+12V Switched (Ignition) Wire Guage

[Moonlight GSX]

I'm trying to figure out the wire gauge size for the switched +12V from the ignition switch. Reason being is I'm running into current problems with my wiring harness that I'm building for a set of individual coils that run roughly 7.8A draw each (if Ip/103mA = 71/1, Ip=7.8A). Problem is 16 AWG is only "capable" of about 20A-25A (before voltage drops occur) and that's why I'm thinking of running two 12GA wire (paired on coils) but wanted to know if there is a way to either run them both from the ignition like normal or possibly add a relay and have them run into that relay with the ignition switch being the trigger for that relay? Thanks guys!

Edit:

Would this work? The use of a 40A DPST relay that closes the path directly from the battery to the ignition coils with the relay's trigger being the ignition switch +12V constant.

 

[TrevorS]

FWIW -- My '90 uses 14AWG in the ignition wire to the coil packs. I'm in the process of converting my OE ignition to COP using dual transistor packs and a direct battery line for the coil primaries though a relay controlled by the original ignition wire. It works.

What coils are you looking at?

 

[wrencher]

Those coils *may* peak @ 7.8 amps a piece.
But they're only sustaning high amperage for milliseconds.
It will average out to 2-6 amps running.

The relay would be a good idea, less stress on the igntion switch itself.
If you want to get fancy & run a double poll single throw it'd work too.

 

[TrevorS]

The real question is how much current/heat those coil packs will tolerate before overheating with an extended duty cycle (OE is waste spark with a minimum of 4ms). The OE design matches the current to the coils, but anything aftermarket is suspect. Yes, there are mechanisms by which the current can be increased in case of lower inductance coils, but that doesn't guarantee the coils won't overheat. That's the problem with converting from OE ignition to something experimental. If the inductance exhibited by the aftermarket coil is less than OE, then there will be greater heating, hence greater likelihood of failure. What is your goal in this scenario?

 

[Moonlight GSX]

In this thread it'll explain kind of the idea I'm workin' with here. As for duration, running 12 AWG or maybe even 16 AWG won't hurt the setup? I mean at 24A (max avg) I can run 14 and be fine, but 16 would still be over it's allowable current rating.

For reference here is the information about the coil(s) themselves along with the specs and operating frequencies and voltages. These coils will allow me to remove my PTU as each one has it's own "internal" PTU in that it'll get the signal straight from the ECU.

As for the relay, so this would be a nice idea? Using the ignition switch as a trigger and then running slightly larger gauge wire straight from the battery to the coils - supplying ample voltage/current to them.

 

[TrevorS]

If you want the voltage and ground to be stiff, choose wire gauge accordingly. I personally used 10 to the relay, 12 to the coil connector, 14 to the COP modules, and a stubby 14 to a nearby engine ground. Nothing special about that of course, it just makes sense to me for what I'm doing.

 

[Moonlight GSX]

If you want the voltage and ground to be stiff, choose wire gauge accordingly. I personally used 10 to the relay, 12 to the coil connector, 14 to the COP modules, and a stubby 14 to a nearby engine ground. Nothing special about that of course, it just makes sense to me for what I'm doing.

After doing the calculations and talking to a few people at work (fellow "glorified" electricians) that's when the current of wire gauges came into sight. I was thinking of running two 12GA (+12V) to the coils (in pairs) and then on the other side, from the battery to the fuse and relay run one either 8GA or 10GA to a fuse block (1 in/2 out), two 12GA to the 40A/50A relay (triggered by switch), and then to the connector which then gets sent to the coils. A similar system would apply to ground, although would it be better to connect it directly to the battery's negative or just chassis ground (initially same thing)?

