Marine Isolation Transformers


In 2017 NEC lowered the maximum permitted ground fault protection for boats on shore-power from 100mA to 30mA

Leakage current varies from boat to boat. Unfortunately, even with all electrical systems in good working order it will often exceed 30mA. This will trip the GFI circuit breaker in the pedestal and leave the boat without power.

The solution is to install a Marine-Puck on your boat

Our Marine-Puck family recently acquired UL Marine certification for the US and Canada.

Marine isolation transformers eliminate shock hazards to persons on board and in the water adjacent to the boat. They also eliminate possible galvanic corrosion of metal hulls and propellers due to DC currents traveling through a shore ground connection, as well as nuisance tripping of GFI (ground fault interrupt) systems when on shore power.


Marine Puck Sales sheet.pdf

Marine-Pucks are toroidal (ring shaped) isolation transformers fully embedded in solid epoxy resin inside a cup-shaped non-metallic enclosure. They are sealed hermetically and can be mounted onto a deck or a bulkhead.

There are many advantages of toroidal transformers versus conventional laminated types.

  • They are up to 50% lighter and much smaller.
  • They are quiet. No annoying hum when on shore power.
  • Marine-Pucks are double-insulated (class II devices) not referenced to ground in any way.
  • They can be adapted to boost output voltage to compensate for drops and fluctuations caused by long dock-runs or in marinas with 208V rather than normal 240V service.



Overload Protection

Isolation transformers must be protected from overload by a dual circuit breaker installed between the shore cable entry point and the transformer primary side.  The breaker rated current must be equal to or less than the transformer continuous current rating, 30Amp for a 120V MP3.6, 25Amp for a 240V MP6 and 50Amp in case of an MP6  wired for 120V.

Inrush Current Pre-Magnetizing

Marine-Puck feature toroidal (ring shaped) isolation transformers which are much lighter and more compact than traditional transformers. When turned on they may experience a brief Inrush Current surge, which will sometimes trip the input circuit breaker. Tripping may be prevented by pre-magnetizing the core briefly as the transformer is turned on.  A convenient way to do it is to install resistors in the form of quartz halogen bulbs across the contacts in the on-board input circuit breaker. This circuit breaker is opened before the shore cable is plugged in causing a small amount of current to magnetize the core before the transformer is fully turned on by closing the on-board breaker. The bulbs (included with the Marine-Puck) are only on at reduced power during the time between plug in and turn on and will never need to be replaced.

Marine-Puck complies with ABYC Standard E11.17.3.5, Polarization Transformers, except for the enclosure being non-metallic. A grounded metallic enclosure would run counter to the general trend towards composite or plastic junction boxes, and ne-gate Marine-Puck’s status as a class II device. The Marine-Puck ABS enclosure combined with the solid epoxy potting com-pound eliminates shock and fire hazards due to broken or loose conductors touching a metallic case besides effectively pre-venting sea water from entering the transformer.


Mounting: Marine-Puck mounts on top of or below a deck or a bulkhead with a central carriage bolt and washer. Input and out-put wiring may exit from the rear through the mounting surface or radially through the lower rim in any orientation. All models feature two secondary 120V secondary windings which may be user connected in parallel for 120V service or in series for 120/240V service.



Coming Soon! Contact Us for Details at (203) 954-0050


Marine Isolation transformers are used on-board vessels or dock-side as part of marina power distribution systems. The isolation transformers totally isolate the earth-bonded shore AC power from on-board electrical systems and greatly reduce the risk of shock caused by defective electrical systems, and largely eliminate corrosion of metal parts caused by stray AC current.

Problems with the AC supply may be caused by insulation breakdown, immersed wiring, incorrect wiring or equipment failure. If the 3rd (green) ground wire is connected to the on-board bonding circuit, corrosion, excessive erosion of zinc electrodes and shock due to stray current may result. In addition corroded wiring in the shore installation may send considerable voltages down the ground wire without tripping the AC circuit breaker causing damage to on-board electronics.


Our five standard design marine isolation transformers are available in either light gray powder coated or stainless steel enclosures.


Advantages of Toroidal Isolation Transformers

The advantage of a toroidal transformer is that it allows the stainless steel enclosure to be much smaller than when a conventional stacked lamination type transformer is used. Smaller dimensions also mean much lower total weight of the enclosed isolation transformer. 
Both features are of paramount importance on-board vessels of any size.

Isolation Transformer Safety Standard Certifications

E509703 UL Marine certification for the US and Canada

UL 60601.1, CSA C22.2 No 601.1-M90 (1990), EN61558-1, IEC60601-1, CE Marking 
Medical Safety Standard with maximum leakage current of 100 to 300 microamperes depending on the power rating of the transformer.

Enclosure Specifications

NEMA 4X  style, 14 gauge light gray powder coated or stainless steel water tight enclosure with hinged and lockable door. Input and output wires are terminated through sealed stress relief cable glands at the bottom of enclosures to a terminal block inside the enclosure.

Reach Us

Contact Us for Information and Sales

6 Waterview Drive, Shelton, CT 06484

(203) 954-0050

(800) 836-5920

Fax: (203) 954-0051