Why Have a Fuse Box Ground Bar?

[Andrew Shadow]

I am curious about why so many of the auxiliary fuse boxes that are sold also have a ground bar included. Does not seem practical to me to run a power wire from a fuse box under the seat to an accessory at the front of the bike, and then run another ground wire the length of the bike all the way back to the fuse block ground bar. What is the advantage of this over simply running the ground wire to a solid engine/frame ground near the accessory that is being added?

I understand that using an existing ground wire is rarely a good option as you risk overloading that ground circuit. I am excluding that option in this scenario and I am assuming that all wiring and connections are proper. If a sufficiently sized wire is run from the accessory to a ground point on the engine/frame you will have a robust ground path back to the battery through the frame and negative battery cable.

The only advantage that I see is that you are adding additional ground carrying capacity back to the battery negative terminal assuming that you wire the ground bar of the fuse block directly to the battery negative terminal as is intended. I don’t see the need for this however. The negative battery cable is sized to handle considerably more load than what can be drawn by the starter motor and all of the other electrical equipment that is powered when the key is turned on and the starter button is engaged. Once the engine is running there is no more demand from the starter motor. This leaves a large unused capacity available in this cable. I would think that this available unused capacity in the negative battery cable far exceeds the capacity being added by running a # 10 wire (what typically seems to be recommended) from the fuse block ground bar to the negative battery terminal. If it isn't wired directly to the battery terminal it is definitely accomplishing nothing.

I cannot possibly imagine adding enough accessories to even approach, let alone exceed, the capacity of the negative battery cable. So why add another ground bar wired directly to the battery?
It seems to me that all it accomplishes is to make the fuse block bigger and more cumbersome without any significant benefit. The only advantage that I see is having a centralized and easy access point to all of the ground connections of your added accessories. I am not sure that this advantage outweighs the disadvantages of a larger fuse block, when space is at a premium, as well as all of the additional wiring that this requires.

What am I missing?

 

[Uncle Phil]

I don't know , I always get the ones without the ground bar. I guess it makes for 'purer' electrical connections.
I'm sure an electrical engineer will jump in here shortly and enlighten us and I'm sure there is probably a 'good' reason.
But I've managed 300,000+ miles on my ST1100s without one and my hair hasn't caught on fire yet … or my ST1100s.

 

[rjs987]

One BIG reason is that using the frame as a ground point is not a very good idea at all. Frame grounds tend to be iffy and the parts of the frame may not always connect reliably to ground. Using frame for ground is an idea held over from 50-60 years ago when there was much less knowledge about wiring a bike by those who did it themselves. Cars too, when it came to wiring a trailer. I've known many old codgers riding bikes and in cars who wired a trailer harness themselves and simply used the trailer frame and hitch ball/coupler as grounds. They all had electrical issues that they couldn't find... until some of them ran a dedicated wire to the battery for ground and then those issues evaporated. There are specific locations on the frame dedicated for grounding, but not the entire frame. And those locations usually are a bolt that also has a heavy wire tied to battery so it's not really the frame as ground but just a bolt that is tied to battery ground.

That said, I also usually make my own fuse block without a ground bar similar to what Uncle Phil stated, BUT I do add my own ground bar that is tied directly to the neg terminal on the battery. When I do this I simply use a 5 or 7 connection household circuit box grounding bar. Never corrodes and never a bad connection. My current scoot I installed an Eastern Beaver 3CS isolated (switched on with relay) fuse block setup that does come with a ground and plugs with both power and ground for each circuit. This fuse block, as most of the fuse blocks I've added to a bike, is located very close or right next to/on top of the battery. For running from one end of the bike to the other I will run a separate power wire as needed but one ground wire to share at the far end.

Again, DON'T use frame for ground. It's not reliable.

 

[dwalby]

The only thing I can think of is the more current you have flowing through any one ground wire, the higher the 'ground float' voltage across that wire. But from a practical perspective you'd likely need a lot of amps flowing through a single wire to create enough of a voltage drop to matter. Agree with the post that came in as I was typing that some frame ground points aren't terribly reliable either.

 

[thekaz]

It would help stop ground loops, can allow a properly sized ground wire and really helps in diagnosing issues

 

[ST Gui]

I imagine it's a redundancy that adds reliability. Fuse boxes are generally added to bikes because of added accessories. Tying them to a dedicated ground instead of relying of an existing frame ground with it's potential points of corrosion and possible voltage drops makes for possibly predictable points of failure.

The point about ground loops is something to consider when adding audio gear powered by the bike.

 

[Larry Fine]

When I added my own wiring upgrades to my '96 Nighthawk 750ST, I ran a #10 from each battery terminal, with an automotive fusible link in the + wire, to the headlight shell, and terminated each one on one of these:

I ran #12 wires from the + one to the headlight relays, the horn compressor relay, and the lighter socket (and ignition-coil relay one day soon ), and also #12 wires for the grounds for each of these items to the - one. I also added a small relay to disable the headlight relays when I hit the kill switch.

Here's why I call the Nighthawk an ST:

 

[oude geit]

Using the framework as a zero volt plane is not a decision of effectiveness, but efficiency, cost, space.

 

[SMSW]

rjs nailed it. In addition to wiring trailers and using the chassis through the ball as a ground (return path), I've seen guys ground stuff to the front forks an/or handlebars (think older Guzzi's, though probably not limited to one marque). Buying a fuse panel with no ground bar makes sense from only one pov. A given circuit might feed several items (e.g. three relays) with each one needing a ground or return path. Using something like Eastern Beav's PC-8 will quickly fill up the ground bar, so using either Larry's (post #8) or rjs's is the cheapest way to go. I always run a separate ground wire from the fusebox ground bar with the circuit conductor.

