Wiring harness current loads

[Keith66]

Hi All

 

Could do with some advice on where to find or how to find out some info.

 

I’m thinking of doing what Ed Hollingsworth has done and putting a couple of connectors in the wiring harness behind the dash.

 

It seems a good idea as it will enable the bench rebuild of the dash then a simlper install of the prebuild dash and just a couple of connectors to connect behind the dash.

 

But to do this I need to obtain the correct type and capacity of connectors and thus need to know the current load each wire is likely to need to carry.

 

Ed’s being a US car and having a largely custom built harness and power module may not be representative so I’m looking for the info for a 72 CP Pi car.

 

I’ve had a quick look through the brown book but cannot find anything obvious and as far as I am aware I cannot simply look at the wiring diagram as potential loads are not present.

 

So any ideas?

 

Cheers

 

Keith

Edited December 5, 2017 by Keith66

[RogerH]

Hi Keith,

look at your wiring diagram and see what each wire feeds.

 

eg - a headligt bulb (60W) is apprx 5 amps

side lights (5W) is apprx 1/2 amp.

 

Shouldn't take long to see what the acutal loads are. You then need to uprate the connector - double it.

 

Or you could look at the cables themselves. These are already over rated (dream on).

 

 

Roger

[Keith66]

Roger

 

Thanks for that, its the bigger loads that might be an issue, useful to know that a headlight at 60watts is approx 5amp, but how is that worked out, i have no idea? although i need to do something specidic with the Ameter.

 

Hopefully the harness should be ok and up to the job as its a new Autosparks one but is the load printed on the wires?

 

I've asked Autosparks but no response as yet.

 

Cheers

 

Keith

[Kevo_6]

Roger

 

Thanks for that, its the bigger loads that might be an issue, useful to know that a headlight at 60watts is approx 5amp, but how is that worked out, i have no idea? although i need to do something specidic with the Ameter.

 

Hopefully the harness should be ok and up to the job as its a new Autosparks one but is the load printed on the wires?

 

I've asked Autosparks but no response as yet.

 

Cheers

 

Keith

Hi Keith

 

You basically devide the Watts by the volts.

You may find this useful. https://www.12voltplanet.co.uk/cable-sizing-selection.html

[astontr6]

Hi All

 

Could do with some advice on where to find or how to find out some info.

 

I’m thinking of doing what Ed Hollingsworth has done and putting a couple of connectors in the wiring harness behind the dash.

 

It seems a good idea as it will enable the bench rebuild of the dash then a simlper install of the prebuild dash and just a couple of connectors to connect behind the dash.

 

But to do this I need to obtain the correct type and capacity of connectors and thus need to know the current load each wire is likely to need to carry.

 

Ed’s being a US car and having a largely custom built harness and power module may not be representative so I’m looking for the info for a 72 CP Pi car.

 

I’ve had a quick look through the brown book but cannot find anything obvious and as far as I am aware I cannot simply look at the wiring diagram as potential loads are not present.

 

So any ideas?

 

Cheers

 

Keith

Hi Keith. Automotive Wire types have moved on in the last 30 years. The original PVC wiring was made by a company called Rists and they supplied the British motor industry with full harness assemblies as well. This wire was known as thick wall then the Japs came along with a new type of PVC wire called thin wall. The advantage of this wire was that it was lighter, more abrasive resistant, smaller o/d and could carry more amps per given size. I would hope that Auosparks have used this grade of wire? Example 1mm sq. thick wall can carry 8.75 amps,2.7mm o/d. 1mm sq.thin wall can carry 16.5 amps.2.1mm o/d. if they have not used thin wall I would question their competence, as this is the new car standard now..

 

Bruce.

[RogerH]

Hi Keith,

to calculate the current with only having the power known

 

Power(W) = Current (I) x Volts(V)

 

60W = I x 12V

 

60/12 = 5 amps (I)

 

 

another good one is

 

Volts(V) = Current (I) x Resistance®

 

If you measure a resistance in a connector (it should ideally be '0') and it is eg 2 Ohms and you know the current being taken eg 5amps (head light bu;

 

V = 5 amps x 2 Ohms = 10V so this small resistance is dropping a lot of volts for 5 amps. This will make the headlights very dim.

 

The connector may get quite hot as it will be dropping 50W = (10V x 5Amps)

 

V=IR

P=IV

 

Roger

[barkerwilliams]

Keith,

If your car is like my 1970 PI then the dashboard only consists of gauges and the wiper switch & windscreen washer switch.

The only high current circuit is the windscreen wiper switch. I believe the wiper motor draws about 3 amps, however as the wipers are used for extended periods and the wiring gets slowly warmer, and will draw more current when used on a dry screen or against snow I would suggest a hefty margin of safety and use a 1 sq mm thin wall cable good for 16 amps.

Two problems remain the ammeter and the oil pressure gauge. The oil pressure gauge could easily be swapped for an electrical gauge at low cost.

The ammeter; for most of its life it only carries a couple of amps but when starting and the first few minutes of running it can carry a high current. If you fit a connector then the contacts in the circuit add resistance and they would represent a point of failure. Why not just bridge out, or replace, the ammeter wiring and take a small wire off to feed a voltmeter.

If however you wanted to have multi -connectors on lights, overdrive, horn and centre console then I believe you would be better to fit headlamp relays. The horn and overdrive already have relays so draw little current. That only leaves the cabin fan which again is used for extended periods and needs a larger cable but once again would be better with a relay or two for the twin speed.

The last console heavy wire is from the ignition switch, if you fit a 100amp relay near the fuse box that circuit will also be low–draw. So all connectors in a multi plug would have the same current carrying capacity.

​The starter solenoid wiring from the ignition switch carries about 5 amps.

In summary I would near the fuse box

Fit a 100 to 120 Amp relay. This also allows you to discard the fuel pump relay.

Fit headlamp relays minimum of 2, possibly four needed – it’s a safety thing.

Fit new cable to bypass ammeter. Fit voltmeter instead of ammeter.

Fit two relays for twin speed cabin fan.

Fit electrical oil pressure gauge & sensor

Wipers fit relays (2).

Think about the dip switch – column or floor.

If you do embark on a relay type route I would also replace the existing horn and overdrive relays so that all relays in the car are of the same type and current capacity. That way you only need to carry one spare and it makes fault finding so much easier than multiple relay types.

Alan

[peejay4A]

If you bridge the ammeter and replace it with a voltmeter you dont want to connect it to the brown wires because it will be reading voltage all the time. Theyre usually hot wire instruments and will drain the battery over time. An ignition switched source is better.

 

Edit: yet again Apple has removed the apostrophes from this post. Here is a row of them: .

Edited December 10, 2017 by peejay4A

[Alec Pringle]

Hi Bruce,

 

Autosparks do offer some information re their harnesses on their website . . . .

 

https://www.autosparks.co.uk/about-our-harnesses

 

Cheers

 

Alec