Inverter for fridge - sizing and wiring options

[TandC]

Hello all,

 

Now that the cruising season is back on us, I am after some thoughts on inveters and my fridge...

 

We live aboard and the vast majority of the time are moored with a landline so permanent 240v hookup - because of our mooring, much of the year we cannot navigate the river due to the flow. As a result, a 240v fridge/freezer is the most practical option.

 

When we to go on extended cruises, I tend to prepare by emptying the freezer and minimising whats in the fridge (apart from beer) and make do with an electric coolbox which i just run off the battery bank with some crocodile clips, but it's capacity is limited.

 

The biggest problem is the fridge/freezer stops us going on those more spontaneous cruises to the pub when the weather is nice but we have a full freezer/fridge which we dont want to have to eat!

 

I was wondering how much of an inverter would be required purely to run the fridge - we dont need anything else. Also, the wiring solution.

 

Its a small Zanussi, looking at the specification plate it claims it is 50hz and 0.6amps, with an annual consumption of 201KWH - but that is as much as it details. Looking online with the model number doesnt yield any electrical information.

 

I've a 440amphour battery bank, plus would probably only have it turned on for a few hours in reality, most of which would be cruising.

 

What size inverter would I need to run just the fridge? Could i just use one of those small ish standalone units which have a socket built in to them?

 

 

And second - wiring.... the fridge is in the centre of the boat, the battery bank is in the rear. I have 12v cable running down to next to the fridge for the water pump - but it is obviously not massively thick. Would it be possible to locate the inverter behind the fridge, wiring it in to the same supply for the water pump?

 

I could then just move the fridge forward, unplug it from the 240v socket and plug it into the inverter for those occasions when i need to run it.

 

Would the inverter draw too much current on those relatively thin cables?? If it is a case of trying to build the inverter in to the rear cupboard closer to the batteries to allow for thicker core cable and running Artic cable down to the fridge for a new socket then I think i may give up on the idea as that is too much of a faff for me at the moment.

 

 

Any thoughts?

Edited May 31, 2012 by TandC

[real_vibes]

I am pondering the same problem, so eaglery await advice!

[Keeping Up]

If it consumes 0.6 amps at 240v, that is 144 watts.

 

This means first of all that the inverter will be drawing about 13 amps from the batteries while the motor is running. I would be surprised if your water-pump cables could supply this current reliably without there being an excessive drop in voltage - particularly as the fridge and pump could be running at the same time.

 

However your real problem is likely to be the high current that the fridge will draw for a short time every time the motor starts up (which is not just when you first switch on, but also every time the thermostat closes). The spec for your fridge won't tell you how much current that will be, and even the supplier or manufacturer is unlikely to be able to tell you this figure. It would not be unusual for you to need a current of up to 5 to 10 times the steady current. So you can be certain that a small 150 watt inverter (which you might expect to be big enough for a 144 watt fridge) will not be big enough, and I would guess that you are unlikely to get away with anything less than 4 times that size (ie 600 watt) and even then it would only cope if it had a very generous short-term (surge) rating. In practice I have known people sometimes have difficulties in starting their fridge from even a 1000 watt inverter. And if you think about the size of inverter we are talking about, then it definitely couldn't run from your water-pump cables.

 

This guess is largely theoretical, my practical experience of running fridges on 240v inverters is rather out-of-date so hopefully some others will be along with practical evidence to confirm (or refute) the figures.

[Chalky]

We've got a 240V fridge running off a Sterling 800W modified sine inverter. The inverter draws about 0.4A continuously and 6A when running which it does for about 40% of the time. It's an A rated fridge - a B&Q £90 special. We chose the Sterling inverter since it was rated at 800W continuous at 40deg ambient and has a surge capability into the kW range. Has worked perfectly for 4 years, unlike the Maplin 800W inverter which didn't make it to the end of the top pound on the South Oxford!

 

The inverter is mounted directly above the batteries in the engine room with an isolator switch so it can be shut down if required. It uses the cables that came with it.

 

Put the inverter close to the batteries and use 240V for the longer run - it's significantly more efficient.

Edited May 31, 2012 by Chalky

[Capt Ahab]

I am not too good on the electical tech specs side of things but I have recently installed a 800watt inverter. Initially I tied to run this off the 12v cables at the front of the boat but the supply was far too low and it tripped out. I then sited it at the rear and drew my power from the main battery supply to the bus bar - feeding the 240 volt around the boat via the square pin circuit and this works perfectly. I can now draw 240 volts from the shoreline or the inverter - but have to make sure I never try to use both the supplies at the same time!

[bottle]

Just to add to Allan and Chalky.

