In short
- Two checks set the wire: carrying the current (ampacity) and keeping the voltage up over the distance.
- On a long dock run, voltage drop usually sets the size, not the current.
- The NEC suggests about 3% drop for one run and 5% overall. These are informational notes, not required.
- We show copper only. Your electrician picks the wiring method, the aluminum option and the breaker.
On this page
- How the wire size is worked out in four steps
- Longest run (feet) each copper size holds to 3% drop, at 240 V
- Ampacity of copper wire at 75 C and the continuous load it can carry
- What to check on the quote
- Questions to ask an electrician about dock wiring
- Next step: get quotes with the wiring spelled out
- Sources
- Frequently asked questions
This dock power wire size calculator is for Cape Coral canal-front owners who are about to ask an electrician or dock builder for a quote. Enter the load in amps, the voltage and how far the dock is from the panel. It shows the copper size for ampacity, the size for voltage drop and the larger of the two. It uses arithmetic and published code tables only, with no prices. Start with the amps from our shore power calculator and see the dock and lift permit checklist for the electrical permit.
En español: calculadora de calibre de cable y caída de tensión para muelle en Cape Coral
| Your numbers | Result |
|---|
Example: 50 A, 240 V, 200 ft one way, 3% limit, load running 3 hours or more.
How the wire size is worked out in four steps
- Start from the load
Take the amps the dock circuit will carry. Our shore power calculator gives the feeder amps.
- Size for current
Multiply by 1.25 for loads that run 3 hours or more, then pick the smallest copper size whose NEC ampacity covers it.
- Check the drop
Voltage drop = 2 x 12.9 x amps x feet / circular mils. Pick the smallest size that stays under your limit.
- Take the larger
The suggested size is whichever of the two is thicker. One size up is shown for comparison.
Longest run (feet) each copper size holds to 3% drop, at 240 V
| Copper size | 20 A | 30 A | 50 A |
|---|---|---|---|
| 10 AWG | 145 | too small | too small |
| 8 AWG | 230 | 154 | too small |
| 6 AWG | 366 | 244 | 146 |
| 4 AWG | 582 | 388 | 233 |
| 2 AWG | 926 | 617 | 370 |
| 1/0 AWG | 1,473 | 982 | 589 |
Loads are figured at 125% for current. Cells that say "too small" cannot carry that load at all, whatever the distance. All figures are for one-way feet.
Ampacity of copper wire at 75 C and the continuous load it can carry
| Copper size | Table ampacity | Continuous load |
|---|---|---|
| 12 AWG | 20 A | 16 A |
| 10 AWG | 30 A | 24 A |
| 8 AWG | 50 A | 40 A |
| 6 AWG | 65 A | 52 A |
| 4 AWG | 85 A | 68 A |
| 2 AWG | 115 A | 92 A |
| 1/0 AWG | 150 A | 120 A |
| 4/0 AWG | 230 A | 184 A |
NEC Table 310.16 copper, 75 C column. The small-conductor limits of 20 A for 12 AWG and 30 A for 10 AWG apply to the breaker. Aluminum and 90 C wire ratings are for your electrician.
This is an estimate, not a wiring plan. Wiring method, conduit, marine-grade wire, ground-fault protection and the breaker are set by your electrician and the permit. See the shore power calculator and the permit checklist.
What to check on the quote
- Wire size on paper. The quote should name the conductor size and the run length, not just "dock power".
- The run is measured. Ask how many feet from panel to dock, and whether that includes the run along the pier.
- Voltage drop stated. Ask what limit the electrician designed to, and compare with the result above.
- Permit included. Dock and lift wiring has its own electrical permit; ask who pulls it.
Questions to ask an electrician about dock wiring
- What copper size and what run length are you pricing? Compare with the result above.
- What voltage drop did you design for? Ask for the percent at full load.
- Is the wire rated for wet locations and marine use? Ask the type and the conduit.
- Who pulls the electrical permit? See the permit checklist.
- How is the dock tied to the seawall work? A new cap or dock may need the conduit set first; see the cap concrete calculator.
Next step: get quotes with the wiring spelled out
This result is a planning estimate, not an engineering design. We are a referral service, not a contractor; we connect visitors with independent local providers. Use the form below to ask independent seawall and dock contractors for quotes.
Sources
- ExpertCE, conductor resistance with NEC Chapter 9, Table 8: circular mils and resistance of copper conductors.
- ExpertCE, how to use NEC Table 310.16: copper ampacity in the 75 C column.
- ExpertCE, acceptable voltage drop under the NEC: 3% and 5% are informational notes, not mandatory.
- Our shore power calculator: NEC Article 555 receptacle and feeder rules for docks.
The constant 12.9 ohm-circular mils per foot is a common approximation for copper at 75 C; exact Table 8 values differ a little between editions. The 125% factor for loads of three hours or more is NEC practice for continuous loads. Ask your electrician which NEC edition your permit follows.
Frequently asked questions
What wire size does a boat dock need?
It depends on the load in amps, the voltage and how far the dock is from the panel. The wire must be big enough to carry the current and long enough runs need a thicker wire so the voltage does not sag. This calculator shows both sizes and picks the larger. A licensed electrician makes the final choice.
What is voltage drop and why does it matter on a dock?
Voltage drop is the voltage lost along the wire. Docks are often 100 ft or more from the panel, and a big drop makes pumps, lift motors and battery chargers run hot or underperform. The NEC suggests about 3 percent for a single run and 5 percent for the whole chain from panel to outlet, but those numbers are informational notes, not required.
Is the 3 percent voltage drop a code rule?
No. In the NEC it appears in informational notes, which are advice. Your inspector or your electrician's specification can still require it. Ask which limit the job uses.
Does this replace the shore power calculator?
No. The shore power calculator tells you which receptacle each boat needs and how many amps the feeder carries. Use that number here as the load, then see how thick the copper must be for the distance.
Why does the calculator size for 125 percent of the load?
The NEC treats a load that runs three hours or more as continuous and asks for conductors rated at 125 percent of it. Boat chargers and air conditioners can run that long, so we use it by default. You can switch it off for light loads, but ask your electrician.