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Jeep CJ7 Wiring Series- MEGA Fuse Location

Over the next few videos, we’re going to continue our theme on good planning.  Specifically, we’re going to talk about picking the best locations for the major components that come with your Infinitybox system.  We’re installing our 20-Circuit Kit with inLINK and inRESERVE in our 1979 CJ7.  To get the most out of our install, we want to pick the best locations for the MASTERCELL, the front & rear POWERCELLs, the MEGA fuse holder and the inRESERVE solenoid.  We’ve broken this up into 5 different videos, talking about what you need to consider for each part.  This video covers picking the best location for the block of MEGA fuses that comes with your 20-Circuit Kit.

 

 

Everything starts with running primary power from the battery.  The POWERCELLs get powered by cables that connect back to the positive terminal of the battery.  These cables need to be protected against short circuits by fuses.  Your 20-Circuit Kit includes a block of 4 high-current MEGA fuses to protect the four 8-AWG cables that bring battery power to the POWERCELLs.  If one of these cables was to short to ground, the MEGA fuse becomes the weakest link in the electrical chain.  The fuse opens to protect the cable from damage.  You want the MEGA holder to be mounted as close to the battery as possible to minimize the length of unprotected cable.

We’re going to mount the MEGA fuse block right in front of the battery, under the hood.  There is a flat spot on the inner passenger fender that would be a great place.  It is a short run of cable from the positive terminal to this location and we can easily route the 8-AWG power cables to the front & rear POWERCELLs.  Watch this video to learn more about these and see where we’re mounting the MEGA fuses in the Jeep.

Picking the best location for the MEGA fuse holder will get you a safe and reliable electrical system in your car or truck.  It will also make running the power cables to your POWERCELLs easier if you plan in advance.  Keep watching for more in our 1979 Jeep CJ7 Install Series.

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Mounting Fuses

Here’s the first step in our 1967 Mustang wiring project.  It is mounting the fuses that protect the power feeds going to the POWERCELLs.  In our 10 and 20-Circuit Harness Kits, we give you a block of Littelfuse Mega fuses to protect the 8-AWG cables that power the cells.  In our Express Racing Kits and with our inMOTION cells, we give you a JCASE holder instead of the Mega block.

These fuses serve a very important purpose.  They protect the 8-AWG cables from short-circuits or low overloads.  The fuse in any circuit is supposed to be the weak link, electrically.  They are thermally operating devices that are designed to open and clear the circuit if too much current flows.  Wires can be fuses too, which is a bad thing.  If too much current flows through a wire, it will heat up.  As the wire heats up, the insulation can melt causing fire or other damage in the car.

As mentioned above, the fuses ultimately protect against two things.  The most common is a short circuit to ground.  If the insulation on the wire were to become damaged and the conductor were to touch the chassis, you have a very low resistance path to ground.  The insulation can become damaged over time by rubbing against a sharp edge or can be connected to ground quickly in an accident.  You can also accidentally drop a tool across an open set of terminals and make a good short to ground.  (We’ll admit that we’ve done that before.)  The second thing that the fuse protects against is a low-overload.  These are usually resistive connections to ground.  They are not as common as a short circuit.

Car batteries have a tremendous amount of stored power in them.  A low resistance path to ground will allow a lot of current to flow.  This current flow causes heating in the wire.  As mentioned above, the fuse is designed to open (blow) before the wire gets hot enough to cause damage.  The Mega fuses in your kit are sized at 60-Amps.  A 60-Amp fuse is the proper rating to protect an 8-AWG cable.

We have many different blog posts about fusing.  Here is a good one for your reference.

In the case of this 1967 Mustang, the battery is in the trunk.  The customer mounted the Mega fuse block against the trunk wall, towards the driver’s side of the car.  Here’s a picture of the fuse block mounted in the trunk.

Location of Mega Fuse Block in 1967 Mustang wired with the Infinitybox system.

Location of Mega Fuse Block in 1967 Mustang wired with the Infinitybox system.

You want to make sure that the Mega fuse block is securely mounted in the car.  There are mounting holes on the top and bottom of the holders.  We recommend using nuts and bolts with lock-washers to mount this to a flat surface like the one shown in the picture above.  Self-tapping screws will work too but aren’t as reliable as a bolt & nut with a lock-washer.

You want to have reasonable access to the primary fuses in the car.  Over its lifetime, you should really never have to change these fuses.  If you ever were to blow one, that means that something bad has happened.  You have either been in an accident and one of the cables has broken to ground or the insulation on one of the cables was worn over time.

You want to mount the fuses as close to the battery as possible.  The purpose of this is to minimize the length of unprotected wire in the circuit.  The unprotected wire is the wire between the power source and the first fuse.  We give you the Mega fuse block, the ring terminals, the 8-AWG cables and a 4-AWG cable in your kit.  You use the 4-AWG cable to connect the battery to the buss bar on the fuse block.

