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Due to the ongoing global health crisis, Bay Marine Supply is still operational, but In-store transactions or consultations are not available until further notice. Orders may still be placed online for will-call pickup by entering coupon code localpickup during checkout. We will call you when your order is ready - PLEASE DO NOT come to will-call before you have received a call indicating that your order is ready.

Will-call pickup is at the rear of the warehouse. Masks are required at all times and hand sanitizer will be available at the pickup desk to help ensure the safety of your families and ours. Thank you for your patience and understanding through these trying times, and we wish you and your families fortune and health!


Bos'ns' Corner - Electrical


bo'sun (boatswain, bosun) (n.): a petty officer on a
merchant vessel, having charge of the hull and all
related equipment. From Old English bat (“boat”)
+ Old Norse sveinn ("swain"), meaning servant.



“Boat Servant” - we think that's a pretty apt description of how we spend a good part of our time! But if we serve our little ship well at the dock, we trust that she'll serve us well underway, and that's a deal we can live with.

Here is a collection of the electrical questions we get most often, and answers which we hope will help you as well.

Back to the Bo'sun's Corner main index

Chargers and Inverters
What's the difference between the Magnum MS2000 and MS2012?
What is the difference between “modified sine wave” and “pure sine wave”?
Some of the Magnum inverters have -15B and -20B variants. What are they?
What are the differences and similarities between ProMariner and Sterling?
How does the wattage draw on my inverter relate to the inverter's draw on my batteries?
What size wiring will I need for an inverter?

Batteries and Cables
Making battery cables and connecting multiple batteries



Q: The Magnum MS2000 and MS2012 look like they have the same specifications. What's the difference between them?
A: The difference is in the transfer switch. All of the MS-series inverter/chargers except the MS2000 have two 30A legs on the 120V input side, while the MS2000 has one. This is allows them to pass through 60 amps when connected to shore (or grid) power, though the amperage output when in inverter mode is the same. The bottom line is that, if you are running on a 30A source, the MS2000 will do the trick. If you want to pass 60A through the unit when connected to the mains, you'll want one of the others.

Q: Some inverters are “modified sine wave”, while others are “pure sine wave”. What's that mean, and which one do I need?
A: The pure sine wave inverters' output is nearly identical to what you'll get from utility power. The modified sine wave is a squared-off wave form, which is much less expensive to produce, but still puts the "alternating" in A/C. The modified wave is fine for motors and less sensitive electronics, including most computers, though there is a minor loss of efficiency. The modified wave is not a good match with sensitive electronics, especially audio. The discussion on which type is best for general use can get long and heated, and the bottom line is that it's a good idea to check with the manufacturer of the devices you are using, or to try them out on a neighbor's setup. That said, pure sine wave is always the best bet, though at higher cost, and converseley there are thousands of boats and RVs out there using modified sine wave units with no complaints. We're sorry we can't give simpler answer, but we're happy to discuss the issue with you - just give us a call.

Q: Some of the Magnum inverters have -15B and -20B variants. What are they?
A: Those models have two breakers built into the inverter housing, with the “15” or “20” indicating the amperage of each breaker. Whether you need them or not depends on your distribution wiring. If you wish to hook outlets directly to the inverter, the internal breakers are handy. If your inverter is powering a system that already has breakers, you probably don't need the built-in ones.

Q: What's the difference and similarities between ProMariner and Sterling?
A: That some of the ProMariner and Sterling Power products appear identical is no coincidence. Those products were co-engineered and jointly manufactured by the two companies for many years, with ProMariner having the North American market, and Sterling Power doing business in their native Great Britain, along with Europe and much of the rest of the world. A few years ago, they decided to do away with their limited marketing areas, and Sterling entered the U.S. market, opening a warehouse and service facility in Maine. Each company has continued to produce many of their highly successful lines at the same facility, with the label, manual, and packaging being the only difference. Both manufacturers have their own products, of course, but the shared designs are:

  ProMariner Sterling
  ProNautic P Chargers ProCharge Ultra Chargers
  ProSport Chargers Aquanautic Chargers
  Digital Mobil Chargers Alternator-to-Battery Chargers
  ProIsoCharge Isolators ProSplit-Zero-Drop Isolators


Q: How does the wattage draw on my inverter relate to the inverter's draw on my batteries?
A: The basic formula is:
    watts / volts = amps
or conversely
    volts x amps = watts
So if we were to have a perfectly efficient 12 VDC / 120 VAC inverter supplying 1000 watts, it would draw about 83 amps (1000W / 12V=83.3A). But since inverters usually operate at 85% - 90% efficiency, we should include that correction in our formula, which gives us:
watts / volts x 1.15 = amps

Q: What size wiring will I need for an inverter?
A: All of the installation manuals have complete wiring guidelines that should be followed, but a rough guide is:

  1000 watt / 12 volt input #2 - #1/0
  2000 watt / 12 volt input #1/0 - #2/0
  2500 watt / 12 volt input #3/0
  4000 watt / 24 volt input #2/0

These sizes are for cable runs of 3 to 6 feet in open air, and at moderate temperatures. For 6 – 10 foot runs, use the next size larger, or double the cables. Keep in mind that most inverters will operate for short periods at more than their continuous load rating, and so will draw more amps when starting some devices. It's best to place the inverter/chargers as close to the batteries as possible, The AC side, running at 110-240 volts, will require much smaller cables, so length is not as critical. You can also check out the discussion on on our battery cable page.

Bay Marine Supply
3235 Hancock St # 12 San Diego, VA CA 92110
Phone: 619-320-5899