Ampacity: I'm Not As Smart As They Think I Am

[ZephyrEFI]

So, I got this Dakota Digital fan controller. It came with a couple of things I'm not used to. First, there is no harness, and second, it's apparently up to you to figure out what size wire and fuses to use. Maybe they figure these are usually going to be installed by pros, but I am in the dark. And I like to think I'm good at electrical!

It did come with directions, but they mostly focus on HOW to wire it, and program it. Here's a link.

http://www.dakotadigital.com/pdf/650411d_PAC-2750.pdf

So, apparently everything going into and out of the controller itself will be no more than 1.5 amp. And that's just from what Jsfrv6 told me, I certainly wasn't able to glean that from the directions myself. So, anyway according to what ampacity charts by wire size charts I've seen, 14-16 gauge wire should be fine for the controller leads. Before buying, I talked to a guy at DD who told me the controller main power lead needs a fuse of 20 amp. So, that all seems to make sense.

But then we have the relays. The controller came with one 70 amp relay, and I bought a second one to use for the fan high setting. The controller uses ground side switching. So for the relays we have 2 power leads each coming in. One for the relay coil and one for the fan. Do they both need fuses? What size? 70 amp?! According to the wire size charts, you need THREE GAUGE wire to carry 70 amps. The relays DID come with harnesses, and there certainly aren't any 3 gauge leads.

The controller directions say you need to find out the amp draw of your fan. Everywhere I've looked about my fan (Cooling Components 17 Series), it says to use 70 amp relays. I also found this about it: "These fans draw 60+ amps on initial startup and have an operating amperage draw of 16-22 amps." So then do I wire it for the operating amp draw? It looks like 8 gauge wire would work for that, and then maybe go with a 25 or 30 amp fuse?

It just seems really strange to me they wouldn't have precise directions about this, leaving it up to the installer. I would think they'd be concerned about having warranty claims up the wazoo. Apparently not though. And the guy I talked to on the phone answered the questions with a sort of "why don't you know this" attitude.

[Ethyl Cat]

Find somebody with an inductive amp clamp and test it for actual amp draw. This speculation is just going to complicate things.

I find it hard to believe it needs a 70 amp relay.

[Ethyl Cat]

Most relay windings only draw about 200mA

[Ethyl Cat]

If that thing draws 70 amps , you better get a big capacitor for like a stereo sub to prevent dimming of lights, alternator issues, etc....

[grabbergreen84]

But then we have the relays. The controller came with one 70 amp relay, and I bought a second one to use for the fan high setting. The controller uses ground side switching. So for the relays we have 2 power leads each coming in. One for the relay coil and one for the fan. Do they both need fuses? What size? 70 amp?! According to the wire size charts, you need THREE GAUGE wire to carry 70 amps. The relays DID come with harnesses, and there certainly aren't any 3 gauge leads.

I don't think you need a fuse between the fan motor windings and the relays in the secondary circuit. If there is a problem with the fan, it might fry the relay, but that relay will continue to protect the controller, due to the isolated primary and secondary circuits. You do want the relays close to the fan, if running that circuit unprotected. Looking at some factory systems' schematics might give you an idea.
I would probably want a fuse between the controller and the primary circuit (coil) sides of the relays, and close to the controller. Dunno if Dakota specs it that way though.

The controller directions say you need to find out the amp draw of your fan. Everywhere I've looked about my fan (Cooling Components 17 Series), it says to use 70 amp relays. I also found this about it: "These fans draw 60+ amps on initial startup and have an operating amperage draw of 16-22 amps." So then do I wire it for the operating amp draw? It looks like 8 gauge wire would work for that, and then maybe go with a 25 or 30 amp fuse?

You want to select a wire gauge fatter than just for the operating/continuous draw, in order to compensate for the spike-draw when the fans start. Since the spike lasts an infinitesimally short duration of time compared to fan run time, you can get away with thinner wire than you could if that 60 or 70 amp spike were a continuous draw. I'm sure real electricians have a go-to equation for selecting the gauge, based on these considerations. Whatever gauge wire runs from your fan's harness is probably OK.

