RV Extension Cord Calculator: How Long Can a 30 Amp Cord Be?

A 30 amp RV extension cord costs about 0.74 volts for every 10 ft at full load — counting the RV’s own shore cord, which is part of the run. A 25 ft shore cord plus a 50 ft extension loses 5.6 V at 30 amps, and a portable EMS at the pedestal never sees any of it.

How long can a 30 amp RV extension cord be?

There is no length at which a 10 AWG cord suddenly fails. What changes is how many volts reach the RV, and that depends on three things you can measure: the total length of cord between the pedestal and the RV, the current flowing through it, and the voltage at the pedestal to start with.

The calculator’s default is a common case. The pedestal is 60 ft from the inlet and the RV has a 25 ft shore cord, so it needs another 35 ft. The shortest 30 amp cord that covers that is 50 ft. At a full 30 amps, the 75 ft of cord loses 5.6 V, and a 120 V pedestal delivers 114.4 V to the RV.

Mike Sokol of RVelectricity puts the danger point for air conditioner and residential fridge compressors at 105 volts, and recommends “the shortest and heaviest extension cord available”.

Why the drop is twice what some guides say

The current goes out along the hot conductor and comes back along the neutral, so a 50 ft cord is 100 ft of copper in the circuit:

drop = 2 × amps × feet × resistance per foot

RVGUARD’s cord guide works a “Real-World Calculation” for “a 100-foot, high-quality 10 AWG cord … for a 30-amp load”, and puts it at “a minimal voltage drop (e.g., 3-4 volts)”. Counting the return on the neutral, the arithmetic gives 7.4 V — about double. A forum rule of thumb that 50 ft of 10 AWG at 30 amps loses 3 V is closer; NEC’s figures give 3.7.

What counts toward the length

That is why the calculator asks for the distance to the pedestal rather than for a cord length, then picks the shortest cord in our records that covers it. If you can park 10 ft closer, the 25 ft cord reaches instead, and the default case loses 3.7 V instead of 5.6.

What a portable EMS cannot see

A portable EMS plugs into the pedestal, and the RV’s cord plugs into it. It measures the voltage there — before every foot of cord. Its low-voltage protection is therefore protection against the pedestal, not against your cord.

• Progressive’s EMS-PT30X disconnects below 104 V. With the pedestal at 104 V and the default cords at 30 amps, the RV is at 98.4 V when it trips.
• Surge Guard’s 34930 waits until 102 V: 96.4 V at the RV.

The more common case is quieter. With the pedestal sagged to 108 V as the park fills, the EMS reads 108 and stays connected — while the RV, on the same cords at 30 amps, runs at 102.4 V, already below the Progressive’s own disconnect point.

Both disconnect points are the makers’ own published figures, in our surge protector records. A hardwired EMS sits inside the RV, after the cord, and sees the drop — but none of the hardwired units in our records carries a low-voltage figure yet, so the calculator does not guess one. Our shore power calculator covers how the pedestal itself sags as a park fills.

A heavier cord for a 30 amp RV

The cords sold for 30 amp service are 10 AWG. The heavier conductor is on the other side of the pedestal: a 50 amp cord’s hots and neutral are 6 AWG, with about 40% of the resistance per foot. At a pedestal with a 50 amp outlet, a 50 amp cord and a 50-to-30 adapter at the RV end — the same “dogbone” many 30 amp owners already carry for 50 amp pedestals — take a 30 amp RV the same distance for much less drop.

On the default case, the 50 ft 50 amp cord loses 3.3 V instead of 5.6. The RV’s own 25 ft of 10 AWG is still in the run, which is why the saving is not bigger. The RV’s breaker still limits it to 30 amps; the heavier cord changes the voltage, not the power available — as our 30 amp vs 50 amp guide explains for adapters.

Volts lost by total cord length

Total cord, pedestal to RV	30 amp RV at 30 A (10 AWG)	50 amp RV at 50 A on one leg (6 AWG)
25 ft	1.9 V	1.2 V
50 ft	3.7 V	2.5 V
75 ft	5.6 V	3.7 V
100 ft	7.4 V	4.9 V
125 ft	9.3 V	6.1 V

Computed from NEC Chapter 9, Table 8, stranded copper at 75°C. Include the RV’s own shore cord. The 50 amp column is the worst case — all the current on one leg, returning on the neutral; balanced legs lose less. Plugs and adapters add resistance on top.

12V is a different calculation

This page is 120V shore power. For battery cable, inverters, DC-DC chargers and anything else on the 12V side, use our 12V wire size calculator: at 12V the current is ten times higher for the same power, and the voltage you can afford to lose is a tenth as much, so the gauges come out very differently.

The cords compared

Model	Service	Length	Conductors	Buy
Conntek 30A 25 ft	30 A, NEMA TT-30P plug to TT-30R connector	25 ft	10 AWG, 3 conductor (10/3), STW jacket	Check price(opens in a new tab)
Conntek 30A 50 ft	30 A, NEMA TT-30P plug to TT-30R connector	50 ft	10 AWG, 3 conductor (10/3), STW jacket	Check price(opens in a new tab)
Conntek 50A 30 ft	50 A, NEMA 14-50P plug to 14-50R connector	30 ft	6/3 plus 8/1 — 6 AWG hots and neutral, 8 AWG ground — STW jacket	Check price(opens in a new tab)
Conntek 50A 50 ft	50 A, NEMA 14-50P plug to 14-50R connector	50 ft	6/3 plus 8/1 — 6 AWG hots and neutral, 8 AWG ground — STW jacket	Check price(opens in a new tab)

Where this goes wrong

1. Leaving out the return conductor. A 50 ft cord is 100 ft of copper in the circuit.
2. Leaving out the shore cord. Twenty-five feet of it at 30 amps is 1.9 V before the extension starts.
3. Buying the longest cord “to be safe”. Every foot counts, coiled or not.
4. Trusting a portable EMS to catch cord drop. It reads the pedestal.
5. Joining cords. Each join adds a connection, and a worn or dirty plug adds resistance that no calculator can see — Sokol recommends cleaning connectors before plugging in.

Questions worth answering