PDM30 wire run lengths and awg selection

Hi All,

I would like to get your input on wire awg and the number of +12v wires for the stated wire lengths for the components below. TO give you a visual, the location of my PDM is under the dash above the trans tunnel, on the passenger side and I'll be using Tefzel wire (M22759/16).

Starting with the in-tank fuel pumps. I am running a Bosch 044 as a lift pump and Aeromotive 340 as the main pump. The peak amp draw on the A340 is 30amps. The length of wire from the PDM to the pumps will be approximately 17 feet. I was initially thinking of running a single 10awg from a 20 amp output, however, after using an online Wire Calculator, at 13.8v with 2% voltage drop the recommended wire size is 7awg. If i were to run two 10 awg wires (one from each 10A output on the PDM) that would approximate a 7awg wire. That said, I don't believe this calculator takes into account the higher heat rating of this wire type. (I have heard many say on this forum that you'll never really draw as many amps as stated on the hardware components spec sheet, but I just want to be careful as to not burn any wires up or worse). When I allow up to 5% voltage drop, then the calculator does indicate a single 10awg wire will be sufficient over the same distance. So, would running two 10 awg wires be overkill for the A340 pump?

Thermo fans: I am running two SPAL (non-brushless) fans and the wire run length from PDM (through bulkhead connector) will be approximately 9 feet. SPAL also states the peak amp draw is 30 and that 10 awg wire should be used. The Wire Calc. also estimates a 10 awg wire for this distance at 13.8v with 2% voltage drop.

AC Compressor: Sanden SD-7, I believe the peak amp draw is less than 20 amps. Distance of wire run also through bulkhead is around 7 feet. Planning on a 12 awg wire here.

GM High torque mini starter: Wire length is approx 5 feet and was planning on running at least a 12awg wire.

General question on twisting these power wires and what is the maximum recommended number of wires to group together? Is grouping fewer wires together preferred to help dissipate heat?

Thank you for your input!

Neil

Here's the link to the Wire Calculator I used: https://www.wirebarn.com/Wire-Calculator-_ep_41.html

Those are all (relatively) high current, power devices and as power is proportional to the square of the voltage, voltage drops are going to be important. I would remind you, also, that there is the added return/ground/earth lengths, if used, to also consider.

As such I would suggest using the largest size wire conductor the connector pins will accept (some are better than others in their gauge range), to minimise the voltage drop, with the heat dissipation factor being largely irrelevant. While one may "get away" with smaller sizes, I don't think the compromise is worth it.

The Sanden A/C clutch seems to have a maximum wattage of 49W, or ~4A, and that's the place you may be fine with a higher voltage drop, because it's just an on/off circuit where the output isn't direclty related to voltage. https://www.sanden.com/catalog.html

With the starter, what are the spec's for it? Again, one may "get away" with it, but up-sizing the cable will turn the engine over faster, with less battery voltage drop, and that 1-200 rpm can make the difference between starting a reluctant engine and not - especially when it's freezing cold, or stinking hot, and it's big engine, like yours, and most especially if running a lot of compression, initial advance or other loads like supercharger belts.

Thank you Gord for your input.

Regarding the ground wires, my plan is to use the engine block as a ground for the fans, for example, and for the fuel pumps was planning to run 1/0 cable from the battery to a junction block to the trunk area (battery is located behind passenger seat) and ground pumps to this terminal. It will be a relatively short run and have plenty of 1/0 cable left over. For other components, lower current draw, like the LED headlights will ground to the body (have a ground strap from engine block to body).

As to the starter, unfortunately I do not have the specs for it and am unable to locate. It is a GM mini starter P/N 19302919. I have seen bench tests of other GM starters where the peak inrush is ~35 amps and then drops to less than 10 amps. I am running 1/0 power cable to the starter motor from the battery (~8-10 feet) as the motor amp draw will be much higher.

Ah, when I was looking for some info' on the starter, there were several mentions of starter problems that were traced to the voltage drop in the solenoid feed wire being high enough to prevent the solenoid 'kicking' the starter into operation against the return spring, especially when hot - it's usually the main power feed that's the problem but the solenoid is just as important.

Even the official GM site doesn't seem to have any actual spec's on the starter solenoid currents, let alone basics like current/voltage and stall torque that is usually given in starter spec's - seems a bit "trust me", which usually means don't... 35A seems high, but it's certainly possible for the initial current - it's going to be better to over estimate than under, regardless, as it may be the reason for many vehicles being hard to start when hot and the solenoid winding are hot, further increasing resistance and current loss.

Yeah, as a precaution and to prevent that very issue of starter heat-soak, I use Thermotec or similar insulation wrap and if there's enough room I'll fabricate an aluminum coated heat barrier to help keep the starter cool. So far, it's worked and hope it will for this build as well. I'll certainly find out this summer in the 100degF+ Texas heat!

Thanks for this discussion Gord, very helpful! Now, going back to the Aeromotive 340 fuel pump, do you think it's going overboard to run dual 10 awg power wires to the pump since i'm traveling 17ish feet with the wire? The plan here would be to connect a very, short run of 16 awg wire to each 10 amp output and then step up to the 10 awg wire via a step-down butt connector.

According to their site - https://aeromotiveinc.com/wp-content/uploads/2020/05/115-6902-0revA.pdf - at a nominal 13.5V* AT THE PUMP it will draw ~12A for 50 PSI and 14A for 70 PSI pressure head, which would mean the proposed wiring is well within the spec'n unless it's a well built engine that needs all the fuel it can get. The TXL 16G is rated for that current from all the info' I can find. You didn't mention if you're also running the lift pump off the same circuit, which will add to it, though.

It's getting away from the PDM's advantages, but have you considered a slight change in your setup, with 2x10G from the battery (or alternator, but if you have a heavy gauge cable from the alt' to the battery the voltage difference should be minimal) to a HD relay (maybe solid state?) and from there to the pumps? That would free up at least one PDM 10A output and gives the option of freeing up both if a lower rated output is used to switch the relay - or switching directly from the ECU if desired.

*It states the power supply be taken off the alt' output, this is because that will have the highest voltage anywhere on the vehicle ('cept coils, of course), and because of wiring losses the battery voltage may be a bit below that, especially if running other higher draw electrical components.

Yes, I did see those specs however, the instructions state to use a 30 amp breaker and 10 awg wire. I presume this is for in rush amperage. See below.

The lift pump/Bosch 044 pump has a similar amp draw to the A340 pump (flows 276 LPH @ 43.5 psi at 12.8V drawing 11 amps) and I will run this pump on an independent circuit with same awg wire.

I do plan on using the PDM to run these pumps as I do have channels available to do so. It's my first time running a PDM and not going to lie, it will be exciting to make it all work!

It's going to be a really fun car and a really interesting project - I've a soft spot for Pontiacs, one of my first cars was a '61.

Thought about putting some "Pontiac" valve covers on from Butler, or other supplier? Bit $$$y but cool.

Nice! Yeah, me too. My Dad had a ‘71 Firebird Esprit with a 400 cu.in. power plant. Loved that car and had dreams of making that bird a TA with the chin spoiler, flares, and trunk spoiler one day. So, 30+ years later, here we are. Ha!

I haven’t seen the Butler valve covers in a while, but I think I know what you’re referring to. Currently, I have a set of tall Kaytech valve covers with the ign coils sitting on top. Not a fan of the location of the coils but it will work for now. Had to fab an aluminum plate to mount the M&W CDI coils because the stock Kaytech brackets for the OE LS coils was a no-go for fitment.

Back to work on her tomorrow. Have a few more holes to drill in the firewall for installation of the Radloc bulk head connectors.