Acushnet Marine ElectricEST. 2008 · MA #ME-12740
Bulletin · 2026-02Grounding & bondingBy Dom Medeiros · 11 min

Grounding and bonding — ABYC E-11 in plain English.

Two terms that get used as if they meant the same thing, by people who know better. Grounding is for safety from a fault current finding the easiest path to earth. Bonding is for keeping every metal part of the boat at the same potential so the dissimilar metals don't eat each other in seawater. Confusing them is why your zinc anodes are gone in three months.

A bonding bus inside the engine room of a steel scalloper, with green-jacketed bonding conductors running to the hull, the keel, and the prop shaft.
Bulletin 2026-02.a · The bonding bus, F/V Providential

§ 01 · Two definitions

The terms are confused because the same green wire often does both jobs. They are not the same job.

Grounding is the deliberate connection of a circuit's neutral or non-current-carrying metal to earth (or, on a boat, to the seawater through a known path), so that a fault current has somewhere safe to go and the breaker has a chance to open. ABYC E-11 §11.5 governs.[1]

Bonding is the deliberate connection of every non-current-carrying piece of metal on the boat — the engine block, the rudder shaft, the prop shaft, the through-hulls, the fuel tank, the keel, the metal cabinetry — so that they all sit at the same potential. ABYC E-11 §11.16 governs.[2]

The reason they are different jobs: grounding only matters when there is a fault. Bonding matters every minute the boat is in saltwater, because dissimilar metals at different potentials will form a galvanic cell and one of them will dissolve.

§ 02 · What a bonding system actually looks like

A proper bonding system on a 50-foot steel scalloper has:

  • A central bonding bus — a 1/4-inch by 2-inch tinned copper bar in the engine room, 18 inches long, mounted with stainless hardware to a non-current-carrying surface.
  • Heavy bonding conductor — #8 AWG tinned copper, green-jacketed — from each through-hull, the rudder shaft, the prop shaft, the steering quadrant, the fuel tank, the engine block, and the keel to the central bus.
  • From the central bus, an underwater connection to the hull (on a steel boat, that's the hull itself; on fiberglass, a bonding plate or a sintered bronze grounding plate).
  • Anodes attached to the underwater bonding network, sized so that the boat is the cathode and the anode is the metal that gets eaten.

What it does not have, by design: a connection between the bonding bus and the AC neutral, except at the dock-side service. ABYC and NEC are clear on this; running it differently is how you get the dock pedestal to trip when nobody is plugged in.

§ 03 · The grounding system (the safety side)

The shore-power safety ground (the green wire) comes aboard at the inlet, runs through the galvanic isolator (or transformer secondary case ground), and lands on the boat's grounding bus. The grounding bus is electrically tied to the bonding bus — that is the one connection between the two systems — and the connection is single-point, in the engine room, near the inlet.

For DC systems, the ground reference is the negative bus of the house battery bank. The negative bus is bonded to the engine block (which is on the bonding bus) at a single point. ABYC E-10 has the wiring details.

§ 04 · Why the anodes are eating themselves

If you are losing zinc anodes faster than the season explains, the cause is one of three things, in order of likelihood:

  1. Stray current from another boat's failed shore-power system entering your boat through the dock's safety ground. The fix is a galvanic isolator on your inlet (ABYC A-28); if the problem is severe, an isolation transformer.
  2. Bonding system has a corroded connection. Some metal on the boat is no longer at the same potential as the rest, and it forms a galvanic cell with the anode. The fix is to megger the bonding system from each connection back to the bus, and to clean every connection.
  3. Wrong anode metal. Aluminum anodes in fresh water; magnesium in saltwater; zinc in brackish — pick the wrong one and the anode will go fast and the boat will, too.[3]

§ 05 · How we diagnose

We use a silver/silver-chloride reference cell, which gives the boat-to-water potential. A healthy steel boat reads −0.85 to −1.1 V; an aluminum hull, −0.95 to −1.15 V; a fiberglass boat with a bronze underwater fitting on a healthy bonding system, −0.55 to −0.65 V at the fitting. Anything significantly above (less negative) those bands means the boat is not adequately protected; anything significantly below (more negative) means it is over-protected and aluminum and bronze fittings start to suffer too.

The cell is a simple thing — we hang it off the bow and drop it just below the waterline — and the meter is a 1 GΩ-input voltmeter (Fluke 87V or 287). The whole walk is a half-day master-rate item on the rate sheet.

Cross-references: Plate P-03, P-07, St. Antônia's bonding work.

Sources & further reading

  1. ABYC. E-11 §11.5 — AC grounding. ↩
  2. ABYC. E-11 §11.16 — Bonding. ↩
  3. BoatUS. Galvanic corrosion. ↩
  4. NACE International. Cathodic protection basics.
  5. Camp Co. Marine anode selection guide.
  6. Pascoe, David. "Bonding and Galvanic Corrosion".
  7. NFPA. NEC Article 250 — Grounding and Bonding.