 

[TrevorS]

A good engine or chassis ground is probably better than running long lengths of wire. My stubby ground is to the intake manifold which has a sturdy ground to the firewall which supportts the battery ground cable. It's easily enough ground for my purposes. I'm running a 15A fuse from the battery with my dual transistor pack COP setup and it hasn't popped ('course, I just completed install and have only driven with it once . Don't yet know what my peak primary current is at idle, but I plan to measure it -- not really expecting it to exceed 9-10A (perhaps less), but with a road voltage of 14V and stiff supply and ground, who knows? I think the only remaining weakness in my setup is I'm using OE 16 AWG wiring from the transistor packs to the coils, not much distance though and so I doubt it's an issue. I prefer to preserve OE color-coding when practical.

 

[Moonlight GSX]

I know in my current setup, although not using OE colour scheme, I have 16 AWG on my trigger, sensor ground, and cylinder head ground connections. I may do the same AWG for the cylinder head ground but I doubt it'll be needed...

Awesome, so I don't have to run long lines for the battery ground. May be a few feet still though due to the length of the wiring harness as the coils themselves will be mounted elsewhere due to size and whatnot.

 

[TrevorS]

Don't yet know what my peak primary current is at idle, but I plan to measure it -- not really expecting it to exceed 9-10A (perhaps less), but with a road voltage of 14V and stiff supply and ground, who knows?

Hooked up a scope yesterday. At ~625 rpm idle and ~12V battery, the primary current peaked at 12.5A. The charge time was ~8ms and the coils were not reaching saturation. I would expect 15A with road 14V, and given another 2 to 4 ms perhaps as high as 18A saturated.

 

[Moonlight GSX]

Hooked up a scope yesterday. At ~625 rpm idle and ~12V battery, the primary current peaked at 12.5A. The charge time was ~8ms and the coils were not reaching saturation. I would expect 15A with road 14V, and given another 2 to 4 ms perhaps as high as 18A saturated.

Hmm, this is from the battery/alternator yes? If so that means I'll be alright with running 8GA from the battery to a a 40/50A fuse that will then divide the 8 into 2 12 AWG and from there those would be tapped into by the four coils (two each) for the +12V signal.

Though I've been running some figures in my head, the ground to cylinder head, this is for the ground going to the secondary winding yes? If this is the case then the maximum current should be, at peak current, ~.412mA as each coil is rated to a peak current of .103mA... would I be correct in this theory or no?

 

[TrevorS]

Hmm, this is from the battery/alternator yes? If so that means I'll be alright with running 8GA from the battery to a a 40/50A fuse that will then divide the 8 into 2 12 AWG and from there those would be tapped into by the four coils (two each) for the +12V signal.

Though I've been running some figures in my head, the ground to cylinder head, this is for the ground going to the secondary winding yes? If this is the case then the maximum current should be, at peak current, ~.412mA as each coil is rated to a peak current of .103mA... would I be correct in this theory or no?

I'm talking about measured battery voltage and the ground I mentioned is at the transistor packs, not the coils. I'm working with Dodge Intrepid 2.7L COP modules, series primaries aprox .8 ohms and 2.9 mH. I've no information regarding the secondary current, but my concerns are exhibited behavior plus having a local shunt to ground (ignition capacitor on the 12V). At low rpm idle, my alternator is unable to charge the battery, instead the battery gradually runs down. Just curious, why a 40/50 ampere fuse?

 

[wrencher]

12.5 amp peak is high but what is the frequency & the average amperage running on the circuit?
I am not super familair with these aftermarket setups, but it is 2 coils simultaneously firing correct?

The current demand of the coils should be more liken to a pulse width modulated circuit.

 

[TrevorS]

12.5 amp peak is high but what is the frequency & the average amperage running on the circuit?
I am not super familair with these aftermarket setups, but it is 2 coils simultaneously firing correct?

The current demand of the coils should be more liken to a pulse width modulated circuit.

