Marine Isolation Transformers


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

Marine-Puck isolation transformers


Marine-Puck boat isolation transformers share the construction and general principles of our Iso-Puck program of Medical grade Isolation transformers. Iso-Puck are used by medical device manufacturers and in health care facilities to protect patients and staff from life threatening chocks caused by faulty insulation in wiring and equipment. Iso-Puck transformers comply with the strict UL/IEC 60601 standard for medical equipment.

Boats on Shore Power

In a boat, plugged in to a shore power pedestal an isolation transformer eliminates any electrical continuity between the electrical grid and the boat. Shore power is fed to the primary windings and electrical systems on board are connected to the secondary windings, completely isolating the boat from Shore Ground and preventing any unsafe situation. The isolation eliminates possible galvanic corrosion of metal hulls and outboard engine components. It also prevents nuisance tripping of shore installed GFI (ground fault interrupt) systems when the boat is on shore power.

Marine-Puck transformers feature a toroidal isolation transformer, which is embedded in solid Epoxy resin and contained in a cup shaped Polycarbonate (LEXAN) enclosure. Shore side and boat side power cords protrude from the epoxy potting material and seal the transformer hermetically from the outside meaning that the transformer can be mounted on board, inside or outside onto a deck or bulkhead wherever convenient. Installing a Marine-Puck lowers leakage current to less than 1 milliampere, far exceeding the protection level of any commercial GFI device.

There are several advantages of using toroidal transformers versus conventional transformers on board a vessel.

  • Toroids are 30-50% lighter and more compact.
  • They are more efficient, cooler running.
  • They are quiet. No annoying hum while drawing shore power.
  • When resin encapsulated in a non-metallic enclosure they are double insulated (class II devices). No ground connection or neutral bond is required on either side.

Marine-Pucks are mounted by means of a central through carriage-bolt or lag screw. They are prevented from rotating by means of a single stainless fastener through a tab at the periphery.

Input and output cables can exit from the rear through pre-bored holes in the deck or bulkhead, or holes can be field drilled (using a step drill) in the Marine-Puck lower rim. Grommets are included.



Specifications M-P 3.6 M-P 6 Dual M-P 6 (12 KVA)
Input voltage 60Hz AC 120V or 240V 120V or 240V 240V
Input current 30A  or 15A 50A or 25A 50A
Output voltage 120V or 240V 120V or 240V 120V or 240V
Output current 30A or 15A 50A or 25A 100 or 50A
Impedance % 2.8 2.8 2.8
KVA Continuous 3.6 6.0 12.0
Insulation class B 130 C B 130C B 130C
Hi-Pot test voltage 4000V AC RMS 4000V AC RMS 4000V AC RMS
Ambient temperature -20 to +40 deg. C. -20 to +40 deg. C -20 to +40 deg. C
Dimensions 10”OD x 5” ht. 12.6”OD x 6.5” ht. 2 x 12.6” OD x 6.5” Ht.
Approximate weight. 45 lbs. 75 lbs. 150 lbs.
Main mount. 3/8” carriage bolt ½” carriage bolt 2 pc. ½” carriage bolt


Marine-Puck mounts on top of or below deck or on a bulkhead, internally or externally. Input and output cables may exit from the rear through the mounting surface or from the side of the Marine-Puck near the lower rim in any orientation

Marine quality sealed input and output cables feature 4 conductor 12AWG stranded, color coded leads.

Installation: Marine-Puck may be user configured by connecting the dual 120V primary and secondary windings in parallel or in series when hard-wiring into input side or output side circuit breakers.   

A pre-magnetizing circuit prevents nuisance tripping of the input circuit breaker due to inrush current surge when plugging



 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.

 Light Gray Powder Coated Enclosure                                                                                                Stainless Steel Enclosure  


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

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.