Installation of a 12v circuit

[ChrisPy]

The implication is that a 1.2v drop is OK - IMHO it isn't. You should aim for a maximum of not more than 0.4v drop.

on the other hand if your battery is giving out, say, 12.8V under load then a small voltage drop can actually protect some bits of kit from overload.

[Guest]

on the other hand if your battery is giving out, say, 12.8V under load then a small voltage drop can actually protect some bits of kit from overload.

Or above 14volts with modern alternators and battery boilers, when under way. Sometimes it is advantageous to lose 1.5 volts or so as you say.

[chris w]

Or above 14volts with modern alternators and battery boilers, when under way. Sometimes it is advantageous to lose 1.5 volts or so as you say.

 

 

Well first of all if a piece of kit cannot run on the output from an alternator then it shouldn't be installed in a "12v" environment anyway. Secondly, even if your statement were correct, you will still have the same high voltage drop when the engine is switched off and the batteries settle down to their normal level so your sensitive piece of kit may not function correctly at all.

 

Fridges and TVs will usually drop out once you get down to about 11v or so. Remember this is the voltage of the battery under load (before someone points out that batteries should not be allowed to drop below 12.2v or ~50% charge state).

 

Chris

[Guest]

Well first of all if a piece of kit cannot run on the output from an alternator then it shouldn't be installed in a "12v" environment anyway. Secondly, even if your statement were correct, you will still have the same high voltage drop when the engine is switched off and the batteries settle down to their normal level so your sensitive piece of kit may not function correctly at all.

 

Fridges and TVs will usually drop out once you get down to about 11v or so. Remember this is the voltage of the battery under load (before someone points out that batteries should not be allowed to drop below 12.2v or ~50% charge state).

 

Chris

Knew youd ccome back with that. I agree 100% or more if possible, sensitive kit such as electronics wouldn't benefit from dropping the voltage in such a way, quite the reverse in fact. You need proper voltage limiting devices to keep to 12V, I have one such device for my LCD 12V TV. But if you cast your mind back a bit, we were talking about rather crude tunnel lamps as per Tony's site. Lamps, some of which especially halogen, dont like 14volts+ up 'em. My statement is correct, and many boats have overcome problems with blowing lamps (not bulbs please) in this manner. A rather old trick. Battery boilers have been known to cause problems with halogen lamps blowing prematurely, fact.

[chris w]

Knew youd ccome back with that. I agree 100% or more if possible, sensitive kit such as electronics wouldn't benefit from dropping the voltage in such a way, quite the reverse in fact. You need proper voltage limiting devices to keep to 12V, I have one such device for my LCD 12V TV. But if you cast your mind back a bit, we were talking about rather crude tunnel lamps as per Tony's site. Lamps, some of which especially halogen, dont like 14volts+ up 'em. My statement is correct, and many boats have overcome problems with blowing lamps (not bulbs please) in this manner. A rather old trick. Battery boilers have been known to cause problems with halogen lamps blowing prematurely, fact.

 

 

Well I have a halogen tunnel light and it's been on 14.8v many times through the likes of Blisworth Tunnel et al and never a dull moment!

 

You shouldn't need voltage regulators for equipment designed to operate in a 12v environment as I said before. Even an alternator regulator nowadays may be as high as 14.2 or 14.4v. I haven't noticed my "12v" halogen car bulbs blowing lately.

 

Chris

Edited September 5, 2007 by chris w

[Guest]

Well I have a halogen tunnel light and it's been on 14.8v many times through the likes of Blisworth Tunnel et al and never a dull moment!

 

You shouldn't need voltage regulators for equipment designed to operate in a 12v environment as I said before. Even an alternator regulator nowadays may be as high as 14.2 or 14.4v. I haven't noticed my "12v" halogen car bulbs blowing lately.

 

Chris

Have the last word Chris, Iwill be the bigger man.

Edited September 5, 2007 by Guest

[John Orentas]

My statement is correct, and many boats have overcome problems with blowing lamps (not bulbs please) in this manner. A rather old trick. Battery boilers have been known to cause problems with halogen lamps blowing prematurely, fact.

 

 

Hi Catty.