I think this is better characterized as home brew than after market -- there's no kit ! My own setup is with direct battery 12V to the coils and local ground to the transistor packs (no OE harness in between). Yes, it's still waste spark. At my aprox 625rpm idle, I measured ~8ms charge time and ~96ms series coil charge cycle (1/4 & 2/3). The 12.5A is honest and should increase with higher alternator output (ie. significantly greater than 625 rpm). As I said, the coils didn't achieve saturation. I haven't calculated average current, but that is very much a question of rpm and load conditions. Idle is surely the easiest !

 

[Moonlight GSX]

Well I don't have any transistor packs so I won't be able to measure the amperage at them, less I tap at the wires going to the coils. As for using a 40/50 ampere fuse It's more of a precaution just in case too much current is drawn by the coils. Even though their peak current is ~31.8A I wanted prevent them from blowing all the time. I could very well use a 30A fuse similar to that of the value of the the ignition... might try it just to see if it blows. But the 31.8A is the cumulative current of the 4 as they will be fed by 8GA and two 12GA wire.

 

[TrevorS]

Well I don't have any transistor packs so I won't be able to measure the amperage at them, less I tap at the wires going to the coils. As for using a 40/50 ampere fuse It's more of a precaution just in case too much current is drawn by the coils. Even though their peak current is ~31.8A I wanted prevent them from blowing all the time. I could very well use a 30A fuse similar to that of the value of the the ignition... might try it just to see if it blows. But the 31.8A is the cumulative current of the 4 as they will be fed by 8GA and two 12GA wire.

Waste spark or not? If you're using the OE ECU, then I would expect waste spark and series coils (two coils fire at the same time). If that's the case, you may be quadrupling the current requirement in your estimate.

If you're not wiring the coils in series, then remember that they don't all fire at the same time. The fuse only needs to allow for two coils firing at once. Likewise, the fuse only needs to accommodate that firing within the demands of the duty cycle. For my own series firing COP modules, as far as I can determine, a 15A fuse is sufficient.

 

[Moonlight GSX]

Waste spark or not? If you're using the OE ECU, then I would expect waste spark and series coils (two coils fire at the same time). If that's the case, you may be quadrupling the current requirement in your estimate.

If you're not wiring the coils in series, then remember that they don't all fire at the same time. The fuse only needs to allow for two coils firing at once. Likewise, the fuse only needs to accommodate that firing within the demands of the duty cycle. For my own series firing COP modules, as far as I can determine, a 15A fuse is sufficient.

Well as stated in the other thread I'll be using the AEM EMS Series II ECU to run them in series. Alright so I can actually use the 16 AWG for my +12V and Battery Negative and have them all tapped into the same wire (respectively - two wires, one for each)? Considering it'll be in sequential in that only one will be firing at any given time? Basically I should be okay with my original design and won't need to run larger gauge wire, a DPST relay, and an inline fuse as the ignition fuse should be okay for these as well yes? Correct me if I'm wrong on this...

 

[TrevorS]

Well as stated in the other thread I'll be using the AEM EMS Series II ECU to run them in series. Alright so I can actually use the 16 AWG for my +12V and Battery Negative and have them all tapped into the same wire (respectively - two wires, one for each)? Considering it'll be in sequential in that only one will be firing at any given time? Basically I should be okay with my original design and won't need to run larger gauge wire, a DPST relay, and an inline fuse as the ignition fuse should be okay for these as well yes? Correct me if I'm wrong on this...

You have to keep in mind that a fuse rating only has to support the average current plus a reasonable margin, not transient peaks. The rated coil peak current is only achieved (if achieved) after the coil current ramps up over the charge time to the point of saturation. In my idle situation above, the charge time is ~8msec and it does not achieve saturation, hence the ave current during the charge time is ~6.3A (12.5A/2), then throw in the ~96ms charge cycle interval and the overall coil current average comes down to 6.3A*8/96 or about .5A. Since I have two sets of coils, the fuse sees twice that or an average ~1A. If my guesstimate of 18A saturation peak with 14V is correct, then the fuse would see an average under those conditions of 1A*18/12.5 or ~1.5A.

