purposes only and does not constitute professional electrical advice. Wire sizing for permanent circuits, sub-panels, and anything tied into your home's electrical system should be verified against your local electrical code and, where required, installed or inspected by a licensed electrician.
What Is Wire Gauge (AWG) and Why Does It Matter?
American Wire Gauge (AWG) is a standardized numbering system for wire thickness, where a lower number means a thicker conductor. A 10 AWG wire is significantly thicker — and can safely carry far more current — than a 16 AWG wire. This runs backward from what most people expect on first encounter, and it trips up a lot of first-time DIYers, myself included.
Wire gauge matters because every conductor has resistance, and resistance does two things as current flows through it: it wastes energy as heat, and it causes voltage drop — a loss of voltage between the source and the device at the end of the run. Undersize the wire and you get two compounding problems at once: the wire itself can overheat (a fire risk, which is what I was flirting with in my garage), and the device at the far end receives less voltage than it needs to run properly.
Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor's resistance over distance. The National Electrical Code (NEC), the standard adopted across most of the United States, recommends keeping voltage drop under 3% for a branch circuit alone, and under 5% for a combined feeder and branch circuit — beyond that, motors run hotter, LED drivers flicker, and heating elements underperform.
Key Takeaway
Ampacity (the maximum safe current) and voltage drop (the loss over distance) are two separate limits. A wire can pass an ampacity check and still fail on voltage drop if the run is long enough — which is exactly what happened with my 100-foot extension cord.
The Math I Should Have Done Before Plugging In
Here's the calculation I ran after the fact, using the standard voltage drop formula electricians use for single-phase and DC circuits:
Where K is the resistivity constant (12.9 for copper at ~75°C), I is the current in amps, L is the one-way length in feet, and CM is the wire's cross-sectional area in circular mils. The "2" accounts for the fact that current travels out and back — a 100-foot cord run means 200 feet of actual conductor.
My 16 AWG cord has a cross-section of 2,581 circular mils. With a 12.5-amp heater and a 100-foot run:
That's a 10.4% drop on a 120V circuit — more than three times the NEC's recommended 3% ceiling. That missing voltage doesn't just vanish; it gets converted into heat right there in the cord, which is exactly the smell and warmth I noticed. Swap in a 12 AWG cord (6,530 circular mils) for the same run and load, and the math changes dramatically:
Still slightly above the ideal 3%, but close enough that the cord runs cool, and well clear of the danger zone. The Wire Gauge Calculator runs exactly this formula for you — enter the current, voltage, length, and material, and it returns the minimum safe AWG size instantly, so you're not doing circular-mil arithmetic in a cold garage.
| Cord gauge | Circular mils | Voltage drop (12.5A, 100 ft) |
|---|---|---|
| 16 AWG | 2,581 | 12.5V (10.4%) |
| 14 AWG | 4,107 | 7.9V (6.6%) |
| 12 AWG | 6,530 | 4.9V (4.1%) |
| 10 AWG | 10,380 | 3.1V (2.6%) |
⚠️ Note
Cheap "big box store" extension cords are very often 16 or even 18 AWG — fine for a lamp or phone charger over a short distance, dangerously undersized for a space heater, table saw, or shop vacuum run more than 25–50 feet. Always check the gauge printed on the cord's jacket before assuming it's adequate.
How Do You Calculate the Right Wire Gauge?
Sizing a wire correctly means checking two things and using whichever result calls for the thicker conductor:
- Ampacity — does the wire's rated current-carrying capacity exceed the load, with margin? Household circuits are typically sized so the continuous load doesn't exceed 80% of the breaker rating.
- Voltage drop — over this specific length, does the wire stay under 3% drop for a branch circuit?
For short runs (under roughly 25 feet), ampacity is almost always the limiting factor. For long runs — garage sub-panels, workshop outbuildings, dock power, RV hookups — voltage drop usually becomes the binding constraint well before you'd hit the ampacity ceiling, which is exactly the trap I fell into.
A quick worked example: say you're running a 20-amp circuit to a table saw 80 feet from the panel, on 120V, with copper wire. Checking ampacity alone, 12 AWG (rated for 20A) looks sufficient. Running the voltage drop formula — VD = (2 × 12.9 × 20 × 80) / 6,530 = 6.3 volts, a 5.3% drop — shows it actually falls short of the 3% target, and you'd want to step up to 10 AWG (10,380 circular mils), which brings the drop down to 3.3%.
💡 Pro Tip
When in doubt, size up one gauge rather than down. A slightly thicker wire costs a little more up front but runs cooler, loses less energy to resistance, and gives you headroom if you ever add load to the same circuit later.
Outside the US, wire is sized by cross-sectional area in square millimetres (mm²) rather than AWG — the standard under BS 7671 in the UK and the AS/NZS 3000 wiring rules in Australia and New Zealand. A 2.5mm² cable, the standard for a UK ring-main socket circuit, is roughly equivalent to 14 AWG and is typically rated around 20–27A depending on the installation method and cable type. The same voltage-drop logic applies regardless of which system you're working in — longer runs and higher currents both push you toward a thicker conductor.
Copper vs Aluminum: What Changes?