 

Lamps especially tungsten halogen types can be highly voltage sensitive, it goes without saying that automotive types are very tolerant of voltage as their designed natural state is pretty similar to what we use them for. However some of the display lighting which is available often with diachronic reflectors are very intolerant, being manufactured for use with mains transformers which output very close to an accurate 12 volts. That is why I have from time to time advised that some types should be under-wired a little to give a small volt drop.

[smileypete]

However some of the display lighting which is available often with diachronic reflectors are very intolerant, being manufactured for use with mains transformers which output very close to an accurate 12 volts.

 

Well, use a tunnel light instead then

 

cheers,

Pete.

[Guest]

Well, use a tunnel light instead then

 

cheers,

Pete.

Tried that, they looked OK over the bed, but not so good in the galley I'm afraid. I'm going back to oil lamps personally; they were good in our caravan when we were kids.

[chris w]

Hi Catty.

 

Lamps especially tungsten halogen types can be highly voltage sensitive, it goes without saying that automotive types are very tolerant of voltage as their designed natural state is pretty similar to what we use them for. However some of the display lighting which is available often with diachronic reflectors are very intolerant, being manufactured for use with mains transformers which output very close to an accurate 12 volts. That is why I have from time to time advised that some types should be under-wired a little to give a small volt drop.

 

Just to be pedantic, the word you meant was "dichroic".

 

The transformers used with low voltage halogen lamps output 13v AC rms which is over 18v peak!!!!

 

Chris

[ChrisPy]

oil lamps sound good, but most boats have a forward gas locker. so the possibility of gas lamps (or even candleabra at a stretch) comes to mind.

[Tortoise]

I want to buy some "fat cable" - 8 or 10mm diameter so I can extend the 12v electrics down one side of the boat. I need about 12m (black & red) but can't find anywhere that sells it in less than 30m or 100m reels.

 

Can anyone recommend somewhere I can buy it in this sort of thickness - pref. online (but that's not essential if there's a shop willing to post it out to me.)

 

Thanks

 

Lisa

 

CPC do 25m reels of 10mm for around £20+VAT. Their website is a nightmare, but order codes are CB09199 for black and CB09201 for red; put those numbers into their search box. I'm not 100% sure about ordering if you don't have an account, but you could do worse than give them a ring. Shame you're not more local to me, but get in touch if you get nowhere with them.

[Tortoise]

CPC do 25m reels of 10mm for around £20+VAT. Their website is a nightmare, but order codes are CB09199 for black and CB09201 for red; put those numbers into their search box. I'm not 100% sure about ordering if you don't have an account, but you could do worse than give them a ring. Shame you're not more local to me, but get in touch if you get nowhere with them.

 

oops - two errors I'm glad no-one else shot me down for - you said 'diameter' and I quoted cable that was 10mm2 (the second error being not correctly quoting my units) - i.e. cross sectional area of ten square millimetres. Should be fine for the cable drops, but do the maths before trusting what anyone says, especially me... ;-)

[chris w]

10mm² cross section cable will cause a voltage drop of 1.7mV per metre per amp. Don't forget that the metre length is OUT and RETURN, ie: TOTAL length.

 

As an example, if you run 15 metres each way (ie: 30 metres total) and carry 20 amps, the total voltage drop will be:

 

1.7x 10^-3 x 30 x 20 = 1 volt This would be too great as you want to aim for a maximum voltage drop of 0.4v maximum.

 

So for 20 amps the maximum length that you could run with 10mm² cable (for a 0.4v voltage drop) would be 12 metres total (ie: 6 metres each way)

 

Alternatively, if you need to run 30 metres total, then the maximum current that 10mm² cable can carry for a 0.4v voltage drop is 8 amps.

 

Chris

[Gibbo]

10mm² cross section cable will cause a voltage drop of 1.7mV per metre per amp. Don't forget that the metre length is OUT and RETURN, ie: TOTAL length.

 

1.8mV

 

 

Gibbo

[Guest]

at what ambient temp?

[Gibbo]

at what ambient temp?

 

Ha ha. 25 deg C for some reason.

 

Gibbo

[carlt]

(or even candleabra at a stretch)

That's some serious headroom in your boat, Chris!

