Towbar wiring

[Biggles]

Hello
I have a 56 plate Accord Tourer and I have just fitted a towbar which was easy enough but I am a bit worried about the wiring, I have wired a standard single socket to the wires that come off the rear lights, it all works when I test it with the trailer board but I have read that I should fit some kind of special relay box, prices range from £16 to over £100 on Ebay, Do I have to use one of these? What will happen if I don't and when?
Any advice welcome

 

[jayok]

The purpose of the "relay" box it so that you can supply additional power to the trailer unit without drawing load from the existing bulb circuit. In other words, the current bulbs (e.g. brake) will be rated for 21W at 12V per side. So 42W at 12V is approx 3.5A of current. Now, if you add another 2 brake lights (to the trailer) you're adding another 42W to the circuit, drawing a total of 7A down that line. This is not "best practice" as the additional loads should be powered via a separate power line.

So what's the worst that can happen? You may blow a fuse. Over time the additional load may cause the wiring to fail (excessive heat), but in all honestly, you'll probably be fine. As all is working fine now, with no blown fuses, it looks like you're ok. Personally, I'd drop a relay in for piece of mind.

 

[Biggles]

The purpose of the "relay" box it so that you can supply additional power to the trailer unit without drawing load from the existing bulb circuit. In other words, the current bulbs (e.g. brake) will be rated for 21W at 12V per side. So 42W at 12V is approx 3.5A of current. Now, if you add another 2 brake lights (to the trailer) you're adding another 42W to the circuit, drawing a total of 7A down that line. This is not "best practice" as the additional loads should be powered via a separate power line.

So what's the worst that can happen? You may blow a fuse. Over time the additional load may cause the wiring to fail (excessive heat), but in all honestly, you'll probably be fine. As all is working fine now, with no blown fuses, it looks like you're ok. Personally, I'd drop a relay in for piece of mind.

Thanks for the advice, it's only used a couple of times a year to tow my boat to the coast for a spot of fishing so I will fit one of those relays which are about £16 on Ebay, I don't think I need the £100 ones, they are probably if you tow a caravan with twin electrics. Cheers

 

[Biggles]

Thanks for the advice, it's only used a couple of times a year to tow my boat to the coast for a spot of fishing so I will fit one of those relays which are about £16 on Ebay, I don't think I need the £100 ones, they are probably if you tow a caravan with twin electrics. Cheers

Change of plan, I don't know why I didn't think of this before, I have now fitted LED light units to my trailer board, the stop light only draws 0.08 amps so this will not put undue load on the car electrics.