A 50A marine isolation transformer is the standard choice for boats running 50-amp, 120/240V shore power. It isolates the vessel from the dock, stops galvanic corrosion, reduces the risk of in-water shock, and gives onboard electronics a cleaner supply. Three names dominate this category: Charles IsoTransformer / Iso-Boost, Hubbell’s 50A isolation transformer, and Bridgeport Magnetics’ Dual MP6.
This comparison looks at how those three solutions actually differ in size, weight, boost, soft start, certifications, and installation. The goal is to help boat builders, yards, and owners pick the right 50A isolation package, not just the most familiar brand.
Why 50A Isolation Matters
A 50A, 125/250V shore connection is common on mid-size cruisers and yachts with air conditioning, electric cooking, and multiple high-load circuits. Without an isolation transformer:
- Shore ground stays bonded through the water to other boats and the dock
- Galvanic current can eat zincs, shafts, and underwater metals
- Dock-side ground-fault devices can nuisance-trip
- Low dock voltage can sag air conditioning and chargers
- A wiring fault on the dock can put voltage on underwater metals
A true isolation transformer breaks that conductive path. Power is transferred magnetically. The boat’s AC system is no longer electrically tied to shore ground.
The Three 50A Options
Charles
Charles Industries has long been the default name in marine isolation. The relevant 50A isolation product is the 12 kVA IsoTransformer (240V, 50A). Charles also sells Iso-Boost models that add automatic voltage boost as a separate, heavier package. Typical 12 kVA isolation units land around 235 pounds in a rectangular metal enclosure.
Hubbell 50A
Hubbell’s HBL50AITW is a 15 kVA, 60 Hz isolation transformer in a white powder-coated steel (or 316 stainless) UL Type 3R enclosure. It weighs about 170 pounds and measures roughly 17 × 14 × 12 inches. Auto-boost is available as an option, not as standard on every unit. Hubbell emphasizes dual current-carrying shields, encapsulation, and ABYC hipot performance.
Dual MP6
The Dual MP6 is two 6 kVA Marine-Puck toroidal isolation transformers used together as a 12 kVA, 50A, 120/240V system. Combined weight is 160 pounds. Each puck is 12.6 inches in diameter and 6.5 inches high. Soft start and 104V / 208V boost are included on every unit. Enclosures are flame-rated non-metallic ABS, hermetically epoxy-potted, Class II double-insulated, UL Listed Marine, and ignition protected.
Side-by-Side Comparison
| Feature | Dual MP6 50A | Charles 12 kVA IsoTransformer | Hubbell 50A (HBL50AITW) |
|---|---|---|---|
| Continuous rating | 12 kVA | 12 kVA | 15 kVA |
| Typical service | Dual 50A, 120/240V | 50A, 240V shore | 50A shore systems |
| Frequency | 50/60 Hz | 50/60 Hz | 60 Hz standard (50/60 optional models) |
| Total weight | 160 lb (two 80 lb pucks) | ~235 lb | ~170 lb |
| Package size | Two units, 12.6″ OD × 6.5″ H each | ~12″ × 15″ × 16″ | ~17″ × 14″ × 12″ |
| Core type | Toroidal | Conventional laminated | Conventional laminated |
| Enclosure | Non-metallic ABS, epoxy sealed | Metal | Powder-coated steel or 316 SS |
| Soft start | Included | Not standard on basic IsoTransformer | Not standard |
| Voltage boost | 104V and 208V included | Separate Iso-Boost model | Optional Auto-Boost / taps |
| Grounding of case | Class II, no ground reference | Shielded metal enclosure | Metal enclosure |
| Ignition protection | Yes | Check specific model | Not the primary listing claim |
| Marine listing | UL Listed Marine E509703 | Marine UL on many Charles models | UL listed, CSA, UL 3R |
| ABYC relevance | Designed for ABYC isolation practice, ignition protected | ABYC E-11 / hipot on boost models | Exceeds ABYC 4000V hipot |
| Acoustic noise | Very low (toroidal) | Typical laminated hum | Listed as quiet vs NEMA, still laminated |
| Origin | Made in USA | USA brand, established marine line | USA brand |
Size and Installation Flexibility
This is the first place Dual MP6 pulls away.
A Charles or Hubbell 50A transformer is one large rectangular box. It needs a single, relatively open mounting location and enough structure to carry 170 to 235 pounds in one lump.
Dual MP6 splits the 12 kVA into two 80-pound pucks. Each puck mounts with a central carriage bolt on a deck or bulkhead. That matters on boats where the only available space is two smaller pockets rather than one large locker. Combined footprint is also lower in height (6.5 inches per unit), which helps under bunks, in lazarettes, and against bulkheads.
Up to 50 percent smaller and lighter than many conventional laminated isolation transformers is the practical result of the toroidal, potted construction.
Weight
| Product | Weight | Notes |
|---|---|---|
| Dual MP6 | 160 lb total | 80 lb per puck |
| Hubbell 50A | ~170 lb | Single enclosure |
| Charles 12 kVA IsoTransformer | ~235 lb | Single enclosure |
| Charles Iso-Boost 50 | ~235 lb | Isolation plus boost in one heavier package |
For yards and owners, 160 pounds split in two is easier to handle than 235 pounds in one box. It also reduces point load on a single mounting surface.
