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ivan&alice

Calculate power requirements for Shoreline Fridge

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Obliquely, we see the occasional snipe at Shoreline for their fridges being so expensive at £500-ish, but is anyone cares to look up the price of a Danfoss BD35F 12V compressor they will find the compressor alone costs £500 delivered in its cardboard box. 

 

This means Shoreline must be relying on bulk buying discount to squeeze enough headroom to also buy a 240v fridge and graft in the 12V compressor and sell it to us. Profit margins must be wafer thin! They probably need to sell on the 240V compressor to make the whole operation viable.

 

Just musing really....

 

 

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We changed from a 12v Shoreline fridge which used the Danfoss compressor in an LEC cabinet, to a 240v LEC fridge in a similar cabinet. Not only was it only 1/3 the price, but it is also at least 30% more efficient - even after taking the inefficiency of the inverter into account.

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Got my Ah meter working last week and used the fridge to test it. Shoreline fridge with icebox, couple of years old. Can’t remember the model offhand, but it’s side radiating, no “radiator grille” on the back. Two very small 12v fans blow bilge air onto the compressor. On a fairly warm day last week it drew 3.3A running, and used 33.4 Ah in 24 hours.

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10 minutes ago, AndrewIC said:

Got my Ah meter working last week and used the fridge to test it. Shoreline fridge with icebox, couple of years old. Can’t remember the model offhand, but it’s side radiating, no “radiator grille” on the back. Two very small 12v fans blow bilge air onto the compressor. On a fairly warm day last week it drew 3.3A running, and used 33.4 Ah in 24 hours.

That would suggest that it is not very efficient and running for almost 50% of the time (ie10 hours per day)

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2 minutes ago, Alan de Enfield said:

That would suggest that it is not very efficient and running for almost 50% of the time (ie10 hours per day)

Well, perhaps. I’m not boating at the moment, so the boat was shut up and it gets pretty warm inside, 30deg+. I’ve no idea how accurate the meter is :)

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10 hours ago, AndrewIC said:

Got my Ah meter working last week and used the fridge to test it. Shoreline fridge with icebox, couple of years old. Can’t remember the model offhand, but it’s side radiating, no “radiator grille” on the back. Two very small 12v fans blow bilge air onto the compressor. On a fairly warm day last week it drew 3.3A running, and used 33.4 Ah in 24 hours.

I doubt the compressor needs the extra cooling but the sides where the radiators are does. How much gap do you have beside and above the fridge?

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1 hour ago, Tony Brooks said:

I doubt the compressor needs the extra cooling but the sides where the radiators are does. How much gap do you have beside and above the fridge?

The instructions for the official Shoreline fan (which these aren’t) are to put it down where the compressor and electronics are, so I arranged mine to blow up into the compressor space from the bilge - they sit in pockets routed into the floor. There’s only about an inch clear each side of the fridge, a couple of inches at the top, and the space behind is open into the void behind the rest of the kitchen cupboards, but there’s nothing I can do about any of that without some major carpentry! Those figures were from a standing start on a very warm boat, I loaded the fridge up with some of the ship’s beer supplies, zeroed the meter, switched it on, and came back 24h later.

 

I’m not unduly concerned about that level of consumption, I assume it’ll be a bit lower when the weather is cooler, and I could probably turn the dial up a bit more. It’s a bit higher than the “official” figures Shoreline quoted at the time, but I suspect those are like official car MPG figures!

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A tip here to reduce the consumption of the fan itself, is to put a room-thermostat in series with it, in the space behind the fridge, set to perhaps 25 degrees. That way the fan only cuts in when it's needed.

  • Greenie 2

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A related but slightly off-topic question. I have completed my spreadsheet (!) of all my existing and planned 12V electrical appliances, their current draw, voltage drops, cable thicknesses required, and energy consumption. I am about to order many squids' worth of cable etc from www.iem-services.co.uk which is the cheapest place I could find and comes recommended by other threads on this very forum.

 

Could I ask someone knowledgeable to check my work?

 

I am planning on placing 3 additional cigarette lighter sockets around the boat to take my device chargers. The chargers I have are max 45W, 3.6A @ 12.5V (which is the average of where my batteries sit, they are 3.75A at 12V). The one-way runs are 10, 16 and 20 metres. I therefore need 50 metres of red and 50 metres of black cable, so although the closer ones could get by with thinner cable I would like to buy a reel of the highest thickness I need to take advantage of the bulk savings and also to provide a decent buffer on the closer ones.

