In short
- A boat dock shore power calculator turns the receptacles on your dock into feeder amps, using the National Electrical Code's Article 555 demand factors.
- Receptacles must be rated 30 amps or more. Three types cover most boats: 30A 125V, 30A 250V and 50A 125/250V.
- Demand factors stay at 100% for 1 to 4 receptacles, drop to 90% at 5 to 8 and keep falling.
- Example: 6 receptacles (4 at 30A, 2 at 50A) come to about 144 to 160 A on the heaviest leg.
- This is a planning figure. A licensed electrician and the permit drawings decide the real feeder.
On this page
This boat dock shore power calculator is for Cape Coral canal-front owners who want to know which pedestal receptacle each boat needs and how many amps the dock feeder must carry before they ask an electrician or dock builder for a quote. It uses code rules and arithmetic only, with no prices. It pairs with our boat lift capacity calculator and our dock and boat lift permit checklist.
En español: calculadora de energía de tierra para muelle en Cape Coral
| Your numbers | Result |
|---|
Example: 4 slips with 30A, 125V and 2 slips with 50A, 125/250V, no other loads.
Which receptacle does a boat need?
The Code sets only a floor: shore power receptacles must be rated at least 30 A (NEC 555.33(A)(4) in the 2020 numbering). It does not tie the rating to boat length, so the choice follows the boat's inlet and what runs on board. A common rule is to keep a continuous load to 80% of the receptacle's rating.
| Receptacle | Assumed kVA | Boat leg, up to (A) |
|---|---|---|
| 30A, 125V | 3.6 | 24 |
| 30A, 250V (rare) | 7.2 | 24 per leg |
| 50A, 125/250V | 12 | 40 per leg |
The kVA figures are our planning assumption (rated amps times 120 V or 240 V). Over 40 A on a leg, ask a marine electrician about two 50A receptacles for that slip or a different design.
How the feeder amps are worked out
- Count receptacles.
Every shore power receptacle on the feeder counts once, whatever its rating. If a slip has two receptacles of different voltage, only the larger counts.
- Find the heaviest leg.
A 120/240 V feeder has two legs. A 30A, 125V receptacle loads one leg with 30 A; the tool alternates them. A 50A, 125/250V or 30A, 250V receptacle loads both.
- Apply the demand factor.
Multiply by the percentage in the table below for your receptacle count.
- Add other loads.
Lights, lift motors and outlets are added at 100%; the factors apply only to the shore power part.
- Round up.
The tool shows the next standard breaker or feeder size from the common list. The tool shows a range: with the factor, and with none.
NEC demand factors for shore power receptacles
| Receptacles on the feeder | Factor |
|---|---|
| 1 - 4 | 100% |
| 5 - 8 | 90% |
| 9 - 14 | 80% |
| 15 - 30 | 70% |
| 31 - 40 | 60% |
| 41 - 50 | 50% |
| 51 - 70 | 40% |
| 71 or more | 30% |
The table sat in 555.12 in the 2017 edition and in 555.6 (Table 555.6) in 2020. A summary of the 2023 changes says the factors moved into Article 220. Which edition your inspector uses is set locally, so the electrician confirms it.
Feeder amps for common docks
| Dock | Heaviest leg (A) | Standard size (A) |
|---|---|---|
| 1 x 30A | 30 | 30 |
| 1 x 50A | 50 | 50 |
| 2 x 30A | 30 | 30 |
| 1 x 30A + 1 x 50A | 80 | 80 |
| 2 x 50A | 100 | 100 |
| 4 x 30A | 60 | 60 |
| 8 x 30A | 108 | 110 |
These figures have no other loads and use our assumptions above. A lift motor or dock lights add to the leg.
Service location, disconnects and permits
The electrician and the permit govern. Article 555 puts the service equipment on land next to the dock, not on the dock (555.4), and requires a disconnect that isolates each boat from its receptacle (555.36). Wire size, ground-fault protection and voltage drop are engineered per job. The city lists dock and lift wiring under its own electrical permit: see our dock and boat lift permit checklist.
Counting pilings for the same dock? Try the dock piling count calculator. Checking the water side first? The dock tide clearance calculator covers depth at low tide.
Questions to ask your electrician
- Which NEC edition are you designing to? Ask which one the city's inspector applies.
- Which receptacle will each slip get, and why? Compare with the boat's inlet label.
- How did you count the demand factor and the legs? Compare with the figures here.
- Who pulls the electrical permit, and is the pedestal listed for marine use?
- How is ground-fault protection handled, and how far is the run from the panel?
Next step: get a written quote
This tool estimates amps, not the price or the job. We are a referral service, not an electrician or dock builder; we connect visitors with independent local providers. Use the form below to ask independent local contractors for a written quote, and read how to choose a seawall contractor first.
Sources
- Mike Holt Enterprises: Marinas and Docks, 2020 NEC Article 555: demand factor table (555.6), service location (555.4), 30A minimum rating (555.33).
- EC&M: Load Calculation Requirements for Shore Power Receptacles: the earlier 555.12 numbering.
- Electrical Association: 2023 NEC Update, Part 2: the move of the factors into Article 220.
- UpCodes: NEC Article 555, Marinas, Boatyards, and Commercial and Noncommercial Docking Facilities: scope of the article.
Frequently asked questions
Which shore power receptacle does my boat need?
Match the boat's own inlet and its biggest AC loads. The Code only sets a floor: shore power receptacles must be rated at least 30 amps. A common rule is to keep a continuous load to 80% of the rating, so about 24 A fits a 30A, 125V receptacle and about 40 A per leg fits a 50A, 125/250V one. Check the boat's inlet label and ask a marine electrician.
Does a private home dock follow NEC Article 555?
Article 555 is titled to cover marinas, boatyards and commercial and noncommercial docking facilities, which includes a dock behind a house. Which NEC edition applies, and how your inspector reads it, is up to the city and the electrician who pulls the permit.
What is the demand factor for shore power receptacles?
It is a percentage of the summed receptacle ratings that you may use when sizing a service or feeder. It is 100% for 1 to 4 receptacles, 90% for 5 to 8, 80% for 9 to 14, and it keeps falling to 30% at 71 or more. The table sat in 555.12 in 2017 and in 555.6 in 2020, and the 2023 edition moved it into Article 220.
Can I run a 30A boat from a 50A pedestal?
Often yes with a listed adapter, but the boat's cord and breaker still limit what it can draw, and adapters are a common source of overheating. Ask the electrician which receptacle each slip gets and which adapters are acceptable.
Do I need a permit to add dock power in Cape Coral?
Plan on one. The City of Cape Coral's own guideline says lift wiring needs a separate electrical permit, and dock power is wiring too. Confirm the exact steps with Development Services.