Friday, January 25, 2013

Living on 30A . . .

How many remember the show Green Acres?  Among the standing story lines was the need to shuffle plugs between the very limited outlets.  Want to iron?  Need to unplug the coffee pot.  Want coffee?  Need to unplug the Refrigerator.

Anyone who lives aboard a boat knows the same drill, and if you are trying to heat using a single 30A shore power cord, even more fun.   Let's look at Viking Star's AC load while 'At Dock'.


  • 1A - Refrigerator
  • 1A - Freezer
  • 14A - Water Heater
  • 8A Block Heater
  • 10A Main Cabin Space Heater
  • 10A Aft Cabin Space Heater
  • 5A Coffee Pot
  • 12A Washer / Dryer
  • 2A  Battery Charger
For  a total of:  63A's!    And if we throw in ironing, or hair dryers....  Compounding this is the fact that a 30A shore power plug is not designed to carry a constant 30A, you really need to limit it to 24A maximum continuous load.  (which is one of the reasons you can see SO many burned out shore power connectors - a primary cause of boat fires).

So, how do we do it?   Add a 2nd shore power line?  (Lots of folks do).  Upgrade to 50A service  or even 50A/220v service?  Again, lots of boats do.   Just live with it and unplug the space heaters when we want hot water, or turn off the hot water heater when we want coffee?   Again . . . .

But on Viking Star we do none of the above.  We have a single 30A shore power cord and do very little of the Shuffle (mostly, just make sure the aft cabin space heater is off when doing laundry).  You may be asking: "Wow, how do you get 63A of service from a single 30A shore power cord.  Must be Magic".   No, no magic.  But the answer is actually very simple:  

    >  We cheat  <

A key here is that not all devices run all the time.  The block heater is on only 25% of the time, the water heater on and off throughout the day.  Coffee pot in the morning,  washer/dryer in the afternoon.  Another key is noting  two of the 'biggest' loads are the water and block heater.  And with that last bit of info is where the Cheat comes:  We run the Water heater and block Heater off of our backup inverter powering them when needed via the massive batteries we have.  And then over the day the other  inverter/charger (the one connected to shore power)  will replenish the batteries. 

Top level summary, click for larger view
Or download .PDF file using this link: PDF File of Viking Star Summary

Above is a block level diagram of Viking Star's electrical system.  Of key note here is the two inverters/chargers, each servicing 4 AC breakers.  By turning off the AC input to Inverter #1 we let it power the water heater and block heater via the batteries.  And by leaving the AC power turned on for the Inverter #2 it will use its 'power share' capabilities to recharge the batteries when less than 25A is being drawn by things like Space Heaters, Coffee Pots, Washer/Dryers, etc.

It is premised on having two inverters, but we have two anyway as they are rather critical for the boat's operation and hence we carry a spare.

In practice this has worked very well for two years now.  A downside is lower overall efficiency having to go through the AC --> DC --> Battery --> DC --> AC conversion cycles for the Water heater and Block Heater, but then that does provide a nice heat source for the lazarette where the inverters are located.

And here, my friend, is how on Viking Star we pack 63A of stuff into a 30A bag...


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