Soft Start
Toroidal transformers can draw a short inrush when first energized. On a dock with a sensitive shore breaker, that inrush can trip the pedestal.
Dual MP6 includes soft start on every unit. That is not an add-on. Charles basic IsoTransformer and Hubbell standard 50A units do not include soft start as a standard feature. Dual MP6 is built so the shore breaker is less likely to open when the boat is first plugged in.
Boost Included, Not Optional
Low dock voltage is common in busy marinas. Air conditioning and battery chargers sag. Many isolation transformers treat boost as a separate SKU or an optional auto-boost board.
Dual MP6 includes 104V and 208V boost taps on every unit. There is no need to buy a second Charles Iso-Boost chassis or a Hubbell Auto-Boost option to cover weak docks. Wiring diagrams for boost and no-boost configurations ship with the units.
Hubbell offers 12.5 percent taps and optional Auto-Boost. Charles offers dedicated Iso-Boost models. Dual MP6 builds the boost into the standard 50A package.
Certifications and ABYC Practice
All three brands aim at marine isolation rules. The details differ.
Dual MP6
- UL Listed Marine (E509703)
- Ignition protected (relevant on gasoline boats and ABYC C-1500 / UL 1500 type concerns)
- Class II double insulation
- Hermetic epoxy potting
- Designed around ABYC isolation-transformer practice
- 50/60 Hz international rating as standard
Hubbell 50A
- UL listed and CSA certified
- UL Type 3R enclosure
- Exceeds ABYC 4000V hipot
- Can be wired as polarization or isolation per ABYC E-11
- Dual full current-carrying shields
Charles
- Long marine UL history on IsoTransformer and Iso-Boost lines
- Full current-carrying shield between windings
- Iso-Boost models marketed to ABYC E-8 / E-11 type requirements depending on generation
For builders working on gasoline boats or tight engine-room adjacent spaces, ignition protection plus a non-metallic, ungrounded enclosure is a Dual MP6 advantage that the steel-box competitors do not match in the same way.
Corrosion and Environment
Steel and stainless enclosures survive if they stay dry and coated. They still present a metal case that must be considered in the bonding and corrosion plan.
Dual MP6 uses flame-rated non-metallic ABS and solid epoxy potting. There is no metal can to rust, and the winding is hermetically sealed against salt air and spray. The enclosure is not used as a ground reference. That combination is purpose-built for the bilge, locker, and lazarette conditions where 50A isolation transformers actually live.
Noise
Laminated EI-core transformers hum. Some are quieter than others. Toroidal cores, by geometry, have much lower audible magnetostriction. Dual MP6 is effectively silent in the cabin compared with a 170 to 235 pound laminated box running next to a stateroom bulkhead.
How to Choose
| If you need… | Better fit |
|---|---|
| Smallest and lightest 12 kVA 50A package | Dual MP6 |
| Soft start included | Dual MP6 |
| Boost included, not a second SKU | Dual MP6 |
| Ignition-protected, non-metallic enclosure | Dual MP6 |
| Split mounting in two tight spaces | Dual MP6 |
| Single familiar steel box, 15 kVA nameplate | Hubbell 50A |
| Legacy Charles footprint or dedicated Iso-Boost behavior | Charles |
| 60 Hz only, optional auto-boost remote | Hubbell |
FAQ
What size isolation transformer does a 50A boat need?
A 50A, 240V shore connection is 12 kVA (50A × 240V). Dual MP6 and the Charles 12 kVA IsoTransformer are sized to that load. Hubbell’s 50A unit is a 15 kVA transformer, which adds headroom.
Does Dual MP6 replace both Charles and Hubbell 50A isolation transformers?
Functionally yes for standard 50A, 120/240V isolation. It is a 12 kVA isolation system with boost and soft start included. It is not a drop-in dimensional copy of either metal enclosure.
Why does Dual MP6 use two transformers?
Each MP6 is a 6 kVA puck. Two units in the Dual MP6 configuration provide 12 kVA for 50A, 240V service while keeping each piece at 80 pounds and 6.5 inches tall.
Is boost standard on Charles and Hubbell?
No. Charles sells boost as Iso-Boost models. Hubbell sells taps and optional Auto-Boost. Dual MP6 includes 104V and 208V boost on every unit.
Is Dual MP6 ABYC compliant?
It is designed for ABYC isolation-transformer practice, is UL Listed Marine, and is ignition protected. Final system compliance still depends on correct installation per ABYC E-11 by the yard or electrician.
Will Dual MP6 stop galvanic corrosion?
Yes, when installed as a true isolation transformer. The shore and boat grounds are not directly connected through the transformer.
Final Thoughts
Charles and Hubbell defined the 50A marine isolation category with large, proven metal-box transformers. They still work. They are also heavier, taller, and more likely to treat boost and soft start as extras.
Dual MP6 takes a different path: two sealed toroidal pucks, 160 pounds total, 6.5 inches high, non-metallic, ignition protected, with soft start and boost included. For yards fitting 50A boats where space, weight, dock-voltage sag, and shore-breaker trips are the real problems, that combination is the reason to specify Dual MP6 instead of defaulting to the older rectangular box.
Product Callout
Bridgeport Magnetics Dual MP6 50A
12 kVA toroidal marine isolation transformer system for 50A, 120/240V shore power. Soft start and 104V/208V boost included. UL Listed Marine, ignition protected, Class II, Made in USA.