 

Inverter as close to the batteries as possible, permanently wired with the correct isolator and 'fusing'.

[RobinJ]

I was wondering how much of an inverter would be required purely to run the fridge - we dont need anything else. Also, the wiring solution.

 

Its a small Zanussi, looking at the specification plate it claims it is 50hz and 0.6amps, with an annual consumption of 201KWH - but that is as much as it details. Looking online with the model number doesnt yield any electrical information.

 

I've a 440amphour battery bank, plus would probably only have it turned on for a few hours in reality, most of which would be cruising.

 

What size inverter would I need to run just the fridge? Could i just use one of those small ish standalone units which have a socket built in to them?

I see two options;

Fit am inverter (smallest will probably be about 600W), but you are better fitting that as close to the batteries as possible with decent size cables. The 240V can simply be plugged in where the landline goes! You should be able to do that for about £100 ish.

Obtain a 12V fridge/freezer unit, there are now quite a few available, it may also be slightly cheaper. However you will need to make sure it will run without any major headaches switching between 240V and 12V, or run it off 12V with a battery charger.

 

I have a small 12V frdige/freezer with 12/24V unit on 550Ah and that hardly affects the batteries after a week without engine running! (though this last week it did seem to draw more than usual).

[TandC]

Thanks folks - I had feared that will be the case...!

 

THis has now gotten me thinking about retro-fitting an inverter to the system.

 

Just to summarise the current set up - 240V its a straightforward distribution board into which the shoreline feeds, running two rings - one for sockets and one for lights (which isnt used), both running through an appropriate fuse/MCB at the distribtion board. The 12v comes off the battery bank, in to a busbar and through fuse panel for the various feeds - shower pump, water pump, lights split into 4 separate 'rings', nav lights, horn, etc.

 

There is a Sterling ProDigital charger, which is running off a 240v socket, so can simply be switched off when there is no shoreline. It acts as the powerpack/transformer for all our 12v needs when on the shoreline.

 

Where i am struggling is the room - the above is all in a tiny cupboard at the rear of the boat, with the batteries in the enginebay directly behind the cabin (cruiserstern). There is barely any room in the cupboard, and additional cabling in would be very challenging.

 

Can i then simply fit an inverter in the engine bay next to the batteries, in a protected box - simply place an extra pair of cables from the battery bank to the inverter, then run a 240v cable from inverter in to the distribution board - an additional supply to that coming off the shoreline? Would i be able to use the same disstribution board - simply wiring in the extra pos/neg/earths? This would be the ideal solution in that all the sockets would then just draw off the inverter - so i wounldnt have to faff about moving the fridge.

 

I could then simply manually turn on the inverter once the shoreline is disconnected. And turn it off before we plug it back in.

 

What happens if my mistake the shoreline was plugged in at the same time as the inverter were still turned on? And vice versa, what happens at the distribution baord? Are there models which sense when there is already a 240v supply and simply turn off - just trying to idiot proof it!

 

If i could do this as simply as described, all i need to overcome is where to locate the inverter in the engine bay, routing some cable through to the distribution board and getting it wired in. That i think i could manage....

 

Thanks v. much

 

Thanks RobinJ - the reason I didnt go for a 12v fridge in the first place is simply that we are on the shoreline for the vast majority of time, we need a decent freezer and reasonable capacity fridge as we live aboard. The 12v models out there seemed extremely expensive for something able to switch between supplies while also giving a decent size/freezer, so I always thought that the money would be better spent on an inverter - which would then upgrade the whole electrical system, as opposed to just the fridge.

[DeanS]

What happens if my mistake the shoreline was plugged in at the same time as the inverter were still turned on? And vice versa, what happens at the distribution baord? Are there models which sense when there is already a 240v supply and simply turn off - just trying to idiot proof it!

 

The one method already mentioned is the best in my book. Your output from the invertor can have a shoreline type plug on. When you're on shoreline, you pull out the invertor plug, and put in the long cable plug. When you're off shoreline, you are forced to remove the long cable plug before putting in your invertor fed plug.

[TandC]

Chalky

 

You say that your inverter draws about 0.4A continuously and 6A when running which it does for about 40% of the time. Is that 0.4a the draw from the fridge just ticking over, and 6amp when the compresser is running? Or 0.4amp is just the residual useage of the inverter, with no load?

 

Do you just turn it off when you dont want to use the inverter for the fridge?

 

When you're on shoreline, you pull out the invertor plug, and put in the long cable plug. When you're off shoreline, you are forced to remove the long cable plug before putting in your invertor fed plug.