[brianj]

70 amps continuous is a whole different thing than 70 amps at start. Quite a few factory fan set ups draw around the same numbers, and they do not run 3 ga wire! Basically, just what grabber just said, I'm just dumbing it down to my level! Lol

[Ethyl Cat]

I stand corrected, it will draw 70 amps for about 250mS.

I should have related my fuel pump testing on rush in current to this. A fuel pump that pulls about 7 amps constant at 13.5V will have a rush in of 20-24 amps. That is roughly 3- 3.5 times the running current. If we take the midpoint of the running current you posted (19.5A) and multiply it by 3.2 it ends up 62.4 amps.

I found some other testing that backs this up and attached it. Most cooling fan circuits are fused at around 40amps. you can get slow blow maxi fuses .

I also attached a pdf from Littlefuse corp. In it it states that it takes about 2.2 seconds of 100 amp current to blow a 30 amp maxi.


Also there is a section on sizing the fuse for the circuit in the Littlefuse doc.

Littelfuse_Application Note Slow vs Fast Auto Fuses.pdf

Current rush in testing.pdf

This should be enough to answer all your questions. Happy reading!

Steve

[ZephyrEFI]

Hahaha, "slow blow"... Thanks, Steve, I'll give those a read!

I don't think you need a fuse between the fan motor windings and the relays in the secondary circuit. If there is a problem with the fan, it might fry the relay, but that relay will continue to protect the controller, due to the isolated primary and secondary circuits. You do want the relays close to the fan, if running that circuit unprotected. Looking at some factory systems' schematics might give you an idea.
I would probably want a fuse between the controller and the primary circuit (coil) sides of the relays, and close to the controller. Dunno if Dakota specs it that way though.

Okay, so here's a diagram from the directions.



You're saying I don't need a fuse on the black circuits, right? I get that. But what about the red? It's a lead coming straight from the battery. The switching is done on the white (grey) circuits, the ground side. "Fan High" and "Fan Low" refer to leads coming from the controller. The directions do say the "Fan Power" green circuit does need to be fused, but what capacity is one of the questions here. They don't say anything about the red circuit.

[Ethyl Cat]

If you don't and they go to ground before the relay you could have a problem...

Put a 1.5 or 3 amp in there. More than enough

[jsfrv6]

You will need to size your wires to handle the "constant" load and then fuse said wire to protect in case of a short....
Here's a chart..
http://www.offroaders.com/tech/12-vo...gauge-amps.htm
In your situation I would use a 10 gauge wire with a 40amp fuse/breaker for the fan supply. The red wire in your diagram..
I used this breaker for the fan power... Got it from Oreillys
http://www.delcity.net/store/Auto-Re...!-12V/p_198669
I ran one 10awg wire from the battery to the breaker and two 10awg wires from the breaker to the relays. One to each. You will never use both relays at the same time so this will suffice...

Your black wire will be protected by this breaker as well so no need to add fuse there.

I used a 14 awg wire for the relay power, your green wire, and fused it with a 10amp fuse.
I used a 14 awg wire for the relay trigger, your gray wire, with no fuse. The fuse for the relay power will offer protection should it need it..

Put your fuse/breakers as close to the power source as possible.

Hang in there.. You will get it an learn some valuable skills in the process...

[Ray Dog]

The red circuit will only see the current of the relay coils, around 1/4 amp.
The green circuit is the one that will see the current draw of the fan motor
one speed will draw more than the other. A clamp on ammeter was suggested to find the current draw, but you need one made for DC.
An AC meter will not work.
I have checked our cars with electric fans. the Saturn has one fan and that has a 30 amp fuse. The Magnum uses 2 and has a 50 amp fuse.
Doing a few reverse calculations on motor fuse sizing at around 150 to 175% of motor full load current., I would take a guess around 14 to 15 amp draw
for a fan motor. I would think a number 10 wire size would do the job.
The relay is sized at 70 amps to take the initial current draw at start. An electric motor is about as close to a direct short as you can get at start up.
Once at speed it actually is generating a reverse voltage at speed which is less than supply volts. They cancel out so the resulting low voltage with the low winding resistance gives a fraction of the starting current for a running current.
That's why the lights dim when the frig or the ac kicks on.
Sorry for the electrical theory ramblings.
I used to get a kick when I started the cardboard compactor at an interim job I had between my real jobs.
You could hear the wires jump in the conduit when I hit the start button. It was probably a 40 or 50 HP motor.

