jddevel Posted May 23, 2017 Report Share Posted May 23, 2017 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. Link to comment Share on other sites More sharing options...
WotEver Posted May 23, 2017 Report Share Posted May 23, 2017 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 Link to comment Share on other sites More sharing options...
Jen-in-Wellies Posted May 23, 2017 Report Share Posted May 23, 2017 (edited) When you say 6mm, do you actually mean 6mm2 cross sectional area? Don't seem to be able to do a superscript 2. Edited May 23, 2017 by Jen-in-Wellies Link to comment Share on other sites More sharing options...
WotEver Posted May 23, 2017 Report Share Posted May 23, 2017 If you only wanted 0.5V drop then you'd need 16mm2. Link to comment Share on other sites More sharing options...
jddevel Posted May 24, 2017 Author Report Share Posted May 24, 2017 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. Link to comment Share on other sites More sharing options...
Robbo Posted May 24, 2017 Report Share Posted May 24, 2017 If you already have 6mm, there is nothing wrong with running two runs as long as it's fused correctly for an individual cable. Link to comment Share on other sites More sharing options...
WotEver Posted May 24, 2017 Report Share Posted May 24, 2017 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 Link to comment Share on other sites More sharing options...
mross Posted May 24, 2017 Report Share Posted May 24, 2017 (edited) 45mm2 OK, it works! but you won't see the result until you post - it doesn't show whilst editing. Edited May 24, 2017 by mross Link to comment Share on other sites More sharing options...
smileypete Posted May 24, 2017 Report Share Posted May 24, 2017 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.... Link to comment Share on other sites More sharing options...
Robbo Posted May 24, 2017 Report Share Posted May 24, 2017 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. Link to comment Share on other sites More sharing options...
mross Posted May 24, 2017 Report Share Posted May 24, 2017 It only takes 25A when it's doing a hard job - if you get my drift. Link to comment Share on other sites More sharing options...
smileypete Posted May 24, 2017 Report Share Posted May 24, 2017 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. Link to comment Share on other sites More sharing options...
Robbo Posted May 24, 2017 Report Share Posted May 24, 2017 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. Link to comment Share on other sites More sharing options...
smileypete Posted May 24, 2017 Report Share Posted May 24, 2017 (edited) 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 May 24, 2017 by smileypete Link to comment Share on other sites More sharing options...
rusty69 Posted May 24, 2017 Report Share Posted May 24, 2017 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 1 Link to comment Share on other sites More sharing options...
jddevel Posted May 24, 2017 Author Report Share Posted May 24, 2017 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. Link to comment Share on other sites More sharing options...
BoatyMacBoatface Posted May 29, 2017 Report Share Posted May 29, 2017 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. Out of interest, what size are the tails? Link to comment Share on other sites More sharing options...
WotEver Posted May 29, 2017 Report Share Posted May 29, 2017 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". Link to comment Share on other sites More sharing options...
BoatyMacBoatface Posted May 29, 2017 Report Share Posted May 29, 2017 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. Link to comment Share on other sites More sharing options...
WotEver Posted May 29, 2017 Report Share Posted May 29, 2017 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. Link to comment Share on other sites More sharing options...
cuthound Posted May 30, 2017 Report Share Posted May 30, 2017 17th edition of BS7671, IET Wiring Regulations, states a maximum of 3% from source to final distribution if I recall correctly. Link to comment Share on other sites More sharing options...
Murflynn Posted May 30, 2017 Report Share Posted May 30, 2017 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? Link to comment Share on other sites More sharing options...
Alan de Enfield Posted May 30, 2017 Report Share Posted May 30, 2017 (edited) 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 May 30, 2017 by Alan de Enfield Link to comment Share on other sites More sharing options...
WotEver Posted May 30, 2017 Report Share Posted May 30, 2017 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. Link to comment Share on other sites More sharing options...
WotEver Posted May 30, 2017 Report Share Posted May 30, 2017 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. Link to comment Share on other sites More sharing options...
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