[grandunion5]

Hi all

 

What size cable would you recommend for a fridge/freezer that is sitting roughly 35ft away from the batteries? Besides, looking at the connectors on the fridge, it would only accept small (possibly 6mm2) female blade connectors. If a larger cable is required, how do you plug it to the fridge? The advertised power consumption is 2.5/6.0 A

 

Thanks

Marc

[ChrisPy]

Hi all

 

What size cable would you recommend for a fridge/freezer that is sitting roughly 35ft away from the batteries? Besides, looking at the connectors on the fridge, it would only accept small (possibly 6mm2) female blade connectors. If a larger cable is required, how do you plug it to the fridge? The advertised power consumption is 2.5/6.0 A

 

Thanks

Marc

the 6mm yellow spades are fine for 6A.

 

you'll need a heavier cable to reduce voltage drop, so you'll have to step down the cable size. of course you could just trim off half the strands and fit a yellow connector.

[justintime]

the 6mm yellow spades are fine for 6A.you'll need a heavier cable to reduce voltage drop, so you'll have to step down the cable size. of course you could just trim off half the strands and fit a yellow connector.

The following is a guide to automotive cable :

Number & Diameter of strands (mm)

Nominal CSA of core(mm)sq

:

Resistance/Metre at 20deg C

9/0.30

0.65mm

0.0294 ohms

 

14/0.30

1.00mm

0.0189 ohms

21/0.30

1.50mm

0.0125 ohms

28/0.30

2.00mm

0.0094 ohms

35/0.30

2.50mm

0.0075 ohms

44/0.30

3.00mm

0.006 ohms

56/0.30

4.00mm

0.00471ohms

65/0.30

4.50mm

0.0041 ohms

84/0.30

6.00mm

0.0031 ohms

97/0.30

7.00mm

0.0027 ohms

120/0.30

8.50mm

0.0022 ohms

 

Thinwall automotive cable

28/0.3

2.00mm

0.0094 ohms

32/0.20

1.00mm

0.0195 ohms

 

Starter cables**

37/0.71

15.00mm

0.0011 ohms

266/0.30

20.00mm

0.0010 ohms

336/0.30

25.00mm

0.0008 ohms

37/0.9

25.00mm

0.0008 ohms

61/0.9

40.00mm

0.0005 ohms

61/1.13

60.00mm

0.0003 ohms

Highflex cable

**

195/0.50

40.00mm

0.0005 ohms

Calculation for expected voltage drop on a cable run is calculated by:

Current Flowing x Total Cable Resistance = Volt Drop

The following is a guide to automotive cableNumber & Diameter of strands **** Nominal CSA of core(mm)sq *Resistance/Metre at 20deg C(mm)9/0.30 ************************* 0.65mm***************** 0.0294 ohms14/0.30*************************1.00mm *****************0.0189 ohms21/0.30*************************1.50mm *****************0.0125 ohms28/0.30*************************2.00mm *****************0.0094 ohms35/0.30*************************2.50mm *****************0.0075 ohms44/0.30*************************3.00mm *****************0.006 ohms56/0.30*************************4.00mm *****************0.00471ohms65/0.30*************************4.50mm *****************0.0041 ohms84/0.30*************************6.00mm *****************0.0031 ohms97/0.30*************************7.00mm *****************0.0027 ohms120/0.30 ***********************8.50mm *****************0.0022 ohmsThinwall automotive cable28/0.3 *************************2.00mm******************0.0094 ohms32/0.20 ************************1.00mm******************0.0195 ohmsStarter cables37/0.71 ************************15.00mm *****************0.0011 ohms266/0.30*********************** 20.00mm *****************0.0010 ohms336/0.30*********************** 25.00mm *****************0.0008 ohms37/0.9 *************************25.00mm *****************0.0008 ohms61/0.9************************* 40.00mm *****************0.0005 ohms61/1.13************************ 60.00mm *****************0.0003 ohmsHighflex cable195/0.50 ***********************40.00mm*****************0.0005 ohmsCalculation for expected voltage drop on a cable run is calculated by: Current Flowing x Total Cable Resistance = Volt Drop

not sure if the text formatting came out quite right on my post above as "20 deg C" should be next to "cable resistance at" Edited October 7, 2007 by justintime