Marine Galvanic Isolator Selector & Sizer
Determine galvanic isolator rating, diode threshold, and fault current duration to block underwater galvanic metal corrosion while maintaining AC safety grounding.
Enter the values above and click Calculate to view recommendations and engineering specifications.
The Galvanic Corrosion Threat in Marinas
Calculation Assumptions
- Solid-state series diode circuit blocks low-voltage DC galvanic stray currents (<1.2V DC) on the AC grounding conductor.
- Conducts immediately during an AC ground fault, safely passing full short-circuit fault current.
Engineering Limitations
- Must carry ABYC A-28 "Fail-Safe" certification with internal capacitor to handle RF noise.
Connecting to a marina dock with steel pilings creates a galvanic cell between your underwater bronze running gear and neighboring vessels. Installing an ABYC A-28 30A galvanic isolator blocks galvanic current while preserving safety grounding.
When your boat connects to shore power, the green grounding wire connects your engine block, shafts, and through-hulls to the marina grounding grid. If a boat three slips away has depleted anodes or an improper DC ground fault, your boat's underwater metals will sacrifice themselves to protect your neighbor's boat!
Because galvanic voltages are small (under 1.0V DC), two back-to-back semiconductor diodes (with a combined 1.2V–1.4V forward conduction threshold) create an electrical brick wall that completely halts galvanic current flow.
ABYC A-28 Fail-Safe Requirements
- Fail-Safe Construction: Older isolators could fail open-circuit if struck by lightning, leaving the boat completely ungrounded and creating a lethal electric shock drowning (ESD) hazard. Modern ABYC A-28 isolators must be rated "Fail-Safe"—if the unit burns out from an extreme surge, it permanently welds into a solid closed circuit, preserving human life safety.
- Dual Shore Inlets: Boats with two 30A inlets (e.g. one for house loads and one for air conditioning) must install a separate galvanic isolator on each shore inlet green wire, or upgrade to a dedicated isolation transformer.
Frequently Asked Questions
What does a marine galvanic isolator do?
A galvanic isolator connects in series with the AC green safety ground wire on a boat's shore power inlet. It blocks low-voltage DC galvanic stray currents (up to 1.2V to 1.4V DC) caused by the interaction of dissimilar underwater metals between neighboring boats connected to the common marina shore ground, while maintaining an uncompromised path for dangerous AC electrical faults to trip shore breakers.
What is an ABYC A-28 'Fail-Safe' galvanic isolator?
Under ABYC Standard A-28, a Fail-Safe galvanic isolator is engineered so that even if internal semiconductor diodes fail under extreme lightning or electrical surge, the device remains permanently welded into a dead-short conduction state. This guarantees that life-saving AC equipment ground continuity is never lost.
What size galvanic isolator do I need (30A vs 50A)?
The galvanic isolator must match or exceed the amperage rating of the boat's shore power inlet service. A boat with a 120V 30-amp shore power connection requires a 30A galvanic isolator. A boat with a 120/240V 50-amp shore power connection requires a 50A galvanic isolator (or dual 50A units for dual inlets).
What is the difference between a galvanic isolator and an isolation transformer?
A galvanic isolator is a compact, solid-state diode block that blocks DC galvanic current on the green wire while physically maintaining the shore ground bond. An isolation transformer completely magnetically isolates the boat's AC electrical system from the shore power grid with zero physical ground wire connection, providing superior galvanic and polarity safety, but at substantial cost, size, and weight.