phase 2 starting - power....

[Jamesb4uk]

ok,

 

So now its time for phase 2 - re-wiring.

One thing i dont want to do is use the wrong cable - as there is already evidance of burnt wiring in the boat from previous wiring faults.

 

I've done a lot of reading & to be honest its confusing me slightly.

 

I'm trying to work out the cable sizes needed for various runs, for lights, pumps *3, fridge, tv, power outlets.

 

take the fridge for example - say its on average using 40 watts... so 40w/12v = 3.34 amps.

 

Taking it along a 6m run requires 10mm2 with 8 AWG

 

However - this clicky reads that a .8mm2 cable with AWG 18 can take max 20 amps.

 

 

A slightly confused James

 

James

[WotEver]

Oohhh... where to start?

 

There are always 2 considerations with cable - the maximum current that it can safely carry bearing in mind its method of installation (free air, clipped to a bulkhead, bundled with other cables etc), and the voltage drop along that cable when carrying the specified current for that circuit.

 

I'll take the fridge first, because you mentioned it and because it's a special case. When starting up, the compressor on a fridge can draw many amps (known as the inrush current). A large current drawn down a small cable will result in significant voltage drop, causing the fridge to cut out with a "Low Voltage" error. This is why fridge manufacturers recommend such fat cables for connecting their fridges. From this pdf you'll see that for a 6 metre distance Shoreline recommend a minimum of 6mm².

 

For other circuits voltage drop may be less significant - an LED lamp running at 12.6V will look no different to an LED lamp running at 11.6V - but if you wire your boat with absolute minimum cable sizes throughout then you are allowing no capacity for future expansion and addition to those circuits.

 

Does that help?

 

Tony

 

Take a look at this page for a more in-depth explanation.

[Jamesb4uk]

Thanks for the reply,

 

The bulk of the wiring will be in a conduite in the ceiling with 1 or 2 run along the gunwales.

 

I understand the theory & initial power up draw which is why im looking at the bigger end of the cable (10mm2).

 

I'm just confused as to the rating of the thinner cable it seems to meet the requirements quite happily appart from cable width..

 

James

[Eeyore]

ok,

 

So now its time for phase 2 - re-wiring.

One thing i dont want to do is use the wrong cable - as there is already evidance of burnt wiring in the boat from previous wiring faults.

 

I've done a lot of reading & to be honest its confusing me slightly.

 

I'm trying to work out the cable sizes needed for various runs, for lights, pumps *3, fridge, tv, power outlets.

 

take the fridge for example - say its on average using 40 watts... so 40w/12v = 3.34 amps.

 

Taking it along a 6m run requires 10mm2 with 8 AWG

 

However - this clicky reads that a .8mm2 cable with AWG 18 can take max 20 amps.

 

 

A slightly confused James

 

James

hello James

Firstly it would be easier to settle on one description of the cable, I would suggest that you stick to mm2 - the cross sectional area of the copper in the cable. AWG and at least two other 2 part systems are common; cables marked with more than one description will be the exact size for one and slightly over size for the others.

A long time ago it was common for voltage to be refered to as "pressure of electricity".

In your link the cable in question is 20 amps at 600 volts (pressure), so looking at it in a simplistic way we can see that 12 volts (pressure) is not going to push as much down the cable.

The reality is a little more complex; but voltage drop tables are available for 12 volt instalation which make the job easier. (anyone got a link handy?).

Personaly I like to think of it as effectively moving the battery post to the appliance, but I know that description doesn't suit everyone.

A max volt drop of 3% per circuit is a good target figure.

Hope that helps

Steve

[WotEver]

Hi again James,

 

I understand the theory & initial power up draw which is why im looking at the bigger end of the cable (10mm2).

Actually >12mm² if the fridge is 6m away from the battery - you have to take the 'round trip' distance, which is 12m.

 

I'm just confused as to the rating of the thinner cable it seems to meet the requirements quite happily appart from cable width..

That's simply because the rating is the rating of the cable, it has no relationship to voltage drop which, for tiny cable like that, would be significant.

