Why does 120 volts need a larger wire than 240 volts?

The same volt drop is a bigger percentage of 120 than of 240. Percent drop is Vdrop divided by the source voltage.

Is 10 AWG always legal for a 30 amp RV outlet?

10 AWG copper meets the 30 A small-conductor rule, but it can miss a 3 percent drop on a long 120 V run. The result shows which limit governs.

What conduit is assumed?

PVC Schedule 40 at an editable price per foot. Changing to EMT or rigid metal changes the cost line, not the voltage-drop math.

30 Amp RV Outlet Wire Gauge and Voltage Drop

A 120 V, 30 A TT-30 style outlet where voltage drop shows up quickly.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

8 AWG · 8.37 mm²

Single conductor set

Voltage drop

3.52 V

2.93% · end 116.5 V

Power loss

105 W

I²R at the operating current

Required ampacity

30.0 A

Each conductor 50 A

Material estimate

$373.50

3 conductors + PVC

Conductor weight

14.4 lb

6.5 kg · approximate insulated weight

Single-line and conductor gaugeLive scaled blueprint

Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.

Bill of materials and shopping list
Item	Net	Waste	Order	Unit price	Line total
copper 8 AWG · 8.37 mm²	75 ft one way	cuts excluded	3 conductors	USD	$302.25
PVC conduit	75 ft	—	75 ft	USD	$71.25

Copper versus aluminum at this run
Material	Minimum size	mm²	Drop	End voltage	Loss
copper	8 AWG · 8.37 mm²	8.4	2.93%	116.5 V	105 W
aluminum	4 AWG · 21.2 mm²	21.2	1.90%	117.7 V	69 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
25 ft	10 AWG · 5.26 mm²	1.86 V	1.55%	118.1 V
50 ft	8 AWG · 8.37 mm²	2.34 V	1.95%	117.7 V
75 ft	8 AWG · 8.37 mm²	3.52 V	2.93%	116.5 V
100 ft	6 AWG · 13.3 mm²	2.95 V	2.46%	117.1 V

Show the deterministic formula

Single phase: Vdrop = 2 × K × I × D / cmil. Three phase: Vdrop = 1.732 × K × I × D / cmil. K is 12.9 for copper and 21.2 for aluminum. D is one-way feet.

Percent drop = Vdrop ÷ source voltage × 100. Green is 3% or less, amber is 3 to 5%, and red is over 5%. Power loss is I²R on the current-carrying conductors.

The smallest conductor that meets both the ampacity (125% when continuous) and the drop limit is selected. Ampacity does not include bundling or temperature correction.

Contractor sign-off

Project: 30 Amp RV Outlet Wire Gauge and Voltage Drop

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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.

Engineering notes

A 30 A RV receptacle on 120 V loses a larger share of its voltage than the same amps on 240 V. The preset is 75 ft of copper in PVC at a 3 percent limit, and the load is not marked continuous. Because the source is only 120 V, the calculator often leaves 10 AWG behind even though 10 AWG is the 30 A small-conductor limit. Confirm the pedestal breaker, the receptacle listing, and whether the site wants a 50 A 240 V outlet instead.

Code reference

NEC 210.19(A) and 215.2(A) informational notes recommend 3 percent on a branch circuit and 5 percent total with the feeder. Ampacity follows the 75°C column of Table 310.16 plus the small-conductor limits. Confirm the adopted edition.

Field questions

Why does 120 volts need a larger wire than 240 volts?: The same volt drop is a bigger percentage of 120 than of 240. Percent drop is Vdrop divided by the source voltage.
Is 10 AWG always legal for a 30 amp RV outlet?: 10 AWG copper meets the 30 A small-conductor rule, but it can miss a 3 percent drop on a long 120 V run. The result shows which limit governs.
What conduit is assumed?: PVC Schedule 40 at an editable price per foot. Changing to EMT or rigid metal changes the cost line, not the voltage-drop math.