Copper conductors have roughly 40% lower electrical resistance than aluminum conductors of the same cross-sectional area, which is why copper is the default choice for most residential branch circuits and extension cords. Aluminum is lighter and cheaper per foot, which makes it common for larger feeder cables and utility service drops, but it needs to be sized up — typically two gauge sizes larger than the equivalent copper run — to carry the same current within the same voltage drop budget.
The resistivity constant (K) in the voltage drop formula reflects this directly: 12.9 for copper versus 21.2 for aluminum at standard operating temperature. Plug the same 20-amp, 80-foot table saw example through with aluminum instead of copper, and the voltage drop on 10 AWG jumps to roughly 5.5% — back over the recommended ceiling — meaning you'd need to step up to 8 AWG aluminum to match what 10 AWG copper achieves.
Key Takeaway
Never assume an aluminum wire and a copper wire of the same AWG number are interchangeable. Aluminum needs a larger gauge number reduction (a thicker wire) to deliver equivalent ampacity and voltage drop performance to copper.
Aluminum wiring also requires connectors and terminations specifically rated for aluminum (marked "AL" or "CU/AL") — using copper-only connectors on aluminum conductors is a well-documented cause of loose connections and overheating over time, which is one reason many electricians still prefer copper for anything inside the finished living space of a home.
Sizing Wire for an EV Charger or Garage Sub-Panel
This is the wire-sizing question I get asked most often now, since home EV charger installs have become one of the most common electrical upgrades homeowners take on. A typical Level 2 home charger draws 32–40 amps continuously on a 240V circuit.
Because EV charging counts as a continuous load — running at full current for three hours or more — NEC 210.19 requires the circuit conductors and breaker to be sized for 125% of that continuous rating, not the raw draw itself.
A 40-amp continuous charger therefore needs a circuit sized for 50 amps (40 × 1.25). At typical residential run lengths (30–60 feet from a US main panel to a garage), that generally lands on 6 AWG copper — which is rated for 65A at 75°C per NEC ampacity tables and comfortably clears both the ampacity and voltage-drop checks for runs in that range. Longer runs, an aluminum feeder, or a detached garage sub-panel can all push that up further, which is exactly the kind of check worth running through the calculator before you buy wire or call an electrician for a quote.
| Continuous load | 125% NEC minimum | Typical copper gauge |
|---|---|---|
| 24A (Level 2, lower amp) | 30A | 10 AWG |
| 32A (common Level 2) | 40A | 8 AWG |
| 40A (higher-output Level 2) | 50A | 6 AWG |
| 48A (near max residential) | 60A | 4 AWG |
⚠️ Note
Table above assumes short-to-moderate copper runs at standard ambient temperature — always re-check voltage drop for your actual run length, and confirm the final circuit against your local code and panel capacity before installing. EV charger circuits are permanent, high-current installations; in most jurisdictions they require a permit and, in many, a licensed electrician.
Frequently Asked Questions
Is it OK to use a slightly thicker wire than the minimum required?
Yes — going thicker than the minimum is always electrically safe and simply reduces voltage drop and heat further. The only downsides are cost and, for larger gauges, stiffness that can make an installation more awkward to work with. There's no "too thick" from a safety standpoint, only diminishing returns on cost.
Why did my extension cord get warm even though it was rated for the amperage?
Most printed ampacity ratings on cords assume a relatively short length. As the run gets longer, resistance increases proportionally, and voltage drop (and the associated heat) climbs even though the ampacity rating hasn't technically been exceeded. This is exactly the scenario that overheated my 16 AWG cord — technically within its amp rating, but far too long a run for that gauge.
Do I need to double the wire length in my voltage drop calculation?
For single-phase AC and DC circuits, yes — current has to travel out to the load and back to the source, so the voltage drop formula multiplies the one-way length by 2. Three-phase circuits use a different multiplier (1.732, from √3) because the phase relationship changes how the return current behaves.
What gauge wire do I need for a 20-amp circuit?
For a standard 20-amp US residential branch circuit at typical lengths (under roughly 50 feet), 12 AWG copper is the standard minimum. Longer runs may need to step up to 10 AWG once voltage drop is factored in — run your specific length and load through the Wire Gauge Calculator to check.
Is AWG the same numbering system everywhere in the world?
No. AWG is the standard in the US and Canada. Most of the rest of the world, including the UK, EU, and Australia/New Zealand, sizes wire by cross-sectional area in square millimetres (mm²) under standards like BS 7671 or AS/NZS 3000. The underlying physics — and the voltage drop formula — is identical; only the labeling convention differs.
Try It Yourself
The gap between "rated for the amps" and "safe for this specific run" is exactly what voltage drop calculations exist to catch — and it's the gap that nearly cost me a melted extension cord and a much worse night in the garage.
Use the Wire Gauge Calculator to enter your current, voltage, run length, and conductor material, and get the minimum safe AWG size and voltage drop instantly.
Also worth checking:
- Ohm's Law Calculator — for the underlying voltage, current, and resistance relationships
- Voltage Divider Calculator — for sizing resistor networks in low-voltage circuits
- Electricity Cost Calculator — to see what running higher-draw appliances actually costs on your bill