 

I took the resistivity ρ of copper to be 0.000000017 Ωm (= 1.7*-10^8 Ωm)

 

The furthest distance one (20 metre one way run) I calculated to need 5.3125mm^2 of copper cross sectional area to keep the voltage drop to 4% (which means around 12.0V at load when the batteries are at 12.5V). So I'm planning on getting 6mm^2 cable to use everywhere, which is a voltage drop of 0.425V / 3.5% for the furthest, 2.6% for the middle and 1.6% for the closest. I know this is a terrible reason but 6mm^2 is also the largest size my crimping tool can handle and I'd like to not have to buy a larger one.

 

The 6mm^2 thin-walled cable I want to buy is 84 strands at 0.30mm each: https://www.iem-services.co.uk/thin-wall-automotive-cable-60mm-500-amps--alternator-and-ammeter-circuit-etc--115-per-metre--buy-a-30-metre-reel-for-2760~1055 The cost is 27.6 GBP for 30 metres or 74.75 GBP for 100 metres.

 

Does this seem reasonable or should I go a size up to be sure?

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8 minutes ago, ivan&alice said:

 

Does this seem reasonable or should I go a size up to be sure?

Near our marina is a 'truck-parts' supplier.

 

https://particmotorspares.co.uk/index.php?page=products

 

Their cable prices were well below the 'electrical wholesalers' prices and their 8.5mm2 (flexible automotive) wire was VERY 'well priced' so I used that throughout for my Cigarette lighters, fridge, solar controller to battery etc.

 

Have a look around your area for a similar company and give them a call before ordering your cable.

 

Maybe :  https://www.cordwallis.com/

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13 minutes ago, Alan de Enfield said:

Their cable prices were well below the 'electrical wholesalers' prices and their 8.5mm2 (flexible automotive) wire was VERY 'well priced' so I used that throughout for my Cigarette lighters, fridge, solar controller to battery etc.

From what I understand the "red/blue/yellow" system does 0.5-1.5mm^2 (red), 1.5-2.5mm^2 (blue), 4-6mm^2 (yellow). So I'm thinking that keeping everything below 6mm^2 would make life much easier, even if it means I have to double up to get the required thicknesses sometimes?

Or am I making too much of the difficulty/expense of working with thicker cable?

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1 minute ago, ivan&alice said:

Or am I making too much of the difficulty/expense of working with thicker cable?

Yes

 

Buy a decent crimp tool (ratchet type) and you'll only need to buy it once

 

It is not 'good practice' to relying 'doubling -up' should (for any reason) one wire break or detached then everything will be passing down one single wire. This is certainly more important when you are 'doubling' to achieve a load rating, but even with doing it for volt-drop reasons you could end up with appliances not working or overheating due to working hard with a low voltage.

 

Remember that W = A x V

 

So A = W / V.

 

To make the point lets say you have a 24 volt system.

 

A 500 watt appliance will draw 500/24 = 20.8 amps.

Now, if the voltage actually getting to that appliance is now only 15 volts 500 / 15 = 33.33 amps.

 

Watts are fixed so as Volts vary so do Amps.

 

Your appliance is now drawing more than 50% higher current than it was designed for and will overheat.

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1 hour ago, Alan de Enfield said:

Watts are fixed so as Volts vary so do Amps

You have to be careful with that statement. With an inductive load the current will tend to increase with decreasing voltage. With a resistive load the current will decrease with decreasing voltage. 

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4 minutes ago, WotEver said:

You have to be careful with that statement. With an inductive load the current will tend to increase with decreasing voltage. With a resistive load the current will decrease with decreasing voltage. 

You are (as usual) correct, however, I was just trying (in a simplistic way) to explain why high volt drop is 'bad', and what could happen to your water pump, fridge etc.

 

If someone has the knowledge to realise the differences between resistive and inductive loads they probably would not be asking for recommendations for cable sizes.

Edited by Alan de Enfield

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3 hours ago, WotEver said:

You have to be careful with that statement. With an inductive load the current will tend to increase with decreasing voltage. With a resistive load the current will decrease with decreasing voltage. 

 

Same happens with any load with a switched mode power supply in it.

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Note that at present I don't have anything that needs more than 6mm^2, according to my calculations above this is enough. Given that no one has pointed out any flaws in my reasoning (which is what I was asking for, rather than recommendations for cable sizes) I am given to believe this is correct. So right now, I don't need to do any doubling, I'm just wondering what I should do with the leftover cable on my reel. I calculated the volt drop to be around 3.5% at the most which should be fine.

