12v Power Box

[Yearofthegoat]

Hi All. After looking for, and failing to find, a 12v splitter that can do more than 10A, I've decided to build my own.

But, I have a couple of questions about fusing things.

The box has 3 marine-type locking cigar sockets. The internals are wired with 15A rated components (am thinking max load of 5A on 3 sockets = max 15A). The box will be connected to either an XM21X DC supply when at home, or an old car battery when out and about.

Each cigar plug will have a fuse in it suited to the appliance (I note my HEQ5 Pro cigar plug, as supplied, has a 5A fuse - off topic but is that right?).

The cable from the supply to the splitter is orange mower flex, rated at 13A.

The dilemma I have is what rating of fuse to put at the supply end of the cable - 13A would match the cable rating and the box, but is too high for what the XM21X can deliver. So do I make a second cable fused at say 7A for the XM21X?

Ultimately I might make a battery box to hold the battery and the 3 sockets (plus...) but not yet.

Thanks!

 

[roblegion]

Hi, if each plug has a suitably rated fuse, fusing the orange cable is probably an overkill, if you get anywhere near drawing enough current to damage that cable, everything else is gonna crispy as you'd be pulling over 200watts!

The power supply itself claims to have a 1.0 amp internal and a 1.5 amp external fuse.

[Yearofthegoat]

Thanks - yes I thought that as well but was considering a battery fault which results in it putting out big ampage. Maybe I should just put a 13A one on for battery use.

Presumably if I overload the XM21X (e.g. by asking to supply >9A for a while) it'll blow one or both of its fuses, or just not work?

[roblegion]

There are guys on the site doing electronics a lot longer than me, but if you are worried about the interconnecting cable, I wouldn't go over 75% of its rating.

If you recon this cable is rated for 13A then I would go with a 10a MAX.

[Physopto]

Inside your box the fuse rating you state is 15 amps. therefore at max 14,4 volts and full power you will get VA or 15 x 14.4 = approx. 225 watts. You will loose a bit due to the Orange cable resistance from the power supply, usually less than about 1 volt. So you can re do the maths at 15 x 13.4 = about 200 watts at the power splitting end. Still you should add a bit for safety for the supply rating. So take 225 watts and add about half that takes you to around 330 watts. So your cable can be fused at 330/14,4=23 amps A 25 amp fuse will do nicely. Or to reduce any chances go slightly less at 20 amps for the orancge cable fuse.

But as roblegion said it really is unimportant as the cable can take much more. If it is 2,5mm csa it will be rated at 13 amps at 240 volts or slightly over 3 KW. In other words you will not be putting more than 1/10 of its rated capacity on the cable in worst case scenario.

Derek

[Yearofthegoat]

Thanks, that helps. The cable is probably not 2.5csa but it's not doorbell cable either (I'll have to double check its rating).

The box has its own fuse in its cigar plug so if I put a 15A in there I guess that should cover it?

[Waldemar]

As a rule of thumb for cable every 0,1 mm2 can handle 1 A  max.
You want to stay under the max limit, so fuses for 1,5 mm2 will be 10A and for 2,5 mm2  it will be 16A (standard fuses) Voltage does not matter in this.
Heat building is determined by Joule's law: Q=0,24.I².R.T in  which Q is the developed heat; I is the current (amps); R is the total resistance of the cable ; T is the time it is engaged. Voltage is no part of the equation. A raise in temperature changes the resistance of the cable, which will influence the voltage at the end of the cable. In cold circumstances more heat will be ceded to the environment of course, so the critical temperature will be reached less fast.

Be advised that cigar plugs are infamous for failures because of the bad connection they make. Far better and safer options are available.
I don't understand why they are still used on such a wide scale...

The above formula is the exact reason why those cigar plugs fail so easily: Because of the very small contact surface of the central connection, a lot of heat may be produced that will carbonate or corrode the small contact surface. More resistance will occur, hence more heat: connective failure will be the result.

Just my 2c

Waldemar
 

[101nut]

Note that the cables being used and quoted for here are mains (240Vac) rated and insulated accordingly.

If you look at 12V specialists (automotive or marine are the obvious ones) you will find much thinner cables used as the electrical insulation requirements are much lower hence physically thinner insulation with less thermal insulation thus the cables themselves are higher current rating - 1mm² at 16A for instance. Just as an example

If you really need to preserve voltage then keep a fat cable. If space, current and tidiness are the only issues then I'd use thinwall cable every time. I would suggest that if you voltage requirements are that tight I'd be worrying about electrically stability.

I totally agree about cigar lighters - cheap and nasty. XLR and the like are much better but you pay for that.  

AndyG

[Physopto]

It is not really a matter of how small the cable CSA you can get away with though is it?

It is more of what the cable is capable of supplying. If it can do the job without reducing the voltage due to resistance effects when carrying the maximum load the system can take. Small CSA is OK for very short cable runs, but useless for long runs where the voltage drop can cause problems. I use 4mm² for my cable from battery to junction boxes, ( 8metres). 2.5mm² for runs in the splitter boxes, usually less than 15 inches long and several to different items of equipment, each carrying lower currents than the main supply, (Dew Heater up to 10 amps uses 2.5mm²). If in the mA ranges 1.5mm² is fine.

It all depends upon the length of cable run and current carried.

Derek

[Yearofthegoat]

I looked at locking XLRs etc. but I took AstroBaby's point about needing the cable to disconnect rather than pull anything over in the event someone (like my 5 year old tasmanian devil aka my son) trips on the cable etc. Hence I went with the twist-locking marine cigar plugs - they are firm connections and won't work loose on their own, but you can still pull them out with a good tug.

I've checked the orange flex and it's 2-core 1mm² stranded, so 10A rated. This is a bit under what I planned for the splitter box so I'll get some 1.5 or 2mm² to allow for 15A overall.

At present I use 10m of the 1mm orange flex to run from the garage where the XM21X is plugged in, to the scope(s). So it looks like that will still be okay for my HEQ5 and the Virtuoso, but not much more if some overhead is included.

I need to double-check the current the two scopes will draw, as I'm still not convinced about the 5A fuse in the HEQ5 plug (should be lower?).

Thanks for all the pointers.

 

[101nut]

20 hours ago, Physopto said:

It is not really a matter of how small the cable CSA you can get away with though is it?

True, that's why I didn't say that.

However, even assuming there's a good reason to spec the conductor CSA according to its BS6500 rating, choosing appropriate cable insulation can significantly reduce the loom size and improve handling no end.

AndyG

[michael8554]

I've seen suggestions that the HEQ5 draws about 1.8Amps when slewing, so a 5Amp is too large, try 2.6Amps

Michael