for electrical experts

[lindsay]

I have just bought one of those 12 volt heating elements, fitted with a cigarette type plug, that will neatly heat up a mug of whatever liquid. This is to replace a Boy Scout type spirit heater that I use when I run out of Calor gas, which always seems to happen at weekends or at anchorage.

On taking my new toy out of the package I see that it is marked 12 volts, 120 Watts. Does this mean that it will be passing a current of 10 amps? From memory my 12 volt system is fused well below that, 3 or 5 I think. So, if I use it will it blow the system?

Thanks

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[malcp]

10amps is right. I'm afraid it will blow the fuse.

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[Evadne]

The short answer is yes. If you've got a meter, put it across the pins and you should see 1.2 ohms. It's surprising how small a wire will pass 10 Amps, it's what you have in your house after all. If all your electrics are <5A circuits, with appropriate wire, I'd consider fitting a dedicated 13A line to your (>10A rated) socket using twin core and an in-line 15A fuse, then you can plug in anything you like.

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[Robin]

Isn't it strange how gas always runs out when you are using it and not whilst you aren't on board! /forums/images/icons/smile.gif

12v x 10A = 120W so you will blow any fuse less than 10A. Also if your boat is small enough not to want to carry a 2nd gas cylinder your battery may also be small and will run flat if you make too many hot drinks. (no details in your profile)

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[andyball]

10 amps or more I'm afraid.

Even if your wiring is up to it & the fuse size increased, cheap cigar type sockets sometimes overheat very badly with this sort of current, so check carefully the 1st time you use it- the centre contact of the plug can get very hot -enough to burn your finger & melt the plastic.

If increasing the fuse size....you need to allow for 10A continuous....a 15A blow fuse will get quite warm at 10-12A

 

[Gunfleet]

This is why I fitted a paraffin stove.... so i could run out of paraffin instead!

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[dralex]

The other option to consider is that if you are only going to use the heater every now and again, put some crocodile clips on the end and attach directly to the battery, that way you don't put too have to worry about too much current through existing circuits.- I have a 12V soldering iron and this works like that. Also means you don't have to do any extra wiring or fusing.

<hr width=100% size=1>Just enjoy it.

 

[Wiggo]

The 'cold' resistance will be almost zero for any heating element or bulb filament. Once current starts to flow, the 'dead short' element/filament rapidly heats up, and its resistance rises until it reaches an equilibrium, in this case at 1.2 Ohms. If you do the same calc for a 240V 100W bulb, yuo find it should have a resistance of 576 Ohms, but again, cold it will be almost zero.

As others have said, a cheapo ciggy lighter socket, even fused correctly, may get hotter than your heating element ;-)

Bearing in mind that a domestic kettle is around 2kW, you may find it faster to rub the mug vigorously with your hands than use a 120W heater...

<hr width=100% size=1>Nihil curo de ista tua stulta superstitione.

 

[bedouin]

But if it is a low wattage element being used only to heat water, then we can assume that it will never be operating at much over 100 Celsius. In those circumstances the resistance of a copper element would be about 30-50% higher than at room temperature. So the "cold" resistance is unlikely to be less than 0.8-0.9 Ohms.

In practice of course they may rate their element (at 120W) in the cold state - in which case the power will decrease as the water heats up.

These comments of course make the assumption that for elements of that size the element does not get much hotter than the fluid in which it is immersed...

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[Wiggo]

Good point, but the conductor will be electrically, and therefore partially thermally, insulated from the metal container, and hence will be somewhat hotter than 100C. It has to be, in any event, to allow for the thermal conductivity of water.

Also bear in mind that the element itself will not be pure copper, but a nickel/chromium alloy, with very different thermal resistance characteristics to copper.

Interesting, but still slower than rubbing two boy scouts together in this instance, I suspect...

<hr width=100% size=1>Nihil curo de ista tua stulta superstitione.

 

[mainshiptom]

can you not have a spare gas bottle?

I have and it never runs out !

T

ps I do have an electric kettle but only use in marina oin shore power !

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[Trevethan]

He's talking about 12 volt not 240! and re: twin and earth -- I have always been told its no good for boats -- the solid core wire doesn't tolerate the condistions -- so you get breaks or resistance buildups which equals heat.

In the U.S. they tried to use aluminium for house wiring anbut the surface corrosion was so great in some cases the heat started fires.




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[Evadne]

I think you'll find 13A refers to the current carrying capability, not the construction. 13A extension wire is stranded, and you don't have to look far to find umpteen recommendations on the preference of using tinned stranded. Solid core wire is ok in theory, if there is no movement or vibration, and you could make a case that it is better than stranded untinned, which will corrode to a nice piece of insulated copper oxide powder if you get it wet, but I wouldn't use it or recommend it.

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[BobOwen]

Just a minor point, Aluminium T+E was used throughout the UK too, around 68 - 70 when world copper prices rocketed. I pulled many a mile of the stuff as a young apprentice. Often wondered how (if) it lasted.

<hr width=100% size=1>Beer - its not just for Breakfast you know.

 

[lindsay]

Thanks guys ( sorry, people!) I am really impressed by your technical wizardry. So much so I have decided to give it away to one of my kids who has a car. Back to the spirit stove in an emergency since at 28 1/2 feet I do not have space for a second gas canister.

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[Robin]

There is another way, though IMO on a 28.5ft boat there should be space for 2nd gas cylinder even if it is a smaller one/different type? However if you carry a small spring balance you can check weigh the cylinder and see how much is left, then exchange it before it runs out. The tare weight (empty cylinder) is stamped on the cylinder. You can buy one from a Calor store or maybe a fishing tackle shop.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>