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Marine Isolation Transformers
At Bridgeport Magnetics Group, we help boat builders, yacht owners, and marina equipment engineers ensure onboard safety, deliver reliable isolated power, and withstand harsh marine environments. Our Marine Puck isolation transformers deliver superior corrosion resistance, vibration and shock protection, low leakage current, and uncompromising reliability - empowering you to meet the rigorous demands of modern marine electrical systems.
Complete Shore Power Safety for Yachts, Trawlers & Cruisers
Every boat that connects to shore power faces the same risks: galvanic corrosion damaging underwater metals, stray AC currents creating shock hazards in the water, and dirty dock power degrading sensitive electronics. A marine isolation transformer solves all three by providing complete electrical separation between the dock and your vessel.
Bridgeport Magnetics Group has designed and manufactured isolation transformers since 1982. Our Marine-Puck line uses toroidal technology to deliver full galvanic and AC isolation while being 50% lighter and more compact than traditional E-I transformers – a critical advantage when every pound and inch of engine room space counts. Whether you’re retrofitting a premium trawler, luxury yacht, or high-end cruiser – or specifying OEM transformers for a new build – the UL 1561 Recognized Marine-Puck MP6 is the professional-grade solution that delivers complete shore power safety and galvanic protection no galvanic isolator can match.
Every US Made Marine-Puck also comes standard with a built-in slow-start thermistor to limit inrush current and ensure reliable performance even on low-voltage docks.
Marine Isolation Transformers
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Marine-Puck DUAL MP6 50A
Marine Isolation Transformers -
Marine-Puck MP6 25A
Marine Isolation Transformers -
Marine-Puck MP3.6 30A
Marine Isolation Transformers
Through our toroidal transformer designs, the Marine-Puck delivers peak efficiency and superior power density in a compact, highly modular package - with extensive configuration options purpose-built for marine shore power.
Integrated Boost Function
Built-in boost configurations (104V to 120V & 208V to 240V) to compensate for low marina voltage.
Toroidal Size Advantage
Toroidal construction delivers up to 50% smaller size and weight than traditional EI transformers, making installation easier in tight engine compartments and saving valuable boat space.
Thermistor Soft-Start
An NTC thermistor is included standard on every MP6, limiting inrush current for clean, nuisance-free startups without extra components or complexity.
Quiet Operation
Toroidal design virtually eliminates magnetostriction hum, providing silent operation that liveaboards and luxury yacht owners consistently praise as a standout feature.
Modular & Configurable
A modular platform with single or dual-unit setups, multiple voltage configurations, and flexible mounting options allows one product family to serve a wide range of boats and electrical services.
How to Size a Marine Isolation Transformer
Selecting the correct kVA rating is the most important decision when installing a Marine-Puck isolation transformer. An undersized unit can overheat under load, trip thermal protection, and leave the boat without shore power. While an oversized unit adds unnecessary weight and cost, a reasonable safety margin is always recommended to handle motor-starting surges from air conditioners, watermakers, and other equipment.
Sizing tip: if your vessel’s loads fall between standard sizes, always size up. A slightly oversized transformer runs cooler, handles motor-starting surges more reliably, and lasts longer.
| Shore Power Service | Required kVA | Marine-Puck Model | Typical Use Case |
|---|---|---|---|
30A, 120V | 3.6 kVA | MP3.6 | Smaller boats, coastal cruisers |
50A, 120/240V | 6 kVA | MP6 | Most mid-size boats & trawlers |
Dual 50A, 120/240V | 12 kVA | Dual MP6 | Larger boats, heavy A/C loads |
Why Your Boat Needs a Marine Isolation Transformer
When you plug into shore power, your boat shares a ground connection with the dock and every other boat on that circuit. This shared ground is the root cause of the most common problems boat owners face: GFCI breaker trips, galvanic corrosion, and electrical safety risks. A marine isolation transformer breaks that connection completely.
Stops GFCI/ELCI Breaker Trips
The most common reason boat owners install an isolation transformer. Normal leakage current from your boat often exceeds 30 mA, which trips the sensitive GFCI/ELCI breaker at the dock and cuts your power. An isolation transformer eliminates these nuisance trips.