[ZephyrEFI]

You will need to size your wires to handle the "constant" load and then fuse said wire to protect in case of a short....
Here's a chart..
http://www.offroaders.com/tech/12-vo...gauge-amps.htm
In your situation I would use a 10 gauge wire with a 40amp fuse/breaker for the fan supply. The red wire in your diagram..
I used this breaker for the fan power... Got it from Oreillys
http://www.delcity.net/store/Auto-Re...!-12V/p_198669
I ran one 10awg wire from the battery to the breaker and two 10awg wires from the breaker to the relays. One to each. You will never use both relays at the same time so this will suffice...

Your black wire will be protected by this breaker as well so no need to add fuse there.

I used a 14 awg wire for the relay power, your green wire, and fused it with a 10amp fuse.
I used a 14 awg wire for the relay trigger, your gray wire, with no fuse. The fuse for the relay power will offer protection should it need it..

Put your fuse/breakers as close to the power source as possible.

Hang in there.. You will get it an learn some valuable skills in the process...

Yeah, I assume you mean red when you say green, and vice versa, right?

I plan to put the relays and their fuses/breakers right next to the battery under the coil cover. Should be a good spot, and keep them away from trouble like my last controller found when I had a coolant overflow during bubbling/topping off.

[jsfrv6]

Yeah, I assume you mean red when you say green, and vice versa, right? .

Yep... My bad..... But looks like you got it!

[ZephyrEFI]

Yeah, I think I'm good to go! Thanks all! Now, I just have to find time to do it. Probably won't be until next weekend...

[grabbergreen84]

The green is the trigger/coil/primary circuit side, and you want those fused as close to the controller as possible, as they are the power wire for the primary circuit, which makes the controller like a battery.

[Ethyl Cat]

The green is the switched fan power. Those wires do not go to the controller they go to the fan.

If we were talking about the red wires, which are the primary/coil/trigger wires, how would a fuse at the controller protect the circuit from a short pre- relay?

That would really be the only reason to put a fuse in the circuit, as a short post relay would only result in the fan staying on and no real damage to the circuit because the current would be limited by the winding resistance.

[jsfrv6]

The green is the trigger/coil/primary circuit side, and you want those fused as close to the controller as possible, as they are the power wire for the primary circuit, which makes the controller like a battery.

This system works a little different. You will have a fused constant or keyed power connected directly to the relays and the ground wire from the relay to the controller. The controller will switch the ground to control the relay...

[jsfrv6]

Attached is a link to an article about wire sizing.
http://www.aeroelectric.com/articles/wiresize.pdf
This guy, Bob Nuckolls, aka Lectric Bob, Is an electrical genius. His forte is aviation but is applicable to automotive as well. I built and flew an experimental aircraft and read volumes from this guy. You need to have all your duck lined up when dealing with aircraft for obvious reason. So check it out. You will learn a thing or two..

[Ray Dog]

A fuse in the relay coil circuit prevents wires burning up if the relay coil or the the wires to the coil short out.

[brianj]

I used to get a kick when I started the cardboard compactor at an interim job I had between my real jobs.
You could hear the wires jump in the conduit when I hit the start button. It was probably a 40 or 50 HP motor.

I watched a 115kv highline get energized against a dead short. (The line was grounded). The physical energy imparted into the grounding cables was enough that they jumped into a fiberglass bucket truck beside them, and snapped the bucket off. Pretty serious energy involved, and a significant screwup by someone!

[grabbergreen84]

The green is the switched fan power. Those wires do not go to the controller they go to the fan.

If we were talking about the red wires, which are the primary/coil/trigger wires, how would a fuse at the controller protect the circuit from a short pre- relay?