 

Of course! Why is it that my mind seems to go for the most complicated route...!!

 

That would be the easiest way. I'd just need to remember to switch off the battery charger, as otherwise i'd be drawing off the inverter to run the battery charger to charge the batteries to run the inverter to run the battery charger to charge the .... i'll stop now.

 

So, considering that when in use, the inverter will run the fridge as described (although I'd likely turn it off when engine isnt running), maybe a phone charger, occasionally run the laptop and I can't imagine much else - will an 800w pure-sine wave model do the job? Ideally, with a remote switch i can mount in the control panel so that I can turn the inverter off/on without fannying around in the engine bay.

 

Any suggestionson model? The Sterling battery charger has been fine, no problems, although I remember their customer service being challenging at times... so I'm tempted to have a look at their models. There is currently a 12v 800w Mastervolt on a popular internet auction site for £300, which is probably getting on for the price range...

 

You can see what's coming next - retro-fitting solar panels and controllers in to the system...!

[smileypete]

Just to summarise the current set up - 240V its a straightforward distribution board into which the shoreline feeds, running two rings - one for sockets and one for lights (which isnt used), both running through an appropriate fuse/MCB at the distribtion board.

If using the inverter like this, it's best to check it doesn't have a 'floating earth', a filament test lamp wired into a plug between live and earth (not neutral) will light up if it's OK.

 

Also fit a fuse at the battery end of the inverters +12V cable to protect the cable from fault currents from the battery.

 

If putting the inverter on a steel bulkhead, try to make it stand off a couple of inches to allow air to circulate behind and avoid condensation inside it, and have a plastic drip shield overhead.

 

cheers, Pete.

~smpt~

Edited June 1, 2012 by smileypete

[TandC]

Thanks Pete -

 

I'm definately thinking that I'll go with the simple option of shoreline fitting on the end of the output from the inverter, as opposed to wiring it straight into the distribution board. In which case, the 'floating earth' shouldnt be an issue - is that right to assume?

 

As for the fuse in the 12v cable from battery bank to inverter - what sort of size would you recommend, and place the fuse in the negative or the positive cables?

 

Really grateful for all the assistance so far.

[RobinJ]

If using the inverter like this, it's best to check it doesn't have a 'floating earth', a filament test lamp wired into a plug between live and earth (not neutral) will light up if it's OK.

 

Also fit a fuse at the battery end of the inverters +12V cable to protect the cable from fault currents from the battery.

Also I assume as alrady has landline that earths etc. are all in place with RCD?

[TandC]

Thats right Robin - the distribution board is an RCD affair, earthed etc.

 

I've looked at the Sterling ProQ quasi-sine inverter, 12v 1000w model - at £200 it seems keenly priced. The instructions recommend fusing the positive battery cable with a 200amp ANL fuse, so that would answer that question.

 

Anyone have any thoughts on that model - it would meet the needs, has remote, is well sized to allow fitting and is well priced.

 

It seems the 240v output is a socket - therefore, I'd have to make up a cable with a standard three-pin plug to go into the inverter, with a shoreline socket on the other end. I trust there are no problems doing that, as long as the inverter is well protected and waterproofed, yet with ventilation.

[TandC]

Ah - one last thought - the alternator is 70amp only... will this cause problems with a 1000w inverter, not being man enough to fully service it....? the fridge wouldnt be drawing that full 1000w continuously, so there would only be brief periods when the alternator wouldnt fully cover the use of the inverter?

 

Sorry!

[Keeping Up]

Ah - one last thought - the alternator is 70amp only... will this cause problems with a 1000w inverter, not being man enough to fully service it....? the fridge wouldnt be drawing that full 1000w continuously, so there would only be brief periods when the alternator wouldnt fully cover the use of the inverter?

 

Sorry!

 

That won't be a problem.

[smileypete]

Thanks Pete -

 

I'm definately thinking that I'll go with the simple option of shoreline fitting on the end of the output from the inverter, as opposed to wiring it straight into the distribution board. In which case, the 'floating earth' shouldnt be an issue - is that right to assume?

I'm afraid not, if the inverter's earth is isolated the RCD downstream won't trip in the event of you getting a shock or electrocution.

 

Somewhat confusingly, the RCD test button will still work, as it checks the integrity of the RCD itself, but not the installation.

 

Commonly these inverters are only used with only one or perhaps two items items of equipment plugged straight into it, which greatly lessens the risk, but supplying a whole boat's installation is completely different.

 

If getting a Sterling model they should be able to advise on it's earthing.

 

cheers, Pete.

