Gauge vs Distance

Part of Power Transmission

Selecting the correct wire gauge for every run length and current level to keep voltage drop within acceptable limits.

Why This Matters

Every conductor has resistance. Current flowing through that resistance produces a voltage drop — the voltage at the far end of a wire run is lower than at the near end. The longer the wire and the higher the current, the greater this voltage drop. If the drop is large enough, equipment at the end of the run sees insufficient voltage to operate properly: motors run slow and overheat, lights dim, and sensitive electronics malfunction or fail.

Wire gauge selection is therefore not just about whether the wire can carry the current without overheating — it is also about keeping the voltage at acceptable levels at the load end. These two criteria often require different gauge selections, and the more demanding requirement governs the choice.

In low-voltage DC systems (12V, 24V, 48V), voltage drop is almost always the controlling criterion. A 5% voltage drop on a 240V AC system (12V drop) is easily tolerable. The same 5% drop on a 12V system (0.6V drop) puts motor and battery charging circuits outside their operating range. This is why short-run, low-voltage systems need surprisingly heavy wire.

Resistance of Wire

Wire resistance depends on material, cross-section, and length:

R = ρ × L / A

Where:
  R = resistance in ohms
  ρ = resistivity of conductor material
  L = total conductor length (both out and return conductors)
  A = cross-sectional area of conductor

Resistivity values:
  Copper: 1.72 × 10⁻⁸ Ω·m
  Aluminum: 2.82 × 10⁻⁸ Ω·m
  Iron: 10.0 × 10⁻⁸ Ω·m (for reference — poor conductor)

The resistance per unit length for common wire gauges (copper):

AWG	Diameter (mm)	Resistance (mΩ/m)
6	4.11	1.30
8	3.26	2.06
10	2.59	3.28
12	2.05	5.21
14	1.63	8.28
16	1.29	13.2
18	1.02	20.9
20	0.81	33.1

Note: AWG decreases with increasing size. 6 AWG is thicker and lower resistance than 12 AWG.

Voltage Drop Calculation

V_drop = I × R_total

Where:
  I = current in amperes
  R_total = resistance of both the outgoing and return conductors

V_drop (%) = (V_drop / V_supply) × 100

Example — 15A load at end of 30m run on 12V system, 12 AWG copper:

Total conductor length = 30m out + 30m return = 60m
R per meter for 12 AWG = 5.21 mΩ/m = 0.00521 Ω/m
R_total = 0.00521 × 60 = 0.313 Ω
V_drop = 15A × 0.313 Ω = 4.69V
V_drop (%) = 4.69/12 × 100 = 39%

Only 7.3V reaches the load — this is completely unacceptable.

Same scenario with 8 AWG copper:

R per meter for 8 AWG = 2.06 mΩ/m
R_total = 0.00206 × 60 = 0.124 Ω
V_drop = 15A × 0.124 = 1.85V
V_drop (%) = 1.85/12 × 100 = 15.4%

Still too high. Try 6 AWG: 1.30 mΩ/m
V_drop = 15A × (0.00130 × 60) = 1.17V
V_drop (%) = 9.8% — marginal but may be acceptable for non-critical loads.

For a 12V system, the only real solution for this scenario is to use 24V or higher voltage. This illustrates why low-voltage DC systems are impractical for runs over about 15m at significant current.

Acceptable Voltage Drop Standards

The acceptable voltage drop depends on the application:

Application	Maximum acceptable drop
Battery charging	1–3% (excessive drop reduces charge current)
Motor loads	3–5% (more causes overheating)
Lighting (incandescent)	5% (slightly dim; acceptable)
LED lighting (with driver)	5–10% (driver accommodates variation)
Electronic equipment	3–5%
General loads on AC systems	3–5% (industry standard)

For transmission lines (high voltage, reduced current), even 10% drop may be acceptable if the step-down transformer’s output voltage is set 10% high to compensate.

Wire Selection Tables

These tables show the maximum one-way run length (in meters) for copper wire at various currents and voltage drop limits.

12V DC System, 3% Maximum Voltage Drop (0.36V drop)

AWG	5A	10A	15A	20A	30A
6	27.7m	13.8m	9.2m	6.9m	4.6m
8	17.5m	8.7m	5.8m	4.4m	2.9m
10	11.0m	5.5m	3.7m	2.7m	1.8m
12	6.9m	3.5m	2.3m	1.7m	1.2m

The short distances here illustrate why 12V DC systems are impractical for community distribution. Even 6 AWG cable can only carry 20A a total of 6.9m at 3% drop.

24V DC System, 3% Maximum Voltage Drop (0.72V drop)

Distances double versus 12V system. Still practical only for on-site single-building use.

120V AC System, 3% Maximum Voltage Drop (3.6V drop)

AWG	5A	10A	15A	20A	30A
10	110m	55m	37m	27m	18m
12	69m	35m	23m	17m	12m
14	44m	22m	15m	11m	7m

These distances are workable for community distribution.

240V AC System, 3% Maximum Voltage Drop (7.2V drop)

AWG	5A	10A	15A	20A	30A
10	220m	110m	73m	55m	37m
12	138m	69m	46m	35m	23m
14	87m	44m	29m	22m	15m

At 240V, even 14 AWG wire can serve loads 87m away at 5A. This is why raising system voltage is the correct solution to long-run distribution.

Using the Calculation in Practice

Step 1: Identify the load current. Add 25% safety margin for starting surges.

Step 2: Measure the actual run length (out and return). Multiply by 2 to get total conductor length.

Step 3: Determine acceptable voltage drop for your application.

Step 4: Calculate maximum permissible resistance:

R_max = V_drop_allowed / I

Step 5: Calculate required cross-section:

A_min = ρ × L / R_max

Step 6: Convert to AWG or metric cross-section and select the next larger standard gauge.

Step 7: Verify the selected gauge also meets ampacity requirements (can carry the current without overheating). Use the larger of the two requirements.

Practical Shortcuts

The doubling rule: Increasing current by 2× or run length by 2× requires moving two AWG numbers larger (from 12 to 10, or from 10 to 8) to maintain the same voltage drop percentage.

The 3% rule for 120V AC: On 120V circuits, 12 AWG wire is adequate for 15A loads up to 30m. 14 AWG is adequate for 15A loads up to 18m. These numbers are memorizable and cover most residential-scale work.

High-voltage for long runs: If a 12V system run must exceed 10m at significant current, the correct solution is usually to switch to 48V and rerun the wiring — not to use ever-larger copper wire. The weight and cost of wire escalates rapidly as gauge increases.

Bundle derating: Wire bundled together with other conductors cannot shed heat as easily. Code standards require derating (reducing the ampacity by 70–80%) for wires bundled together. A single 12 AWG wire carries 20A safely; six 12 AWG wires bundled together in conduit must each be limited to 12–14A.