Looking at 6Krpm, the charge cycle interval will be 10msec and the ECU will probably not exceed a 60% coil duty cycle (stress considerations) and a 6ms charge time is even further from achieving saturation. If I assume a worst case peak current of 15A, then the max interval average would be under 4.5A. Add the other coil pair to that and the fuse sees an average under 9A. As I said, my fuse is actually 15A -- I really doubt I'll have any fuse problems. Since your COP modules are paralleled and you have a different saturation current, your numbers will be different. You can choose to wire it for the average current if you wish, I have peak current in mind.

 

[wrencher]

Another fact to keep in mind for electrical circuits.
Resistance or a weak link in excess loads will be the connections.
The coils will still have (cannot recall exactly) 16-18 gauge terminals.
That would be a bottle neck in a high amperage circuit.

OEM configuration they never meant for these coils to run that much amperage for an extended period.
The coil driver in the OE PCM ramps them quickly & they usually build to 6-7 amps in about 6 milliseconds.

 

[Moonlight GSX]

Another fact to keep in mind for electrical circuits.
Resistance or a weak link in excess loads will be the connections.
The coils will still have (cannot recall exactly) 16-18 gauge terminals.
That would be a bottle neck in a high amperage circuit.

OEM configuration they never meant for these coils to run that much amperage for an extended period.
The coil driver in the OE PCM ramps them quickly & they usually build to 6-7 amps in about 6 milliseconds.

They currently have 16 gauge wire, as this was the highest gauge they can take within the connector. Although I've been doing some research on current limits of these wires (NEC Ampacity Chart) just to see what they are efficient at before getting severe voltage drops. That's why I planned to change the wires out and pretty much my own cable runs to the ECU, battery, and cylinder head (respectively) - to possibly eliminate the possibility of running into these current limiting conditions. As for the coil driver each inductive coil contains it's own coil driver or PCM.

Unless that was regarding Trevor?

 

[TrevorS]

Another fact to keep in mind for electrical circuits.
Resistance or a weak link in excess loads will be the connections.
The coils will still have (cannot recall exactly) 16-18 gauge terminals.
That would be a bottle neck in a high amperage circuit.

OEM configuration they never meant for these coils to run that much amperage for an extended period.
The coil driver in the OE PCM ramps them quickly & they usually build to 6-7 amps in about 6 milliseconds.

My COP modules are manufactured for Chrysler Corporation, not Mitsubishi. The DSM ignition module has no control over how fast the coils ramp, that's a function of the series resistance, the coil primary inductance, and the supplied voltage. Chrysler doesn't have these coil modules wired in series (shedding .4 ohms in comparison), each independently sees the battery voltage. However, I don't know if the Chrysler ignition module provides any current limiting, I do know the DSM doesn't outside of its inherent resistance.

It's really not too important that the COP module connector wires are only 16 AWG, one can choose to reduce them to pigtails replacing the rest with say 14 AWG, thereby reducing their effect on peak currents. Either way, current heating of the lighter gauge wire isn't a concern since the peak currents are reached very briefly, so that doesn't inherently entail a reliability issue. I'm still debating on replacing the longer 16 AWG with 14 AWG between the COP modules and the ignition modules, but I honestly doubt switching to the heavier gauge will make a material difference given the fairly short length remaining in my setup.

If I were seeing extended saturation, I would be more concerned, but as I mentioned, not even 8ms is enough to saturate these COP modules. I think the key is the fact they are in series. However, it's appropriate to recognize the OE coil inductance is about 3.9mH, whereas two series Intrepid COP modules are about 2.9 mH. The only way to even match the OE energy is to boost the current some 16%, consequently people generally recommend CDI with COP. This approach is a lot cheaper and no more risky than using increased voltage to push the higher peak current through the OE DSM harness and non-OE COP module primaries. It's just an alternative solution, that's all.

 

[wrencher]

That's what I was refering to in my last post is the OEM chrysler set up coils.

