Shore Power Cable & Voltage Drop Sizer
Calculates voltage drop, delivered voltage, and thermal dissipation across 120V 30A and 120/240V 50A RV pedestal and marina dock shore cords over long runs. For interior 120V/240V branch wiring, use the AC Branch Circuit Wire Sizer.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Engineering Principles: Shore Power Drop & Heat
Calculation Assumptions
- Standard unity power factor (cos φ = 1.0) for resistive RV/marine branch circuits.
- Round-trip loop length includes line and neutral conductors per NEC Table 8.
Engineering Limitations
- Inductive motor loads (A/C compressors) exhibit lower power factor (0.85), increasing voltage drop.
A 30A 120V continuous load over a 50-foot 10 AWG extension cord causes a 6.0V drop (5.0%), dropping pedestal voltage from 120V to 114V and dissipating 181W of heat.
Shore power extension cords operate in harsh outdoor environments and frequently carry high continuous loads (air conditioning, water heaters, and lithium battery chargers). Unlike DC wiring where pure resistance dominates, AC shore cords exhibit both copper ohmic resistance ($R$) and 60 Hz inductive reactance ($X_L$):
When RV parks or marinas experience summer heat waves, pedestal voltage can sag to 110V or lower before the cord even starts. An additional 5% drop across a long or undersized cord can deliver less than 104V to your rig, causing A/C compressor motor overheating and locked-rotor stall cycles.
Best Practice Guidelines for Shore Cords
- The 3.0% Rule: Keep shore cord voltage drop below 3.0% under maximum load to preserve headroom for internal branch wiring.
- Cord Coiling Warning: Never leave a heavily loaded shore power cord tightly coiled on a reel or in a storage bay. Concentrated heat dissipation (50W–120W) cannot escape, leading to thermal jacket breakdown and melting.
- 50A Split-Phase Advantage: 50A 120/240V services provide four times the power of a 30A service ($12,000\text{W}$ vs $3,600\text{W}$) with significantly lower line loss on balanced 240V loads.
Frequently Asked Questions
How much voltage drop is acceptable on a shore power cord?
Standard electrical design recommends keeping shore power cord voltage drop below 2% to 3%. With nominal marina or campground utility voltage often dipping to 110V-114V under heavy summer air conditioning loads, excessive cord voltage drop can deliver sub-105V power to the RV or boat, damaging compressor motors and sensitive electronics.
Can you use a 100-foot extension cord for a 30A RV?
A standard 10 AWG 30A RV power cord running 100 feet under a continuous 24-amp load (80% continuous breaker limit) experiences approximately 5.8V of drop (4.8% voltage drop). For runs of 75 to 100+ feet, upgrading to an 8 AWG heavy-duty cord is strongly recommended to maintain line voltage above 115V.
Why do shore power plugs melt or burn at the pedestal?
Burnt shore power pins are caused by worn, loose receptacle blades in the dock or RV park pedestal. Loose mechanical contact creates high contact resistance, which under a sustained 24A-30A load generates intense heat, melting plug plastic and oxidizing electrical prongs.
What is the difference between a 30A and 50A RV shore power connection?
A 30A RV connection is a single 120V hot leg delivering a maximum of 3,600 Watts (120V * 30A). A 50A RV connection is a 120/240V split-phase service providing TWO separate 50A hot legs, delivering a massive 12,000 Watts (120V * 50A * 2 legs) — more than triple the power of a 30A service.