Prevents Galvanic Corrosion
When your boat shares a ground with the dock and other vessels, your underwater metals can become the sacrificial anode for boats nearby. An isolation transformer breaks this connection completely, so your zincs only protect your own boat.
Improves Electrical Safety
It eliminates the path for AC fault current to travel through your boat’s bonding system and into the water. This significantly reduces the risk of electric shock drowning around your vessel.
Protects Sensitive Electronics
Modern boats have chartplotters, radar, AIS, and other electronics that are sensitive to dirty shore power. An isolation transformer provides a clean, isolated power source and reduces noise, voltage spikes, and ground-loop issues.
Marine Isolation Transformer vs Galvanic Isolator
Boat owners typically choose between a galvanic isolator and a marine isolation transformer. Both help with galvanic corrosion, but they work very differently. A galvanic isolator only blocks low-voltage DC current. An isolation transformer fully separates your boat from shore power, solving corrosion, GFCI trips, and other electrical risks.
Key Differences
A Galvanic Isolator
is a small inline device (typically a pair of diodes or a diode bridge) installed in series with the shore power ground conductor. It blocks low-voltage DC galvanic currents below approximately 1.2 volts. This is enough to interrupt the typical 0.5V-0.9V galvanic potential between dissimilar metals on connected boats. It costs $150-$400 installed, weighs 1-3 pounds, and takes minutes to wire in.
A Marine Isolation Transformer
provides complete electrical separation. The shore power feeds the transformer's primary winding. The boat's electrical system connects to the secondary winding. There is no electrical conductor between the two: energy transfers magnetically. The shore ground terminates at the transformer, and the boat's grounding system becomes fully independent.
Reliable Marine Isolation, Manufactured to Standards
25A, 30A, 50A Marine Isolation Transformers – UL 1561 Recognized. Designed for compliance with ABYC E-11 recommendations for isolation transformers when installed according to the supplied wiring diagrams (including secondary neutral-to-ground bond).
Frequently Asked Questions
Do I need a marine isolation transformer?
You likely need one if your boat keeps tripping the GFCI/ELCI breaker at the dock, even when everything on the boat is working properly.
An isolation transformer also protects against galvanic corrosion and improves electrical safety. If you’re dealing with frequent power trips at the marina, it’s usually the best solution.
What size marine isolation transformer do I need?
Choosing the right size is straightforward:
- 30-amp shore power (120V) → Use the MP3.6 (3.6 kVA)
- 50-amp shore power (120/240V) → Use the MP6 25A (6 kVA)
- Dual 50-amp shore power or boats with heavy air conditioning and high electrical loads → Use the Dual MP6 50A (12 kVA)
Quick tip: Match the transformer to your shore power service first. If your boat regularly runs near the limit of its shore power (especially with air conditioning on), go up one size for better performance and cooler operation.
What is the difference between an isolation transformer and a galvanic isolator?
A galvanic isolator only blocks the small currents that cause corrosion. It does not stop GFI breakers from tripping.
An isolation transformer completely separates your boat from shore power. It stops both corrosion and GFI trips, while also providing better electrical safety.
Can I install a marine isolation transformer myself?
Yes, it is possible if you have solid electrical experience and follow the wiring diagrams carefully. However, we strongly recommend having a qualified marine electrician install it. Proper installation ensures the system is safe, meets marine standards, and works correctly with your boat’s existing shore power setup.
Why does my boat keep tripping the GFCI breaker?
Most boats naturally leak a small amount of current, even when everything is working correctly. Modern marina GFCI/ELCI breakers are very sensitive and trip around 30 milliamps. When your boat’s leakage goes over that limit, the dock breaker shuts off power. An isolation transformer stops this by separating your boat from the dock’s ground.
Will a marine isolation transformer stop GFCI trips?
Yes. A marine isolation transformer fully separates your boat from the dock’s ground connection. This prevents the leakage current that causes GFCI/ELCI breakers to trip, so you can stay connected to shore power without constant power loss.
