Wire Size Calculator: Stop the Voltage Drop Before It Stops You
Wire Size Calculator: Stop the Voltage Drop Before It Stops You
Electricity is like water flowing through a pipe. If the pipe is too narrow (thin wire) or too long (distance), the pressure drops. In plumbing, this means a weak shower. In electricity, it means Heat.
Resistance generates heat, melting insulation and starting fires. Whether you are wiring a shed 100 feet away or installing a solar bank, use this NEC-compliant tool by ShockTrail to ensure your electrons arrive safely.
The AWG Estimator
Calculate the correct Wire Gauge (AWG) based on Load, Distance, and acceptable Voltage Drop.
DC Wire Sizing Tool
Engineering Logic by ShockTrailRecommended Minimum Size:
⚠️ Always use a fuse close to the battery matching this wire rating.
PART 1: The Physics of Voltage Drop
Every foot of wire has internal resistance. As current flows, some voltage is lost to this resistance.
The NEC 3% Rule
The National Electrical Code (NEC) recommends a maximum voltage drop of:
- 3% for Branch Circuits (Outlet to Breaker).
- 5% for Feeder + Branch combined (Outlet to Main Panel).
Example: If you start with 120V and lose 10% (12V) over a long run, your drill will run slow, your lights will dim, and the wire inside your wall will act like a toaster element.
DC vs. AC (Why 12V is Hard)
Low voltage systems (12V Solar/Van Life) are much more sensitive to distance. Losing 2 Volts on a 120V system is barely noticeable (1.6%). Losing 2 Volts on a 12V system is a 16% drop, which will shut down your inverter instantly.
PART 2: Real-World Case Studies
Distance kills voltage. Let’s analyze three scenarios where standard wire tables fail.
Case Study 1: The Backyard Shed (150ft Run)
The Project: Running a standard 20 Amp circuit (120V) to a workshop shed located 150 feet from the house.
The Standard Mistake: Most DIYers grab 12/2 Romex (Standard for 20A) and run it.
The Math:
- Voltage: 120V. Amps: 20A. Distance: 150ft.
- Drop with 12 AWG: ~7.8 Volts (6.5%). FAIL. The voltage at the shed will be 112V under load. Motors will overheat.
- The Fix: Upsize to 8 AWG wire to keep the drop under 3%.
Case Study 2: The EV Charger (Continuous Load)
The Project: Installing a Tesla Wall Connector. 48 Amps charging rate. 50ft run.
The Continuous Load Rule: For loads that run 3+ hours (like EV charging), you must size the breaker/wire at 125% of the load.
The Math:
- 48 Amps × 1.25 = 60 Amps Required Capacity.
- Standard: 6 AWG Copper is rated for 55A (NM-B/Romex) or 75A (THHN in conduit).
- Decision: If using Romex inside walls, you MUST use 4 AWG. If using THHN in conduit, 6 AWG is acceptable.
Case Study 3: The Solar Van (12V High Current)
The Project: Connecting a 2000W Inverter to a 12V Battery Bank. Distance is only 5 feet.
The Math:
- Amps: Watts / Volts = 2000W / 12V = 166 Amps!
- Even at 5 feet, the resistance is massive.
- Standard: A 10 AWG house wire would melt instantly.
- Requirement: You need 2/0 AWG (00 Gauge) welding cable to handle this current without catching fire.
PART 3: Copper vs. Aluminum
Copper is the gold standard (pun intended), but it is expensive. Aluminum is cheaper but less conductive.
The “Upsize Rule”
If you switch from Copper to Aluminum to save money (common for Subpanel Feeders), you usually need to go 2 sizes bigger.
- 100 Amp Service (Copper): #4 AWG (THHN).
- 100 Amp Service (Aluminum): #2 AWG or #1 AWG.
Warning: Never connect Aluminum wire to a standard outlet designed for Copper. It causes galvanic corrosion and fires. Use “CO/ALR” rated devices or special connectors (Noalox).
PART 4: Wire Types (Romex vs THHN vs UF-B)
The insulation matters as much as the metal.
| Type | Location | Amp Rating (6 AWG Example) |
|---|---|---|
| NM-B (Romex) | Indoor / Dry Walls | 55 Amps (60°C column) |
| THHN / THWN | Inside Conduit / Wet or Dry | 75 Amps (75°C column) |
| UF-B (Underground Feeder) | Direct Burial (Dirt) | 55 Amps (60°C column) |
| Welding Cable | DC Battery Banks (Flexible) | High (Fine strands) |
PART 5: Conduit Fill (Bonus)
You cannot stuff a pipe 100% full of wires. They need air to cool down.
- Rule: Maximum 40% fill for 3+ wires.
- If you pull too many wires, they overheat, and you violate code.
Frequently Asked Questions (FAQ)
Does 120V vs 240V change the wire size?
For Ampacity: No. 20 Amps is 20 Amps.
For Voltage Drop: Yes! 240V pushes twice as hard, so you can run the same wire twice as far with the same % drop.
What does AWG stand for?
American Wire Gauge. It is a reverse scale: Smaller numbers = Thicker wire. 12 AWG is thin; 0 AWG is thick.
Can I use extension cords for permanent wiring?
No. Extension cords are stranded wire not rated for permanent wall enclosure. They degrade and cause fires.
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