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1 hour ago, ivan&alice said:

Note that at present I don't have anything that needs more than 6mm^2, according to my calculations above this is enough. Given that no one has pointed out any flaws in my reasoning (which is what I was asking for, rather than recommendations for cable sizes) I am given to believe this is correct. So right now, I don't need to do any doubling, I'm just wondering what I should do with the leftover cable on my reel. I calculated the volt drop to be around 3.5% at the most which should be fine.

What to do with left over cable??  Save it for all the mods and additions that will be requested.  I soon added a usb charge point just inside by the helm and another by the bed. Use them more than the ones I first put in.

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2 minutes ago, Chewbacka said:

What to do with left over cable??  Save it for all the mods and additions that will be requested.  I soon added a usb charge point just inside by the helm and another by the bed. Use them more than the ones I first put in.

Yes exactly, I just want to make sure that the left over cable is of a useful thickness :)

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On 03/08/2019 at 16:33, Bobbybass said:

Even in the hot with door opening ..it was rare the compressor ran for any period.

Try switching it on! :D

 

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On 03/08/2019 at 19:30, Tony Brooks said:

And Danfoss the compressor makers figures. It really is not worth skimping on cable size for a fridge. However I think there is another company making 12V DC fridge compressors but I doubt the consumption will be any less and may be a bit more.

Ye canna change the laws of physics Captain.

 

  • Haha 1

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Just now, BEngo said:

Ye canna change the laws of physics Captain.

 

Agreed, but just maybe they have more powerful permanent magnets or even a different type of motor.

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11 hours ago, ivan&alice said:

A related but slightly off-topic question. I have completed my spreadsheet (!) of all my existing and planned 12V electrical appliances, their current draw, voltage drops, cable thicknesses required, and energy consumption. I am about to order many squids' worth of cable etc from www.iem-services.co.uk which is the cheapest place I could find and comes recommended by other threads on this very forum.

Another recommended supplier is http://www.vehicle-wiring-products.eu.

  • Greenie 1

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16 hours ago, David Mack said:

Another recommended supplier is http://www.vehicle-wiring-products.eu.

Yes, I looked at them too but their cables were around 32% more expensive than iem-services... 43.92 GBP inc VAT vs 33.12 GBP inc VAT for the 30 metres of 6mm^2 thin wall.

Edited by ivan&alice

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I think what one or two of you have said about fridge consumption is wrong. And this was queried back on the first page - but my two pene'rth...

When the fridge is running it may take 6-8a. It may run for approximately 30% of the time, giving rise to an average figure of 2ah per hour. Around the 40 or so a day. (a/h pedants have I said that right?).

Some shoreline fridges (I'm thinking of my fridge/freezer) have the heat dissipation at the sides rather than the back, though of course there will be heat from the compressor too. Something to think about when positioning.

In the hot weather I find that mine runs quite a lot though the consumption does seem to be a couple amps an hour on average and I have it set to three or four on the stat. It's typically 7c inside.

Once the other week I thought I saw the led come on (low voltage) so I need to investigate - but as the led is positioned inside the fridge, behind the salad basket (which is always full of salad) it can't normally be seen. Another great design. Out of sight, out of mind.

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1 hour ago, Johny London said:

I think what one or two of you have said about fridge consumption is wrong. And this was queried back on the first page - but my two pene'rth...

When the fridge is running it may take 6-8a. It may run for approximately 30% of the time, giving rise to an average figure of 2ah per hour. Around the 40 or so a day. (a/h pedants have I said that right?).

Some shoreline fridges (I'm thinking of my fridge/freezer) have the heat dissipation at the sides rather than the back, though of course there will be heat from the compressor too. Something to think about when positioning.

In the hot weather I find that mine runs quite a lot though the consumption does seem to be a couple amps an hour on average and I have it set to three or four on the stat. It's typically 7c inside.

Once the other week I thought I saw the led come on (low voltage) so I need to investigate - but as the led is positioned inside the fridge, behind the salad basket (which is always full of salad) it can't normally be seen. Another great design. Out of sight, out of mind.

Which bit is wrong?

 

We work on an AVERAGE consumption of between 30 and 50Ah per day. As you knowledge the actual consumption will vary with fridge design, loading, installation and ambient temperature. Your figure puts the consumption towards the top of that range but there are other 12V fridges out there and more than one compressor maker.

 

The starting current surge will be transitory so can be ignored for consumption purposes but it can not be ignored for cable sizing and 1 sq mm CCSA for each metre between fridge battery and fridge is specified by the compressor makers. On "professional" loom maker did his own thing (thought he knew better than the makers) and the customers had lots of supposed battery problems that were only "cured" when the undersized cable problems were hidden by messing with the control box cut out voltage.

 

I think that is all there is to it and all was said.

 

 

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