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Cable sizing


jddevel

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I`ve a Vetus macerator toilet in my sailaway which according to the technical detail is rated at 25 amps. However it also refers to a cable size of 6mm. Being 8 metres from the distribution panel on my 12 volt circuit this seems rather small or is my understanding still got a gaping hole. By my calculations 16mm would be correct. Help please.

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The size of cable required can be calculated with the following simple formula:
cablesize in mm2 = 18*metres*amps/(V*1000
Where metres = entire cable length there and back and V = Maximum permitted voltage drop. 

So, plugging in your numbers, and assuming 1V maximum voltage drop, we get

18*16*25/1000 = 7200/1000 = 7.2

Round up the 7.2 to 8.5mm2 (readily available size). 

Tony

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8 hours ago, Jen-in-Wellies said:

When you say 6mm, do you actually mean 6mm2 cross sectional area

Quoting from the Vetus literature and again a failure on my part to fully describe. Should have written "use wires with a minimum cross section of 6mm2 (for 12v)"

 

8 hours ago, WotEver said:

If you only wanted 0.5V drop then you'd need 16mm2.

Seeing your earlier comment is the general consensus that a one volt drop is O.K bearing in mind that a macerator toilet means the toilet would not be running continuously. Having said that the conduit laid in is 25mm diameter so large enough to accommodate two by 16mm2 and the difference in cost I would find acceptable. Fit once and fit the best.

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3 hours ago, jddevel said:

Having said that the conduit laid in is 25mm diameter so large enough to accommodate two by 16mm2 and the difference in cost I would find acceptable. Fit once and fit the best.

Absolutely :)  So 16mm2 it is. 

11 hours ago, Jen-in-Wellies said:

Don't seem to be able to do a superscript 2.

[ sup ] 2 [ /sup ] without the spaces :)

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14 hours ago, jddevel said:

I`ve a Vetus macerator toilet in my sailaway which according to the technical detail is rated at 25 amps. However it also refers to a cable size of 6mm. Being 8 metres from the distribution panel on my 12 volt circuit this seems rather small or is my understanding still got a gaping hole. By my calculations 16mm would be correct. Help please.

They should specify a max distance for the 6mm in the instructions, if not why not call Vetus and get their take on it, would be interesting to know....

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1 hour ago, smileypete said:

They should specify a max distance for the 6mm in the instructions, if not why not call Vetus and get their take on it, would be interesting to know....

The calcs may also be on a fully charged battery, so going over the recommend spec is good.

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3 hours ago, Robbo said:

The calcs may also be on a fully charged battery, so going over the recommend spec is good.

Would have thought a premium mfr should know their product well enough. At the end of the day it comes down to cost/benefit, not much point spending ££££ for little or no benefit.

A good wiring design can save ££££ and make changes easier, but it sounds like the OP is committed with the existing premade loom.

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29 minutes ago, smileypete said:

Would have thought a premium mfr should know their product well enough. At the end of the day it comes down to cost/benefit, not much point spending ££££ for little or no benefit.

A good wiring design can save ££££ and make changes easier, but it sounds like the OP is committed with the existing premade loom.

Aye, but they would have specced it from a good charged battery.  Now hands up who has one of them.  

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2 hours ago, Robbo said:

Aye, but they would have specced it from a good charged battery.  Now hands up who has one of them.  

I am sure Vetus are aware of the phenomenom of changing battery voltage.

I thought everyone on here religiously recharged their batt as soon as the S****gauge read 49.99999% ? :)

Edited by smileypete
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20 minutes ago, smileypete said:

I am sure Vetus are aware of the phenomenom of changing battery voltage.

I thought everyone on here religiously recharged their batt as soon as the S****gauge read 49.99999% ? :)

No. Only mtb

  • Greenie 1
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LoomTec designed the toilet cable for a Dometic Vacuflush. 7.5amps. Decided on a macerator hence the required upgrade of cabling. The toilet has tails around 600mm long to which I will now add the two cables back to the fuse panel/distribution panel.

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1 minute ago, BoatyMacBoatface said:

There's an online cable size calculator at https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html which gives the correct cable size as 16mm2

For what voltage drop? Cable size without specifying the voltage drop is pretty meaningless.

Out of interest, what figures did you enter into that calculator?  I used NYY-J cable,  0.3kW, 12V, 16m, and it results in 'No suitable cable found". 

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Just now, WotEver said:

For what voltage drop? Cable size without specifying the voltage drop is pretty meaningless.

Out of interest, what figures did you enter into that calculator?  I used NYY-J cable,  0.3kW, 12V, 16m, and it results in 'No suitable cable found". 

The voltage drop is 4.7% or 0.56v. The IEE regs specify maximum voltage drop from the source. I forget the exact value as I haven't been a qualified electrician this century :)

I'd have thought 16mm PVC insulated singles were adequate as the cable is to be run in 25mm conduit. The distance from the distribution panel is given by the OP as 8m. 

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16mm is indeed fine, as established earlier in the thread it will give <0.5V drop. 8 metres is the one-way distance, so 16m for the calculations as in post #2. 

What figures did you enter into that calculator that you linked? I can't find any that work. 

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13 hours ago, BoatyMacBoatface said:

The voltage drop is 4.7% or 0.56v. The IEE regs specify maximum voltage drop from the source. I forget the exact value as I haven't been a qualified electrician this century :)

I'd have thought 16mm PVC insulated singles were adequate as the cable is to be run in 25mm conduit. The distance from the distribution panel is given by the OP as 8m. 

what on earth have the IEE wiring regs got to do with a 12v boat system?

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 A useful volt-drop calculator here (about 2/3rd of the way down the page)

http://www.12voltplanet.co.uk/cable-sizing-selection.html

 

Using 12 volts, 24 amps (300w @ 12.5v) and 8 metres (16m total) 16mm2 cable will give a VD of 0.41v / 3.42%

 

They suggest that 3-4% is the 'norm' but their calculator allows you to change the % drop by changing the cable size so you can achieve whatever you want.

eg - 25mm2 would result in 0.26v / 2.17%

Edited by Alan de Enfield
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Yup, the above figures confirm what I wrote in posts #2 and #4. 

31 minutes ago, Murflynn said:

what on earth have the IEE wiring regs got to do with a 12v boat system?

Bugger all, but it was an interesting aside, I thought. 

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17 minutes ago, Alan de Enfield said:

A useful volt-drop calculator here (about 2/3rd of the way down the page)

I'd disagree with their assertion that "the generally acceptable voltage drop for DC circuits is around 3%" It entirely depends on the circuit. Even a 10% drop would be acceptable for some little used circuits such as a horn, and 1% would be the maximum I'd want to see on a charging circuit. 

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