Wire gauge for wires to battery

[Barnsey]

The previous owner of my bike has connected to the battery with pieces of mains wire and lashings of gummy insulation tape, so I am looking to do a more professional job.  However I really don't know the current draw on the battery.  I am guessing that 18 AWG wire with a current rating of 16A should be enough.  What do people think?   

[Barnsey]

OK,  Maybe answering my own question, but, digging around the net, it seems that 1mm² or 17awg is considered sufficient.

[Spaceman]

With everything possible turned on with the engine off, the battery current draw would be around 10 A and 1 mm sq cable would easily take that. However, you also need to consider voltage drop along the cable especially with only 6 V to start with. I would therefore recommend the use of slightly thicker cable to connect the battery to the ammeter and to earth to minimise the voltage drop.

[Greenfield]

To some extent it depends on the bike, 6 or 12v and what electrics are fitted. If you assume everything is on ie headlamp, brake light, tail lamp, horn, ignition, etc, add up all the wattage and divide by your voltage. This gives your maximum amps. A standard 1mm square conductor (14/0.3 OD 2.6mm) is good for 8.75A; 105W on a 12v system but only 53W on 6v. Thin wall cable takes up less space and can be much tidier - a 1mm conductor (32/0.2, OD 2mm) has a capacity of 16.5A. Its always worth fitting the next size up and consider fitting fuses.

[Spaceman]

You have to be a bit careful with thin wall cable especially on a C15 inside the headlight or under the saddle because the PVC insulation on it is not as flexible or tolerant to flexing as standard PVC cable. The reason thin wall cable has a higher current capacity is purely because its PVC insulation is rated up to 105 deg C rather than the 70 deg C for standard cable. 

[AltcarBob]

Not an elastictometrician but I have always gone for the chunkiest wire I can get through the grommets or clips. Got to give the little electric Imps plenty of room to run about.

[Rog1]

This is the wiring diagram I came up with for my M33. I've added an earth wire for reliability rather than earthing everything through the frame. I haven't actually rewired the bike yet but I'm an electrician so I know what I'm doing. I'm showing you this because it's the clearest way to see where I think a 2mm sq. cable is needed; I'm not saying this is the diagram you should wire to. All my bulbs are LED, but it's the dynamo output that dictates cable sizes. I plan to use the thick wall stuff; it's more robust.

Every wire is 1mm sq. except for:

Dynamo 'D' to regulator 'D'. Maximum dynamo output is 60w; 10 amps.
Regulator 'A' to ammeter. Carries dynamo output of up to 10 amps to ammeter.
Battery negative to ammeter. Wire to ammeter connects battery to system so 2mm sq.; nice and durable. May also have to carry large charging current.
Battery positive to frame earth and to main earth in headlight. Dynamo earths through frame and also earths my big, car type horn.

The parking bulb (as I call it) is the underslung type, (I have the cowl) but my headlamp reflector also has a pilot bulb socket, so I'm probably going to have a bulb there that comes on with the headlight to give a bit more general light. Most bikes will not have this. I might not bother to put a light in the ammeter; they are not fitted as standard.

[Spaceman]

The bike in question here is a C15 as I understand it. The two wires that will carry the maximum current in the wiring harness are between the battery and the ammeter, and between the rectifier and the other side of the ammeter.

With the engine off and the dipped headlight on, the max current draw from the battery will be about 9 A for the C15. In contrast, with the engine running at 5,000 rpm, the maximum current output from the rectifier will be about 9.5 A.

In principle, the two wires in question could end up being operated above the maximum current of 8.75 A specified for 1mm sq standard PVC cable but, since that current will not be maintained for very long, this size of cable should be ok in practice and not over-heat.

However, to avoid these wires and the earth connection to the battery getting hot to the touch (up to 70 deg C), the use of 2 mm sq cable will prevent this.

[Barnsey]

Woww!   What a lot of responses.  Thank you everyone.  It looks like I will be ordering some 2mm wire to supplement the 1mm² which I already have on order. 

   Regards,
     Ian

[Spaceman]

It's definitely worth considering the earthing arrangement for the bike. I tend to take Rog1's approach and never rely on using an earth return through the frame or tinware, especially on a 6V bike. The key problem areas are the earth return from the headlight shell and the earth return from the rear lights and I therefore always run a separate earth return wire from each to a common point near the battery. You only need a bit of corrosion around a bolt along the way to produce a slight voltage drop resulting in 5.5V or worse rather than 6V at the lights, especially at the headlight.

[Greenfield]

On my 6v B33 I've brought the earths together on the regulator mount accessible, but not in an obvious position under the saddle.