Can someone check my wiring maths please?

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Critical Mick

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I'm putting a pair of 100w wipacs on the front of my 300 and I'd be most appreciative if someone could check I'm about to use the correct cables before I set the whole thing on fire. So this is what I've calculated should be sufficient...

From the battery positive to the relay (pin 30) 27 amp cable with a 25amp fuse.

From the relay (pin 87) to each of the lamps 17 amp cable.

From the relay (pin 85) to earth. I wasn't 100% sure but I didn't think it had to be to thick so I was just going to use some 8 amp cable.

Simliarly the feed to activate the relay (pin 86) I was just going to use some 8 amp cable (Incidentally I'm using the fog light switch, having just circumcised the bumper)

So am I on the right track or am I about to be left with a blackened charred mess?

Thanks in advance for any help or advice.
 
each lamp will draw 8.3A so 27 amp cable to each lamp individually will:

A) Be very expensive for no gain
B) make it damn near impossible to get the cables into a crimp terminal to fit to the relay...

20A cable to feed (20A blade fuse)
10A cables to lamps (or 20A from relay to first lamp, then to second)
 
I was only going to use 27 amp cable from the battery to the relay as it will be drawing near enough 17 amps. I chose 27 because its easy to get hold of but if you think 20 would be up to the job I'll probably go with that.
 
I'm an industrial spark by trade so my auto wiring is done with cable measured in metric not americanese or current... as long as the cable is rated to 20A it'll be fine, but as i mentioned - too big and you'll struggle to terminate it and a bad connection is more likely to end up charred than a cable which is rated only just above the load!
 
27A would be ~2.5mm CSA though at a guess

If so - that'll be fine to fit in a normal insulated spade connector (blue)
to fit two cables into one for the load you'd struggle to get two 2.5mm's into a yellow spade connector - 1mm would be enough for wiring to each lamp individually and you'd get two of them into a blue connector fairly easily (don't be tempted to twist the wires together just shove the two cores separately into the terminal) 1mm would also be fine for the signal wiring...
 
If I could trouble you with a further question. Does the cable for the earth leads need to match the ampage of the positive lead, ie 20A from the relay to earth and 10A from the lamps to earth?
 
The purpose of the fuse is to protect the wiring not what's at the end of it so if you only have one 25A fuse you need to use 25A+ cable for everything after the fuse except for the relay coil feeds. If you use smaller cable to each lamp and get a short the cable will melt before the fuse blows. Alternatively you could fit an extra 10A fuse to each lamp and use 10A cable from the relay or you could use two relays two 10A fuses and 10A cable for everything which would at least mean that if you had a relay fail or a fuse blow you would still have the use of one light.

I don't think many cars are fused and wired properly when it comes to add ons but if you want to do it right and be safe its worth the little extra effort.

If it was me I would use two relays and wire everything in 10A cable as I would only have two reals of cable to buy (red and black) and its not that expensive for another relay. I would even use the same wire for the feeds to the relay coils and stick a 10A fuse in that as well.
 
I would suggest one cable from battery to a single relay. Then single feed to the first light and then a piggy back spade connector supplying the next light.

Feed would be 27amp with a 20amp fuse within 3 inches of the battery and fused relay too if you wish.

Feed could be taken from an auxiliary fuse box which are easy to get and once installed provide good supplies to all accessories.

One wire around the vehicle is always easier to manage IMO
 
IanFiTheDwarf said:
The purpose of the fuse is to protect the wiring not what's at the end of it so if you only have one 25A fuse you need to use 25A+ cable for everything after the fuse except for the relay coil feeds. If you use smaller cable to each lamp and get a short the cable will melt before the fuse blows. Alternatively you could fit an extra 10A fuse to each lamp and use 10A cable from the relay or you could use two relays two 10A fuses and 10A cable for everything which would at least mean that if you had a relay fail or a fuse blow you would still have the use of one light.

I don't think many cars are fused and wired properly when it comes to add ons but if you want to do it right and be safe its worth the little extra effort.

If it was me I would use two relays and wire everything in 10A cable as I would only have two reals of cable to buy (red and black) and its not that expensive for another relay. I would even use the same wire for the feeds to the relay coils and stick a 10A fuse in that as well.
Click to expand...

True, but each cable to the lamp would only be carrying 8.3A unless shorted to earth - in which case a 20A fuse would still blow before any damage occurred to the smaller cables. ;)
 
lerk said:
True, but each cable to the lamp would only be carrying 8.3A unless shorted to earth - in which case a 20A fuse would still blow before any damage occurred to the smaller cables. ;)
Click to expand...

A 20A fuse will not blow before 10A cable melts! 20A is the current the fuse can carry indefinitely under all conditions including high tempretures. An automotive fuse will carry two or three times its rated load for tens of minutes if not an hour and will carry 10 times its rated load for a short time. This is definitely enough current for a long enough time to turn a 10A cable into a red hot glowing fire starter.
 
Explain ring main circuits to me then... cable with a maximum current capacity of 27A before any derating factors, supplied by 32A breaker. This is perfectly acceptable due to the nature of PFC's

If the cable were to short then there would be >100A flowing through the fuse which would cause it to pop in milliseconds. Granted the cable would also be flowing this too but unlike a fuse, the excess current will take longer to have any noticeable effect on the cable.
 
coz its a RING mains - therefore there are always two cables going to each plug, so the current flowing thru each cable is only half the total.
 
Until a live conductor comes adrift and hits the backbox which is earthed.
At that point in time you have hundreds of amps flowing (through one side of the ring) which blows the fuse in <0.4s
 
PFC's have naff all to do with it. i hope you are not an electrician, I am and have been an auto electrician as well and a plant engineer, avionics engineer and an authorizing engineer for electrical installations.

house and industrial wiring has a whole load of calculations to be done to determine if an installation (cable, fuse ect) is safe. when it comes to wiring in cars a lot of assumptions are made and all the calculations have been done and factored into the ratings of the cable and fuses already so a 10A cable must be protected by a 10A or less fuse and that is that. if your car was to catch fire after you did some wiring and you had some 10A cable protected by a 25A fuse do you think your insurance will pay out?
 
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