Charles Marine’s Iso-Boost 50 is out of production. Yards still have them on 50A boats. When the unit fails, or when a new build would have spec’d one, the question is which isolation transformer replaces it.
The MarinePuck that matches an Iso-Boost 50 is one Dual MP6. That is a 12 kVA, 50A, 120/240V toroidal isolation pair. Soft start is included. 104V and 208V boost taps are included. It is not a bolt-for-bolt copy of the 235 lb Charles box. It is the electrical replacement for that 12 kVA / 50A job.
If the boat has two Iso-Boost 50s, or an Iso-Boost 100, stop. That is 24 kVA. You need two Dual MP6 systems. One Dual MP6 is only 12 kVA.
Which MarinePuck you actually need
| What is on the boat | Rating | Order this |
|---|---|---|
| Charles Iso-Boost 50 | 12 kVA / 50A + boost | One Dual MP6 |
| Charles 12 kVA IsoTransformer (93-IXFMR12I / 12T) | 12 kVA / 50A isolation | One Dual MP6 |
| Two Iso-Boost 50s | 24 kVA / two 50A cords | Two Dual MP6 systems |
| Charles Iso-Boost 100 | 24 kVA / 100A | Two Dual MP6 systems |
| Charles 3.6 kVA / 30A | 3.6 kVA / 30A | MP3.6, not Dual MP6 |
| Single 50A / 120V inlet, mid-size boat | 6 kVA class | MP6, not Dual MP6 |
Read the nameplate before you quote. Iso-Boost 50 and Iso-Boost 100 share a family name. They are not the same kVA.
What an Iso-Boost 50 was
Charles published the Iso-Boost 50 as an automatic isolation and boosting transformer for boats with 50A, 208/240V shore service.
| Iso-Boost 50 spec (Charles) | Value |
|---|---|
| Continuous rating | 12.0 kVA |
| Input | 208/240 VAC, 50A |
| Output | 120/240 VAC nominal |
| Non-boost output current | 50A |
| Boost-mode max output current | 42.5A |
| Boost | Automatic, about 15% when dock voltage is low |
| Frequency | 50/60 Hz |
| Published weight | 235 lb |
| Package | One metal box, about 18 × 15 × 12 in |
The boost existed because many marinas feed 208V phase-to-phase, or a long dock run sags 240V. Air conditioning and chargers want closer to 240V. Charles switched tappings automatically. In boost mode the unit was not a full 50A machine. Charles published 42.5A as the boost-mode ceiling on the 50.
Soft start lived on the Iso-Boost SKU. The plain 12 kVA IsoTransformer did not always include it.
Dual MP6 as the Iso-Boost 50 replacement
| Spec | Charles Iso-Boost 50 | MarinePuck Dual MP6 |
|---|---|---|
| Continuous kVA | 12 kVA | 12 kVA |
| Shore class | 50A, 208/240V | Dual 50A, 120/240V |
| Weight | 235 lb | 160 lb (two 80 lb pucks) |
| Size | One 18 × 15 × 12 in box | Two pucks, 12.6 in OD × 6.5 in H each |
| Core | Laminated | Toroidal |
| Enclosure | Metal | Flame-rated ABS, epoxy potted |
| Soft start | On the Iso-Boost SKU | Included |
| Boost | Automatic ~15% | 104V and 208V taps included |
| Boost-mode current | 42.5A published | Size the load. Taps are not an auto 15% box |
| Case grounding | Metal can, shield scheme | Class II, green capped |
| Listing | Marine UL on Charles boost models | UL Listed Marine E509703, ignition protected |
| Origin | Charles Marine line, out of production | Made in Shelton, Connecticut |
Electrical job: same class.
Mechanical job: not the same holes.
Boost: what you keep, what you do not
This is the sentence that keeps a yard from being surprised.
Charles Iso-Boost 50 watched dock voltage and switched itself into a higher ratio. Dual MP6 includes 104V and 208V boost taps plus a soft-start thermistor. Same problem (low dock, 208V pedestal). Different method.
| Need | Iso-Boost 50 | Dual MP6 |
|---|---|---|
| Isolation from shore ground | Yes | Yes |
| Raise low dock voltage | Automatic 15% | Wire the 104/208 taps |
| Soft start so the pedestal ELCI does not trip on inrush | Yes | Yes |
| One steel box that decides boost by itself | Yes | No |
If the captain’s only requirement is “when the dock is 208V, the boat still sees usable 240V,” Dual MP6 taps do that job. If the specification is “must clone Charles automatic SAFEtech switching,” Dual MP6 is not a clone. Say that in the quote.
Do not ignore Charles’s 42.5A boost-mode limit when you compare. A boat that ran near 50A on a 208V dock was already on the edge of that SKU.
Soft start
A dead 12 kVA transformer inrush will open a marina ELCI. That is why Iso-Boost included soft start and why Dual MP6 includes it on every unit.
If you replace an Iso-Boost 50 with a plain isolation transformer that has no inrush limit, the first complaint is not corrosion. It is the pedestal that will not stay closed.
What “drop-in” means on this swap
A replacement is an Iso-Boost 50 drop-in when:
- Shore is still one 50A, 240V class cord
- The new gear is still 12 kVA continuous
- Isolation still breaks the shore-ground galvanic path
- The panel still sees 120/240V
- Low-dock voltage has a boost path
A replacement is not drop-in when you expect Dual MP6 to use the Iso-Boost 50 bolt pattern, hoist points, and automatic tap board. You gain two 80 lb pucks that can sit in two tight spaces instead of hoisting 235 lb into one stringer bay.