I have scoped & ramped different COP set up's than I can count.

The stock configuration the PCM contains the coil drivers.
The design of the coil lets them ramp quickly, they are not current limited in OEM form.
Because of no circuit protection in the OEM application I've had 2.7/3.2/3.5's with PCM problems melt a coil down.
The fuse held up just fine right on thru to the ASD relay .

The circuit shouldn't have a problem maintaining 5-20 millsecond build to 8 amps a coil.
High RPM is where I would be checking the draw & frequency of the circuit.

It's great you guys are taking the ignition system to the next level.
I am really hopeful for the new igntion plasma based system finally from a main line manufacturer.
Google Advanced Corona Ignition System (ACIS)
They are in the final stages for it's release. Many OEM's testing in their fleets currently.
Oh & it's backwards compatable with older vehicles

 

[Moonlight GSX]

It's great you guys are taking the ignition system to the next level.
I am really hopeful for the new igntion plasma based system finally from a main line manufacturer.
Google Advanced Corona Ignition System (ACIS)
They are in the final stages for it's release. Many OEM's testing in their fleets currently.
Oh & it's backwards compatable with older vehicles

This seems interesting I'll look into that. Although I'm not running a COP setup still something to look into...

 

[TrevorS]

That's what I was refering to in my last post is the OEM chrysler set up coils.

I have scoped & ramped different COP set up's than I can count.

The stock configuration the PCM contains the coil drivers.
The design of the coil lets them ramp quickly, they are not current limited in OEM form.
Because of no circuit protection in the OEM application I've had 2.7/3.2/3.5's with PCM problems melt a coil down.
The fuse held up just fine right on thru to the ASD relay .

The circuit shouldn't have a problem maintaining 5-20 millsecond build to 8 amps a coil.
High RPM is where I would be checking the draw & frequency of the circuit.

The reason they ramp quickly is their relatively low inductance (1.5mH) and plenty of available current given no limiting. From what I see with them in series, our OE 3.9mH coils must ramp very slowly and may need as much as 14 msec to saturate. I would imagine the single COP modules saturate in maybe 3-4 msec.

4500 rpm would be the point were a 60% duty cycle would yield an 8msec charge time, which as I said, doesn't achieve saturation. At 7Krpm, the charge cycle period is down to 8.6msec and 60% would be ~5msec which is maybe a peak of 10A most. I'd guess maybe 4A average over the charge time (it's not linear) for 2.4A ave per COP module pair or 4.8A ave battery draw. The less time is available for the coil to charge when it isn't reaching saturation, the lower it will be on the charge curve and the less the average current.

The main indicator of distress to me is elevated temperature. I measured my transistor pack temps and they're well down, easily under 100F with the car moving and not more than 43C idling with the hood open. I measure the COP module temps at about 120F at idle with the COP plate at 140F and the valve cover at 170F (outside ambient 85F). I expect their rating is 120C (248F) so there's plenty of margin for them. Still, it's a new install and I need to spend time with it to see if any issues crop up.

PS. I'm noticing one complication, with the higher ignition currents, it's more work for the alternator to keep the battery charged. Average rpm has to be kept higher (I let it idle when coasting, and do so frequently). That suggests there's no reason at all for me to swap out any more of the original ignition wiring.

PPS. I was studying my service manual wiring diagrams and discovered the OE wiring is metric. The closest gauge to the OE ignition wire is 17 AWG with a 1.05 mm^2 cross-sectional area !

http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

 

[TrevorS]

Just a follow up, I returned the transistor module ground to the OE harness a couple weeks ago to reduce the battery load. Today I remeasured the COP primary current and came up with 10.5A with a 13.8 battery voltage (roughly 1/3 less current draw than before). I've little experience with the Dodge COP modules and so don't know if there are issues with a 10.5A drive current Vs 8A, but the engine seems to run fine. Only complication seems to be keeping an adequate charge on the battery at idle.