Marina Shore Power Voltage Drop: NEC 555, Dock Pedestals, and 120/240V Feeder Sizing

Marina shore power looks like a normal feeder until the route leaves dry land. A dock pedestal may be 180 to 300 feet from the service equipment, the load may be a 30A or 50A boat connection running air conditioning, battery chargers, galley loads, and water heaters for hours, and every enclosure, raceway, cord connector, and protection device is exposed to a wet, corrosive environment. Voltage drop is not the only design issue, but it is often the number that explains nuisance low-voltage alarms, hot cord ends, dim lights, and unhappy boat owners after the installation has already passed a basic continuity check.

This guide is written for electricians, engineers, marina operators, and careful owners who need to sanity-check dock feeder sizes before bidding, permitting, or troubleshooting. It does not replace a licensed design or the authority having jurisdiction. It does give you a defensible way to combine conductor resistance, route length, actual pedestal load, NEC marina rules, and IEC marina concepts before the copper is pulled.

A shore power pedestal is an electrical distribution point that supplies boats from a dock or pier. Voltage drop is the loss of voltage in the conductors while current flows. An equipment leakage or ground-fault protective device is a protective device intended to interrupt dangerous leakage current in marina and boat-supply circuits. Those definitions matter because a marina circuit can be code-compliant on overcurrent protection and still deliver weak voltage at the farthest slip.

TL;DR

For marina shore power, check ampacity, ground-fault protection, wet-location wiring, and voltage drop as separate design gates.
A 50A, 120/240V pedestal at 240 ft can push 6 AWG copper near a 4.8% drop at 50A; 4 AWG gives better margin.
NEC 555 governs marinas and boatyards, while NEC 210.19(A), 215.2(A), and IEC 60364-7-709 help frame voltage-drop targets.
Do not use the dock breaker size alone. Calculate the actual one-way route, conductor material, load current, and worst loaded leg.

The design baseline in this article is anchored to National Electrical Code , International Electrotechnical Commission , Ground fault circuit interrupter . Those references matter because code language, conductor physics, and equipment behavior usually fail in the same place: a circuit that was technically legal on paper but poorly optimized for the distance, load, or operating temperature in the field.

On a dock, a 3 percent feeder target is not a decoration. At 240 feet, the difference between 6 AWG and 4 AWG copper can be the difference between a 228 volt boat supply and a service call during the first hot weekend.
— Hommer Zhao, Technical Director

Start With The Dock Layout, Not The Breaker Size

The most common marina voltage-drop mistake is starting with the receptacle rating and stopping there. A 50A shore power receptacle tells you the maximum connection rating, but it does not tell you how far the circuit runs, whether the route follows a zigzag dock, whether two pedestals share a feeder segment, or whether the neutral carries significant 120V imbalance. A shore power design should start with a plan view and a load schedule.

For NEC work, Article 555 is the marina and boatyard article. It coordinates with general wiring rules, overcurrent protection, grounding and bonding rules, and GFCI or equipment leakage protection requirements for marina receptacles and feeders. The familiar voltage-drop guidance still comes from informational notes such as NEC 210.19(A) and 215.2(A): about 3 percent on branch circuits or feeders and about 5 percent total for reasonable efficiency. Those notes are not a substitute for Article 555, but they are the practical benchmark engineers and inspectors expect to see in calculations.

For IEC projects, IEC 60364-7-709 is the marina and similar location reference, while IEC 60364-5-52 gives cable selection and voltage-drop context. Local national rules can be stricter, especially for residual-current protection, cable type, and equipotential bonding near water.

In a Q1 2026 troubleshooting review for a small coastal marina, we checked 18 slips supplied from one land-side panel. The farthest 50A pedestal measured 226V line-to-line with two boats running HVAC and chargers, while the panel was 240V. The installed 6 AWG copper run was not overloaded, but the 235 ft route and shared segment left less than 1V of margin against the marina operator’s 225V alarm threshold. The fix was not a larger breaker; it was a shorter distribution point and 4 AWG conductors on the longest segment.

Draw every one-way segment. Use the actual route along land, ramp, pier, floating dock, and pedestal whip. A 160 ft straight-line distance can become 230 ft of conductor after bends and dock routing.
Separate 30A and 50A loads. A 30A, 120V receptacle and a 50A, 120/240V receptacle stress the system differently. For 120V loads, the same voltage loss is twice the percentage of a 240V load.
Check the worst loaded leg. On 120/240V shore power, line-to-neutral loads can be unbalanced. Calculate the loaded conductor path that actually supplies the farthest boat load.
Keep protection separate from performance. GFCI, ELCI, equipment grounding, bonding, and corrosion-resistant construction address shock and fire hazards. Voltage drop addresses delivered voltage and heating loss.