That would really be the only reason to put a fuse in the circuit, as a short post relay would only result in the fan staying on and no real damage to the circuit because the current would be limited by the winding resistance.

Ah OK, I was reading the schematic wrong. (Assumed the red was the high current side.) I still like a fuse between the relay coil and the controller, and close to the controller - unless shorting the controller's switched ground wires to the vehicle/battery ground is safe, and won't fry the controller. (Thinking of some car audio systems which instruct that you ground the speakers only to the speaker grounds on the amp and not straight to the car.)

A fuse in the relay coil circuit prevents wires burning up if the relay coil or the the wires to the coil short out.

That's how I see it too. Green comes from the battery, and flows through the secondary circuit to the fan motor windings, while the red comes from the controller and powers the primary circuit.
If there's a short in the primary circuit, you want the circuit to open somehow before the controller opens it up - although it might well be internally protected.

[ZephyrEFI]

Actually, the grey comes from the controller. The red also comes from the battery. Ground side switching. Sorry, that's not very clear on the diagram. "Fan Low" and "Fan High" refer to leads from the controller.

I sent this email to Cooling Components.

"Hello

I have one of your 17 series 2-speed fans, and I just bought the Dakota Digital PAC-2750 fan controller. I see it is one of your recommended units. I did buy a second Dakota Digital 70 amp relay to use for the Fan High setting as well. I'm wondering if the way Dakota Digital recommends you wire a single fan for high and low with two relays is the same way you recommend doing it. I've attached their diagram. I got the idea somewhere that the low and high positive leads to the fan motor don't work in the typical manor with your fans, so that's the reason I ask.

Secondly, neither Dakota Digital's nor your literature that came with fan, controller, and relay recommend a size of fuse or a gauge of wire to use for any of the circuits. Putting aside the wiring for the controller itself, I assume they will all be pretty low amperage, I'm mostly concerned about the relays. For the red and white circuits (see attached diagram), again, I figure the amp load will be pretty low so, something like a 5 amp fuse on the red lead should do fine. For the green Fan PWR lead, which will provide power to the fan motor itself, I've seen max amp draw for your fan of ~60 amps, and a constant draw of ~18-22 amps. I've seen recommendations of the stated constant amp draw as large as 8 gauge. What are your thoughts on that? And what of the max amp draw? Do I go even bigger for that reason? What about the fuse? Do I fuse it for the constant draw, or go larger because of the max draw? I would like to use a circuit breaker to protect this circuit. I'm thinking 25, 30, or 40 amp. What do you think? Also, should I protect each relay green lead (high and low) separately with its own dedicated circuit breaker?

Thanks for your time"

I found this other potentially helpful bit of information on Speedway Motors page for Cooling Components fans: "This fan has two speeds. It has two positive wires going to the motor, one is gray and one is white. If both recieve 12 volts, the fan will be on high speed. If only one recieves 12 volts, the fan will be on low speed. The black wire is ground."

So, that says to me, wiring it the way Dakota Digital suggests (the diagram from above) won't work. Maybe the one relay should get the "Fan Low" grounding lead, and the other relay should get both "Fan Low" and "Fan High"? Would that even work?

[jsfrv6]

So, that says to me, wiring it the way Dakota Digital suggests (the diagram from above) won't work. Maybe the one relay should get the "Fan Low" grounding lead, and the other relay should get both "Fan Low" and "Fan High"? Would that even work? :confused

It will work fine with that fan set-up. You will need to use the "Two single speed fan" mode... That uses one relay for low speed and both for high speed... See below..

[ZephyrEFI]

Cool! I missed that, thank you!

[ZephyrEFI]

Anyone here have any thoughts on wire gauge size when considering "for chassis wiring" vs. "for power transmission"? It's a pretty big difference, and it's pretty much the only thing throwing a wrench in the works right now. I mean, what's the difference? Chassis wiring carries power. I'm confused. The chart I found says 8 gauge wire will carry 73 amps "for chassis wiring", but only 24 "for power transmission".