 

Take a look at Method 1 Example in Tony's notes that I linked above to see what I mean.

 

Tony

Edited April 24, 2012 by WotEver

[WotEver]

Let's try to make this clearer with a real world example.

 

The cable linked to in the OP stated that it's rated at 20A.

 

We'll use the formula for calculating voltage drop, which is (0.0164 x A x distance in metres)/cable size.

 

Let's say we're feeding something which draws 20A which is 12 metres from the batteries.

 

That gives us a voltage drop of (0.0164 x 20 x 24)/0.8 = 7.872/0.8 = 9.84V.

 

So the cable is perfectly capable of carrying 20A but if you were to use it in this example you'd be dropping 9.84V down it.

 

Tony

[Jamesb4uk]

ahhh ok i think im understaning things now after reading your link Tony.

 

so basically ignore the rating & stick to the size to minimise voltage drop.

 

so another example (just so i confirm understanding!)- Nav lights : 18w/12v = 1.5amps 12m(ish)from battery.

 

round trip 24m so 4mm2? thats with a 3% acceptable loss in Steve's post.

 

James

 

ps - any recommended places to buy 4/6/10mm2 cables?

[WotEver]

Gee, I wrote a long reply and then Windoze deleted it as I posted... oh well, here goes again...

 

Vdrop = (0.0164 x A x distance in metres)/cable_size can be re-written as:

Cable_size = (0.0164 x A x distance in metres)/Vdrop

 

3% of 12.6V is 0.38V

 

So with your example of 1.5amps 12m(ish)from battery we have:

(0.0164 x 1.5 x 24)/0.38 = 0.59/0.38 = just over 1.5mm². So use 3mm².

 

Tony

 

ps - any recommended places to buy 4/6/10mm2 cables?

Vehicle Wiring Products generally gets mentioned in a favourable light, but there may well be better/cheaper out there.

 

Tony

[Tony Brooks]

Hi again James,

 

 

Actually >12mm² if the fridge is 6m away from the battery - you have to take the 'round trip' distance, which is 12m.

 

 

That's simply because the rating is the rating of the cable, it has no relationship to voltage drop which, for tiny cable like that, would be significant.

 

Take a look at Method 1 Example in Tony's notes that I linked above to see what I mean.

 

Tony

 

 

Sorry Tony,

 

The Danfoss/Shoreline statement is that you use 1 sq mm of conductor CSA for each metre between battery & fridge and then use the same for the return. In reality they mean 0.5sq mm for each metre of total cable run but probably think the fitters could not calculate using fractions The correct cable (according to the companies) is 6 sq mm.

[WotEver]

Sorry Tony,

 

The Danfoss/Shoreline statement is that you use 1 sq mm of conductor CSA for each metre between battery & fridge and then use the same for the return. In reality they mean 0.5sq mm for each metre of total cable run but probably think the fitters could not calculate using fractions The correct cable (according to the companies) is 6 sq mm.

Well then, they should be clearer

 

Thanks for the clarification, Tony

[Arthur Brown]

Remember also that cable installed now may not be the last you will need, and the installation will need periodic inspection (-does it get BSS'd closely? ) SO please leave access for inspection and system expansion without needing to rip panelling out later.

[Chris Pink]

The cheapest way to buy cable is at an electrical wholesaler in 100m reels. It is a lot cheaper. Really, a lot.

 

The practical implication of this is that it is sensible to compromise on the gauges.

 

To do this you assess which circuits you don't mind a bit of voltage drop and those that you do.

 

another way of dealing with this is to lay 16mm2 cables all the way down the boat and take fused spurs to the equipment.

 

As I have just looked at 16mm tri-rated cable and it's now £200 a reel (it was under £50 the last time I bought some) it becomes a real cost issue how to get high current down your boat.

Edited April 24, 2012 by Chris Pink

[Jamesb4uk]

thanks for all the help.

 

I've elected to try Loomtech & see what they can price up.

 

as much as i want to do as much of my boat as i can by myself i'd rather be safe than sorry!

 

James