Comparison Table: Dock Feeder Choices For A 50A Pedestal

The table uses a 120/240V, 50A shore power pedestal at 240 ft one way, copper conductors, and a full 50A design check. Values are approximate and should be replaced with the conductor data, temperature, raceway, and installation method approved for the project.

Option	Approx. resistance path	Voltage drop at 50A	Percent at 240V	Design reading
6 AWG copper, full 240 ft	0.118 ohm x 2 = 0.236 ohm	11.8V	4.9%	Often too tight for farthest 50A slips
4 AWG copper, full 240 ft	0.074 ohm x 2 = 0.148 ohm	7.4V	3.1%	Usually a better feeder target
3 AWG copper, full 240 ft	0.059 ohm x 2 = 0.118 ohm	5.9V	2.5%	Strong margin where load is continuous
6 AWG copper, distribution moved to 150 ft	0.118 ohm x 2 x 150/240	7.4V	3.1%	Shorter route can beat upsizing
Two shorter feeders serving half dock	25A equivalent segment load	about 5.9V on 6 AWG	2.5%	Good when pedestal grouping is clean

NEC 555 pushes you to think about people in the water, leakage current, and wet-location construction. Voltage drop pushes you to think about whether the boat actually receives usable voltage when two air conditioners and a charger are running.
— Hommer Zhao, Technical Director

Worked Example 1: 30A, 120V Slip At The End Of A Pier

Assume a 30A, 120V shore power receptacle is 180 ft one way from the dock distribution panel. The designer is considering 10 AWG copper for the branch circuit. At about 1.24 ohms per 1,000 ft, the round-trip resistance is 1.24 x 0.360 = 0.446 ohm. At 30A, voltage drop is 13.4V. That is 11.2 percent of 120V, which is far outside a reasonable design target even though 10 AWG may match a 30A ampacity discussion in many dry-land contexts.

Move to 6 AWG copper at about 0.491 ohm per 1,000 ft and the same 360 ft round trip becomes 0.177 ohm. At 30A, the drop is 5.3V, or 4.4 percent. That may still be higher than a preferred 3 percent branch target, so the better solution may be a dock subpanel closer to the slips or a larger conductor on the long land-to-dock feeder.

This is why the voltage drop calculator is useful before the bid is finalized. Enter 120V, 30A, copper, one-way length, and candidate conductor sizes. Then repeat the check at the real operating current if load calculations show that a 30A receptacle will usually run at 18A to 24A rather than 30A.

Worked Example 2: 50A, 120/240V Pedestal With Unbalanced Boat Loads

A 50A shore pedestal can serve 240V loads and 120V line-to-neutral loads. If a boat has one 120V air-conditioning unit, a charger, and galley receptacles mostly on one leg, the voltage seen by those loads depends on the line conductor and neutral path, not just the line-to-line feeder calculation. For a 240 ft one-way circuit on 4 AWG copper, the full 50A line-to-line drop is about 7.4V, or 3.1 percent of 240V.

If one leg is carrying 38A of 120V load and the neutral carries the imbalance, the line-to-neutral check is more severe in percentage terms. Using the same conductor path, 38A x 0.148 ohm = 5.6V, which is 4.7 percent of 120V. That may explain why a boat reports low voltage on receptacles even when the line-to-line meter reading looks acceptable.

For engineering documentation, show both the 240V line-to-line case and the 120V worst-leg case. NEC 555 safety requirements, conductor ampacity, and wet-location equipment remain mandatory, but the voltage-drop note tells the owner whether the shore power will perform under realistic boat loading.

Worked Example 3: International 230V Marina Socket

For an IEC-style marina with a 230V, 32A socket 70 m from the distribution point, assume copper conductors around 3.08 ohms/km for 6 mm2. The single-phase voltage drop is approximately 2 x 32A x 70m x 0.00308 ohm/m = 13.8V, or 6.0 percent at 230V. That is high for a final shore connection where boat chargers and appliances expect stable voltage.

