Fitting air horns
QI have a set of air horns I want to fit on my boat, they don’t have a compressor so do I need to use a relay.
ATONY REPLIES: If they have no compressor I suspect they are simply “long trumpet” conventional horns. Their volume will be indicative of the current they draw with an initial surge perhaps 20% higher than their rated amperage. However a switch may be described it will still have a current rating but it might be difficult to obtain. If it came as a kit with the horns and no relay was supplied then one hopes it will be robust enough for the job. The problem comes with the length of cable runs. For any given current the longer the cable run the thicker the cable needs to be. Otherwise insufficient current will reach the horns so they may just give a quack when you push the button or not work at all. If the horns are close to the batteries then a relay may be appropriate but if they are a long way away then a relay would demand two thinner cables to signal the relay and two thick ones to feed the horns. You might just as well use two thick cables feeding the horn directly. I can’t advise on cable size without the total length of the positive and negative cables plus the horn current. Once you know those two things you can find tables on the internet to advise the conductor cross sectional area you need. Try to keep the total volt drop on the cables to less than 0.5 volt.