kenso_ukyahoo.co.uk Posted November 18, 2009 Share Posted November 18, 2009 Could someone help me with the size of cable to use for installing the above. Would 0.75 2 core flex be O.K. Thanks. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted November 19, 2009 Share Posted November 19, 2009 My Omnistor 2006 catalogue indicates that an Omni-Vent can draw up to 5.0A. On that basis, as long as your cable length is shortish (say, up to 4 metres), you should be able to get away with a 1.0mm-square cross section, though 1.5mm-square cable might be preferable. Obviously, you'll need to ensure that sufficiently 'thick' cable is used all the way downstream from the Omni-Vent back to its power source. Link to comment Share on other sites More sharing options...
Brambles Posted November 19, 2009 Share Posted November 19, 2009 Derek Uzzell - 2009-11-19 8:53 AM My Omnistor 2006 catalogue indicates that an Omni-Vent can draw up to 5.0A. On that basis, as long as your cable length is shortish (say, up to 4 metres), you should be able to get away with a 1.0mm-square cross section, though 1.5mm-square cable might be preferable. Obviously, you'll need to ensure that sufficiently 'thick' cable is used all the way downstream from the Omni-Vent back to its power source. You are correct Derek, but the fuse rating should also be taken into account and fault conditions. To allow for surge currents for the motor a 10 Amp fuse might be appropriate, therefore the wiring shoud be able to take 10 Amps. So 1.5 mm is required. Now take into acount wiring might be through roof insulation and so temp rise cable insulation has will be a lot higher so again cable size needs to be larger. I will guess at 2.5mm for safety. Something many installers overlook, and builders get around using a lighter cable as they place in plastic conduit rather direct contact with roof insulation surrounding the actual cable. Watcha think, is what I am saying make sense? What am I saying...if cable is exposed then 1.5, but if fed through insulating material then 2.5. and use a 10 amp fuse. Jon. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted November 19, 2009 Share Posted November 19, 2009 2.5mm square cable and a 10A fuse should certainly cover any eventuality in this instance. Personally, I would fight shy of asking this sort of question on a forum - I'd get hold of the Omnistor installastion instructions for an Omni-Vent and follow them. Link to comment Share on other sites More sharing options...
kenso_ukyahoo.co.uk Posted November 19, 2009 Author Share Posted November 19, 2009 Thanks very much for the info. Link to comment Share on other sites More sharing options...
colin Posted November 19, 2009 Share Posted November 19, 2009 Lets get some perspective on this, 2.5mm.sg for an Omnivent! This vent takes surge loads up to 5amp, the correct conductor for this if in a loom would be 0.65mm.sg, 1.5mm.sg will give more than ample capacity insulation or not. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted November 20, 2009 Share Posted November 20, 2009 colin - 2009-11-19 7:56 PM This vent takes surge loads up to 5amp, the correct conductor for this if in a loom would be 0.65mm.sg... Surely this will depend on the length of the cable within the loom? John Wickersham's book "Build Your Own Motorcaravan" contains an "Electrical Systems" chapter that includes tables on cable maximum Amperage ratings and maximum permitted Amperage on the basis of cable length. Maximum permitted Amperages of 1.0mm² (14 strand) cable by length are given as 3.1A (12 metres), 4.7A (8 metres) and 9.4A (4 metres). For 1.5mm² (21 strand) cable the data provided are 4.7A (12m), 7.0A (8m) and 14.1A (4m). For motorhome applications, 1.0mm² cable is advised for interior lights and 1.5mm² cable for 'extractor fans'. Another table that gives typical fuse ratings suggests a 10A fuse as the norm for an Omni-Vent fan. If I didn't have Omni-Vent installation instructions, I'd use 1.5mm² cable provided that the cable-run length was not excessive. If I had Omni-Vent instructions, then I'd adhere to them. Link to comment Share on other sites More sharing options...
Brian Kirby Posted November 20, 2009 Share Posted November 20, 2009 Brambles - 2009-11-19 11:21 AM ............Now take into account wiring might be through roof insulation and so temp rise cable insulation has will be a lot higher so again cable size needs to be larger. I will guess at 2.5mm for safety. Something many installers overlook, and builders get around using a lighter cable as they place in plastic conduit rather direct contact with roof insulation surrounding the actual cable. ........... One point - although it probably has limited relevance to motorhomes due to their relatively shorter lives - is that it is generally bad practise to place PVC insulated wires in close proximity to expanded polyethylene (EPS) insulants. The EPS (which is a commonly used insulant in motorhomes) reacts with the the PVC, causing it to become brittle. More important with mains wiring (do NOT do this at home!) but still possibly a factor for 12V wiring installed early in the life of a motorhome.The consequence can be fire, as the degraded insulation will allow leakage currents sufficient to heat cables within the insulation, but insufficient to trip MCB's/blow fuses.A slightly larger hole, with a protective non-PVC sleeve for the cables, is the ideal. Link to comment Share on other sites More sharing options...
Brambles Posted November 20, 2009 Share Posted November 20, 2009 That a very interesting point brian, never thought about that one. material compatibility, something probably overlooked by many Motorhome builders as well. Link to comment Share on other sites More sharing options...
colin Posted November 20, 2009 Share Posted November 20, 2009 Derek Uzzell - 2009-11-20 8:19 AM Surely this will depend on the length of the cable within the loom? John Wickersham's book "Build Your Own Motorcaravan" contains an "Electrical Systems" chapter that includes tables on cable maximum Amperage ratings and maximum permitted Amperage on the basis of cable length. Maximum permitted Amperages of 1.0mm² (14 strand) cable by length are given as 3.1A (12 metres), 4.7A (8 metres) and 9.4A (4 metres). For 1.5mm² (21 strand) cable the data provided are 4.7A (12m), 7.0A (8m) and 14.1A (4m). For motorhome applications, 1.0mm² cable is advised for interior lights and 1.5mm² cable for 'extractor fans'. Another table that gives typical fuse ratings suggests a 10A fuse as the norm for an Omni-Vent fan. If I didn't have Omni-Vent installation instructions, I'd use 1.5mm² cable provided that the cable-run length was not excessive. If I had Omni-Vent instructions, then I'd adhere to them. Yes Derek, that is why I aggreed with your 1.5mm sq to cover most eventualilies, I would note that 'most' vent fans draw around 30w when up to speed, the same as a 30w halogen light. Link to comment Share on other sites More sharing options...
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