~smpt~

Edited June 1, 2012 by smileypete

[RobinJ]

I'm afraid not, if the inverter's earth is isolated the RCD downstream won't trip in the event of you getting a shock or electrocution.

 

Somewhat confusingly, the RCD test button will still work, as it checks the integrity of the RCD itself, but not the installation.

Not a problem provided the inverter is N-E bonded. Landline - earth at substation. Inverter - earth on boat. Saves all that complicated switching!

[TandC]

Hmm - if i remember rightly, there is a galvonic isolator fitted between the RCD earth

and a bolt on the engine bearers. Aside from that, I dont think there is any further hull bonding/earthing... Would the inverter need an additional earth or will that suffice?

 

Just checked - this is how the Galvonic Isolator is wired in - Earth from shoreline socket into Galvonic Isolator, then into RCD earth busbar, with a cable coming off the bus bar back down to the hull - as per in gibbo's page: http://www.smartgauge.co.uk/nb_ac_sys.html

 

If i am simply going to swap shoreline for inverter supply, would this earthing arrangement be okay?

Edited June 1, 2012 by TandC

[Paul C]

Hmm - if i remember rightly, there is a galvonic isolator fitted between the RCD earth

and a bolt on the engine bearers. Aside from that, I dont think there is any further hull bonding/earthing... Would the inverter need an additional earth or will that suffice?

 

Just checked - this is how the Galvonic Isolator is wired in - Earth from shoreline socket into Galvonic Isolator, then into RCD earth busbar, with a cable coming off the bus bar back down to the hull - as per in gibbo's page: http://www.smartgauge.co.uk/nb_ac_sys.html

 

If i am simply going to swap shoreline for inverter supply, would this earthing arrangement be okay?

 

Yes, in that an inverter's earth can (and should) be directly connected to the earth hull bonding on a boat, ie it doesn't need to go through a galvanic isolator.

[smileypete]

Hmm - if i remember rightly, there is a galvonic isolator fitted between the RCD earth

and a bolt on the engine bearers. Aside from that, I dont think there is any further hull bonding/earthing... Would the inverter need an additional earth or will that suffice?

 

Just checked - this is how the Galvonic Isolator is wired in - Earth from shoreline socket into Galvonic Isolator, then into RCD earth busbar, with a cable coming off the bus bar back down to the hull - as per in gibbo's page: http://www.smartgauge.co.uk/nb_ac_sys.html

 

If i am simply going to swap shoreline for inverter supply, would this earthing arrangement be okay?

Just check the inverter's earthing with a test lamp wired up as described above, and/or ask Sterling if the inverter's earth is connected internally.

 

Edit: To make a suitable test lamp buy one of these from Screwfix:

 

 

Disconnect the big round bit, wire the end to live and earth of a three pin plug. Put a 60 watt bulb in, plug into inverter, see if it lights up...or not.

 

cheers, Pete.

~smpt~

Edited June 1, 2012 by smileypete

[RobinJ]

Just checked - this is how the Galvonic Isolator is wired in - Earth from shoreline socket into Galvonic Isolator, then into RCD earth busbar, with a cable coming off the bus bar back down to the hull - as per in gibbo's page: http://www.smartgauge.co.uk/nb_ac_sys.html

But nowhere on there does it show the inverter being N-E bonded!

The RCD won't work unless the inverter is N-E bonded.

[TandC]

Just to confirm that thanks to all the help from here I've picked up a Sterling 1000w inverter and installed it now. I have been able to install it inside a cupboard in the rear of the cabin, which is going to be cooler than in the engine bay. I've had to drill a hole in the rear bulkhead into the bay to allow me to connect up to the battery bank.

 

The unit comes with generous length cables to attach to the batteries, but no fuse for the positive. So, I purchased a 200amp mega fuse and holder from VWP, along with a short length of battery cable with ring terminals to run to the battery bank, so that covers the fusing arrangements.

 

Earthing-wise, there is an earthing terminal on the back of the inverter for a ring terminal, which allowed me to run a length of cable from that to the hull earth bonding point.

 

A shoreline plug, length of Arctic cable and a 3-pin plug running back into the engine bay and out of the deck board means I can simply plug it in when I unplug the shoreline to go cruising. I've not needed to fit the remote as its perfectly accessible in the cupboard, which I have to open to switch off the battery charger anyway so its a good reminder - if the inverter is being switched on, i need to turn off the battery charger!

 

Although the weather precludes much in the way of cruising, in a test fire, the fridge was running fine over a period of around an hour in reasonably warm weather so although I didnt sit here listening, I guess the fridge compressor must have fired up in that time, with no problems....

 

There shall never be another warm beer!

 

Many thanks for the advice....