Wiring in parallel - wire size

[benjy_b]

Hello there!

Quick question:

When wiring in parallel, is it necessary to increase the wire in size the closer you get to the power source?

For example, I am working on an engine loom for my car. Many of the sensors earth through only 2 earthing points on the loom. They are currently spliced together randomly. I am wishing to neaten the loom and splice them in a similar location (near the ECU plug). Do I require a larger gauge wire for the one terminating wire?


The current OEM loom uses the same gauge wire throughout that specific circuit, so I guess the answer may be NO.
Would someone explain why?

Edit:

Have a picture of my first practice concentrically twisting a loom.

[cobblers]

Sensors use almost zero current, so from a safety perspective, generally you should be OK to just tie into the same wire.

Depending on the sensor, they can be affected by any slight voltage drop on the ground wire so ideally you would keep the layout of the wires the same unless you really know what you are doing. It's unlikely to cause problems, but if it does they will do your head in and be impossible to diagnose.

[benjy_b]

Hi Dave, thanks for the reply.

All the grounds will be spliced to each other in the same way as OE, except their location. They will all be spliced at the very end of the wire, near the ECU connector, rather than in random locations throughout the loom.

For example,

Here you can see the braided shielding crimped together (white wires):



Again here (green wires):



And again here:





The only difference between the above examples and the OEM splices will be the lengths of wire, will this cause the grounding issue you speak of?

Further more, as you can see in the images above, much of the wire is the same diameter. I have one larger diameter wire that feeds the starter solenoid. I was wondering, instead of using one large diameter wire, could I use multiple smaller diameter wires to the starter solenoid? So instead of one 10mm2 cable, use ten 1mm2 cables?

[cobblers]

In practice your splicing method will probably actually be slightly better than OE, so I'm sure you'll be fine. The wire diameter used for sensors from the factory is dictated by physical strength more than current handling, really so there's plenty of headroom, and running them individually right up to the ECU will actually improve things. Just make sure that stuff like crank/cam sensors etc continue to use twisted pairs or shielded wires if they do from stock.

Running two thinner wires in parallel to increase current handling works fine electrically, but in practice it's not ideal.
For example, if you have a circuit fused at 15A, and use two "10A" wires to handle the current there is no problem.
The issue is that if the loom is damaged (perhaps a chafe through the insulation on an steel edge) it's more than likely that just one wire of the pair will get shorted out, so you'll end up with the wire burning out instead of the fuse going.

Also, if one wire breaks/falls out of the connector etc etc, the other will bear the full current.

Ideally you need to fuse lower than the weakest wire on the circuit, irrespective of how it is connected. In practice a lot of stuff doesn't work like this, but if at all possible you must.

[benjy_b]

Thanks again.

My logic told me I could do it, but common sense was telling me it wasn't the best of ideas.

I was looking into using a multipoint bulkhead connector, which only has a maximum amperage rating of 7.5A per terminal. This got me into thinking around the idea of 'splitting the load' through more wires. It turns out that although the connectors aren't too expensive, the tools to crimp the terminals are all above £200!! So I'm just going through a bulkhead grommet instead.

[cobblers]

Yup, we use those connectors at work. If I remember right, it's just the dies to fit into the tool that are £200. The rest of the tool is £1k+
In a pinch you "can" crimp them by hand but it's not something I would consider doing other than for repairing one or two wires, it's not reliable enough.

Depending on how many pins you need, consider using AMP/TE connectivity circular military connectors, they go up to about 40 pins IIRC. They are solder cup, so you don't need a crimper, but they are hard to seal the wire side of so maybe not ideal, it depends on what use the car will get. I wouldn't use them on a daily all winter, but in practice they are used on the gearboxes of almost every bus for the last 30 years.

[benjy_b]

The connectors I was looking at are the same as the one clamped in the vice on the picture above. Deutsch Autosport I believe. To be honest, It's over kill for my application. The engine harness on my car is only 2.7m long at it's furthest and it simply terminates at 2 of the 3 ECU plugs and a chassis plug (of which all fit through the hole in the bulkhead). So it makes more sense to save the money and use a large rubber grommet to go through.

[benjy_b]

I've been cracking on with my engine loom. It was a lot cheaper to buy one colour than many. I'll just have to triple check all the wires with continuity whilst wiring the sensors up.