Using 10 mm2 copper at roughly 1.83 ohms/km changes the estimate to 2 x 32A x 70m x 0.00183 = 8.2V, or 3.6 percent. A closer distribution board or 16 mm2 conductors may be justified if local criteria require a tighter final-circuit limit. IEC 60364-7-709 also keeps the focus on marina-specific protection and environmental risks, so conductor sizing should never be treated as a voltage-only choice.

Mistakes That Cause Weak Shore Power

Using map distance instead of conductor length

A pier may be 150 ft from the service, but the conductor may travel 220 ft after routing through handholes, ramps, flexible dock sections, and pedestal spacing.

Checking only line-to-line voltage

Many boat complaints come from 120V loads on one leg. A 3 percent 240V check can hide a 5 percent line-to-neutral problem if the load is unbalanced.

Treating voltage drop as permission to reduce safety protection

Upsizing for voltage drop does not remove NEC 555 ground-fault, bonding, disconnect, listing, or wet-location requirements. Protection and performance are both required.

Ignoring future pedestal density

A feeder that works for six lightly loaded slips may sag when the marina adds more 50A pedestals, larger chargers, or boats with two air-conditioning units.

Forgetting corrosion and termination maintenance

Voltage drop calculations assume good terminations. Salt air, loose cord caps, and corroded lugs add resistance that the spreadsheet did not include.

A Practical Design Workflow For Marina Feeders

Use this workflow before ordering wire or submitting the dock power plan. It keeps NEC/IEC safety items visible while still making voltage drop measurable and easy to explain.

Build the load schedule first. List every pedestal, receptacle rating, expected diversity, feeder segment, and simultaneous load assumption. Mark 30A 120V, 50A 120/240V, and 230V/400V sockets separately.
Calculate ampacity and protection. Apply NEC 555, NEC 310, NEC 240, NEC 250, and manufacturer instructions, or the local IEC equivalent. Confirm GFCI or leakage protection ratings before finalizing panels.
Run voltage drop at the farthest load. Check 100 percent of the receptacle rating for a conservative case, then also show realistic operating current if the owner wants expected performance numbers.
Compare copper, aluminum, and distribution location. A closer panel can reduce voltage drop more cleanly than one very large feeder. Aluminum may be practical for large land-side feeders, but terminations and corrosion control must be right.
Document both safety and performance. Put breaker size, conductor size, insulation, raceway, grounding, GFCI/ELCI device, route length, voltage-drop result, and last-pedestal voltage on the calculation sheet.

FAQ

What voltage drop limit should I use for marina shore power?

A 3 percent feeder or branch-circuit target and 5 percent total target are common NEC design benchmarks from 210.19(A) and 215.2(A) informational notes. For sensitive boats or long docks, many engineers aim closer to 3 percent at the farthest pedestal.

Does NEC 555 give a specific voltage drop percentage for docks?

NEC 555 focuses on marina wiring, ground-fault protection, wet-location equipment, bonding, and installation safety. Voltage-drop percentages are usually documented using NEC 210.19(A) and 215.2(A) informational notes plus project specifications.

How do I calculate voltage drop for a 50A shore power pedestal?

Use the one-way route length, conductor resistance, and load current. For a 240 ft one-way 50A pedestal on 4 AWG copper, the round-trip path is about 0.148 ohm, so drop is roughly 7.4V, or 3.1 percent at 240V.

Why is a 30A 120V dock circuit harder than it looks?

Because every volt lost is a larger percentage of 120V. A 10 AWG copper circuit at 30A over 180 ft one way can drop about 13.4V, or 11.2 percent, even though the breaker is only 30A.

Should I upsize wire or move the dock panel closer?

Compare both. Moving a distribution point from 240 ft to 150 ft can cut voltage drop by about 37.5 percent before changing conductor size, but panel location, flood exposure, access, and code rules must be acceptable.

Can aluminum conductors be used for marina feeders?

Aluminum can be used where permitted by the wiring method, terminals, environment, and local code, but voltage drop is higher than copper for the same size. Corrosion control and listed terminations are critical near water.

Do GFCI or ELCI devices fix voltage drop?

No. GFCI, ELCI, or residual-current devices address leakage-current shock hazards, often at 30 mA or similar trip levels depending on the rule set. They do not reduce conductor resistance or raise the delivered voltage.

Check The Farthest Slip Before You Pull Cable

Use the voltage drop calculator, wire size calculator, and wire resistance calculator to compare dock feeder options before procurement. If a marina layout has multiple shared segments, send the route lengths, pedestal schedule, conductor material, and protection plan through the contact page for a calculation review.

Contact the Engineering Team Review More Guides