Charles manuals called out hoisting hardware and a structure that could hold that box. Dual MP6 is two-person staging, not a crane ticket, if the path is clear.
Install notes the electrician has to own
MarinePuck Dual MP6 is UL 1561 Recognized, UL Listed Marine (E509703), and ignition protected. It is designed for ABYC E-11 isolation-transformer practice when installed to the supplied diagrams, including the secondary neutral-to-ground bond. The green shore ground is capped. That is Class II isolation, not a metal-case shield scheme.
Have a marine electrician:
- Confirm 50A / 240V (or the actual inlet) matches 12 kVA
- Land primary and secondary per the Dual MP6 diagram, boost or no-boost
- Bond secondary neutral to ground on the boat side as the diagram shows
- Cap and insulate the shore green
- Size shore and boat breakers for 50A isolation service
- Put ELCI / GFPE where the yard and ABYC / marina rules require it
Wrong bond is not a branding issue. It is an isolation issue.
Weight, heat, and where it sits
| Iso-Boost 50 | Dual MP6 | |
|---|---|---|
| One lift | 235 lb box | Two 80 lb pucks |
| Height in a locker | ~12 in plus clearance Charles called out | 6.5 in per puck |
| Split mount | No | Yes |
| Hum | Laminated stack | Toroidal, much quieter |
| Corrosion of the can | Metal enclosure in a bilge | Non-metallic ABS |
Larger vessels spec isolation because aluminum running gear and wet exhausts do not survive a shore-ground loop. They also spec it because a 235 lb box is hard to place. Dual MP6 is how you keep 12 kVA without giving the entire lazarette to one can.
How this sits next to Hubbell 50A
Hubbell’s common 50A unit is a 15 kVA isolation transformer (HBL50AIT class), about 170 lb, optional auto-boost. That is still a 50A shore product, with more nameplate kVA.
If the search was Iso-Boost 50, do not automatically jump to Hubbell 15 kVA or to two Dual MP6 kits. Iso-Boost 50 was 12 kVA. Dual MP6 is 12 kVA. Hubbell 15 kVA is the next size up if the yard wants a single steel 15 kVA box. Different post.
50A vs 100A, one more time
50A × 240V = 12 kVA. That is Iso-Boost 50. That is one Dual MP6.
100A × 240V = 24 kVA. That is Iso-Boost 100. That is two Dual MP6 systems.
Two Iso-Boost 50s on two 50A cords is also 24 kVA. Same order: two Dual MP6 systems.
A buyer who says “we had Charles boost transformers” is not done specifying. Count the cans. Read 50 or 100 on the label.
Sizing check before the PO
| Shore on the boat | Charles name people use | MarinePuck |
|---|---|---|
| 30A, 120V | 3.6 kVA IsoTransformer | MP3.6 (3.6 kVA, about 45 lb) |
| 50A, 120V | 6 kVA class | MP6 (6 kVA, about 80 lb) |
| 50A, 240V | Iso-Boost 50 or 12 kVA IsoTransformer | One Dual MP6 |
| Two 50A / 240V cords | Two Iso-Boost 50s | Two Dual MP6 systems |
| 100A / 240V | Iso-Boost 100 | Two Dual MP6 systems |
Soft start and boost taps ship on Dual MP6. Do not add a second Charles SmartBoost-style box to “complete” the quote unless the customer insisted on automatic tap switching.
FAQ
What replaces an Iso-Boost 50?
One Bridgeport Magnetics Dual MP6. 12 kVA, 50A class, soft start and 104/208V boost included.
Is Dual MP6 the same as Charles automatic boost?
No. Dual MP6 uses included taps plus soft start. Iso-Boost 50 switched boost automatically and limited boost-mode current to 42.5A.
Can one Dual MP6 replace an Iso-Boost 100?
No. Iso-Boost 100 is 24 kVA / 100A. Order two Dual MP6 systems.
Can one Dual MP6 replace two Iso-Boost 50s?
No. That pair is 24 kVA.
Will Dual MP6 bolt into the Iso-Boost 50 footprint?
No. Same kVA class. Two pucks vs one 235 lb metal box.
Does Dual MP6 stop galvanic corrosion?
Yes, when installed as a true isolation transformer with the boat-side bond per the diagram and the shore green capped.
Does Dual MP6 stop marina GFI / ELCI trips?
Isolation removes the boat from the shore-ground leakage loop. Soft start reduces inrush trips. Pedestal and cord condition still matter.
Is Dual MP6 ignition protected?
Yes. UL Listed Marine E509703 and ignition protected.
Where is Dual MP6 made?
Shelton, Connecticut.
Product Callout
Bridgeport Magnetics MarinePuck Dual MP6
Iso-Boost 50 replacement. 12 kVA, 50A, 160 lb, soft start and 104/208V boost included. Two Dual MP6 systems replace Iso-Boost 100 or a pair of Iso-Boost 50s (24 kVA / 100A). UL Listed Marine, ignition protected, made in the USA.
Phone: (203) 954-0050
Email: [email protected]
bridgeportmagnetics.com/product/marine-puck-dual-mp6-50a