This picture shows the 4-AWG wire connected to the buss bar on the Mega fuse block.  The customer crimped the included ring terminal on the end of the 4-AWG wire and connected that to the buss bar.  The other side of this 4-AWG cable connects to the positive side of the battery.  This connection can be made directly to the battery, to a primary disconnect switch or to our inRESERVE battery management solenoid.  Please note that we strongly recommend a battery disconnect, either a manual switch or our  inRESERVE kit.

Also, note that they used heat shrink tubing to protect the exposed part of the terminal.  This is recommended to reduce the chance of shorting this side of the fuse block to the chassis.

Primary battery cable connected to Mega Fuse block in 1967 Mustang wired with the Infinitybox system.

Primary battery cable connected to Mega Fuse block in 1967 Mustang wired with the Infinitybox system.

Just a quick warning about connecting the Mega fuse and 8-AWG power feeds.  You want to connect all of the POWERCELL feeds back to the same Mega fuse holder.  This Mega fuse holder should be connected to the battery either directly or through a disconnect solenoid.  You must not connect power to your Infinitybox system to the starter cable at the starter motor.  This is our recommendation and that of most other electronics manufacturers.  There are surges and transients associated with the starter motor that could interfere with your ability to start the engine if you are powering the system from the starter cable at the starter motor.

Once the 4-AWG cable is connected, you need to connect the four 8-AWG feeds that go to the POWERCELLs.  Each POWERCELL needs two of these feeds.  In the case of this Mustang, two of the 8-AWG feeds route to the POWERCELL in the trunk.  The other two 8-AWG feeds run to the front of the car to connect to the front POWERCELL.  This picture shows the 8-AWG cable connected and the covers snapped in place on the Mega fuse block.  You must keep these covers in place to protect the terminals on the fuses from getting accidentally shorted to ground.  (Again, we’ve done that before with a wrench.)

Assembled Mega Fuse block in 1967 Mustang wired with our Infinitybox system

Assembled Mega Fuse block in 1967 Mustang wired with our Infinitybox system

Make sure that all of the bolted connections are tight.  Don’t over-tighten these bolts.  The recommended bolting torque is 10 foot-pounds.  Once that’s done, you’ve finished the job of mounting the fuses.  Stay tuned for the next step in the process of wiring a 1967 Mustang with our Infinitybox 20-Circuit Harness Kit.  Click on this link to contact us with comments and questions.

Picture of the Littelfuse Minifuse

Why A Fuse?

We get this question a lot.

Why do the POWERCELL and inMOTION cells use fuses to protect the outputs?  Some say that we’re already using smart MOSFETs, why not rely on them to protect against over-current on the output wires?  Others ask, why not use circuit breakers?  There are several important and practical reasons why we rely on a traditional fuse to protect the outputs on our Infinitybox hardware.

With respect to MOSFETs, they are tried and true technology.  They are used in practically every application for switching and current control.  The MOSFETs used in the POWERCELLs are automotive-grade and designed to carry up to 270-amps.  The chances of them failing in a typical customer car are minute.  However, if a MOSFET fails, it doesn’t fail gracefully.  If a MOSFET were to fail, it fails resistive.  That means that they are going to generate a significant amount of heat, quickly.  No intelligence built into the circuitry can interrupt the current flow if the silicon die in the MOSFET package has become a resistor.  In that case, the fuse is your last line of defense to protect the system from thermal runaway.

You can use the same logic for inMOTION.  It has sensors on the board that monitor the total amount of current flowing out to the load.  inMOTION already shuts off the relay coils when it sees too much current, why not rely on that alone without the fuses?  The practical reality is that a common failure mode for relays is for the contacts to weld together, especially in inductive motor application.  If this were to happen, no intelligence on the inMOTION board can open the circuit.  Again, the fuse becomes the last line of defense.

With the above being said, you can see the need for some circuit protection component in the system if other components fail.  That leads to the debate between fuses and circuit breakers.  Circuit breakers can certainly do the job, however a fuse will out perform a breaker in all categories.

  1. Fuses are more cost effective than circuit breakers.
  2. Fuses are smaller and package better than circuit breakers.
  3. Fuses have better low-overload protection characteristics than circuit breakers.  This is very important in cases of resistive shorts like a failed MOSFET.
  4. Fuses respond quicker than circuit breakers under short-circuit conditions.

Most importantly, a fuse drives better end user behavior than a circuit breaker.  At the end of the day, there is a reason why a fuse opens or a breaker trips.  A motor has failed.  Insulation on a wire has shorted to ground.  An electronics module has failed.  In all these cases, you do not necessarily want to blindly reset a breaker and keep on driving.  The cause of the over-current condition needs to be identified and fixed before resuming normal operation of the vehicle or equipment.  Otherwise, more severe thermal events can occur in the wiring harness causing damage or injury.

See our blog for additional posts on proper fusing and other circuit protection lessons.  If you have any questions, reach out to our power distribution experts at (847) 232-1991 or email our team at sales@infinitybox.com.  You can also click this link to contact our team directly.