A couple of notes. Depending on what I am installing, I sometimes power all the relay's from one circuit - this comes from the so called control circuit you see on push button controlled industrial equipment. Relays rarely short out and blow a fuse, have very low current requirements and thus you can use smaller wire and a 5 amp fuse (or less). The other way to wire a circuit with a relay would be to power the relay and the equipment it controls. In this instance, all of the wiring must be capable of carrying the current allowed by the fuse plus a 20% safety factor.

 

[Bmacleod]

... also, sometimes engines mounted with vibration dampers (rubber mounted), grounds for items on the motor run the risk of inefficient grounds. If you have frame members bolted together, especially with differing materials (steel bolted to aluminum), there is a risk of bad connections in grounds also

 

[Trapperdog]

Those multiway connectors (are they yellow or blue ?)

One yellow under the nose (problematic), one pink wrapped in the harness under the tank.
Interesting that we do have one (3 wire?) oem frame ground unheralded the tank. I don’t remember where those wires go to but on one of my ST’s they were quite corroded.

 

[Andrew Shadow]

Good stuff- you guys are making me examine this from different angles. I still have not had an epiphany in seeing a significant technical reason yet so keep going.

In my original post I was attempting to set a stage where proper ground points are being used to allow for a strong connection to ground. i.e. no points that go through rubber mounts, use of existing ground wires, poorly or corroded connectors, undersized wires, etc.. The reason I did so is because I wanted to take shoddy workmanship and poorly designed circuitry out of the discussion because I am trying to determine the theory behind having an additional ground bar. Towards that end I edited the original post to read engine/frame rather than the previous frame ground. Potential failure points will always exist when grounding through the engine/frame is not done properly just as there is always the possibility of a poor connection and overloaded wiring on a fuse box ground bar as well.

One BIG reason is that using the frame as a ground point is not a very good idea at all. Frame grounds tend to be iffy and the parts of the frame may not always connect reliably to ground.

I don't recall ever actually noticing the connection point between the battery negative cable and the engine on the ST1300 so I don't know for sure but isn't there only the one connection point between the battery negative terminal and the rest of the electrical system? If so, every electrical item on the motorcycle uses the engine/frame as a ground path back to the negative battery cable on the engine on its path back to the negative battery terminal. Using your theory wouldn't this mean that the entire bike is not reliably connected to ground?

and simply used the trailer frame and hitch ball/coupler as grounds.

For the purpose of teasing out the technical advantage of a ground bar let us assume that no such shoddy workmanship has been employed and proper ground connections are being used.

The only thing I can think of is the more current you have flowing through any one ground wire, the higher the 'ground float' voltage across that wire.

No existing ground wires being used. In this scenario all accessories have their own properly sized wire directly to a strong ground point directing the current through the engine/frame on to the battery negative cable. Unless of course the one wire you mean is the negative battery cable.

can allow a properly sized ground wire

A properly sized ground wire is not exclusive to a fuse block ground bar. A proper sized wire can also be routed from the accessory to a proper ground point on the engine/frame.

Tying them to a dedicated ground instead of relying of an existing frame ground with it's potential points of corrosion and possible voltage drops makes for possibly predictable points of failure.

A dedicated ground does not eliminate the conditions that cause these potential failures and they can occur on the dedicated ground as well. No question, a dedicated ground certainly makes it easier to diagnose and repair a failure in the ground circuit and I understand those benefits. I am interested in knowing if there is an electrical reason for having one as opposed to the reasons of convenience.

You cover the point that all grounded current has to return to the battery, and you summise that the earth cable to the -ve post has enough capacity to handle all such additional loads. I really don't know if that is the case or not. Add a loud horn (20A), two 60w (10A) running lamps, two lots of heated gear (10A) it is easy to see how you could be adding another 40 amps to a system that is designed for only 60 amps. But leave that aside for now and lets assume that the cable has enough capacity.

It takes about 40 amps to run the ST1300 as equipped from the factory. In its stock form there is another 20 amps. or so beyond that of available power from the alternator. I couldn't find what the current draw of the starter motor is but generally speaking larger engine motorcycle starter motors are claimed to draw 85 amps and over. Given these numbers I am assuming that the battery cables must conservatively be good for at least 150 amps. but you are correct that I don't really know. I do know that when the starter is not drawing any current at all there is the same amount of current that it does draw available as unused capacity in the battery cables. I would argue that before you exceeded this capacity you would fry your alternator.

rjs nailed it. In addition to wiring trailers and using the chassis through the ball as a ground (return path), I've seen guys ground stuff to the front forks an/or handlebars (think older Guzzi's, though probably not limited to one marque). Buying a fuse panel with no ground bar makes sense from only one pov. A given circuit might feed several items (e.g. three relays) with each one needing a ground or return path. Using something like Eastern Beav's PC-8 will quickly fill up the ground bar, so using either Larry's (post #8) or rjs's is the cheapest way to go. I always run a separate ground wire from the fusebox ground bar with the circuit conductor.

For the purpose of teasing out the technical advantage of a ground bar let us assume that no such shoddy workmanship has been employed and proper ground connections are being used.

... also, sometimes engines mounted with vibration dampers (rubber mounted), grounds for items on the motor run the risk of inefficient grounds. If you have frame members bolted together, especially with differing materials (steel bolted to aluminum), there is a risk of bad connections in grounds also

Agreed, but I am referring to the use of quality connections made to proper ground points just as Honda has made- otherwise wouldn't the entire electrical system as delivered by Honda be suspect?

 

[Larry Fine]

I agree - with poor connections left out of the equation, the frame is a good ground. Much of a car's electrical system uses the body/chassis with no issues.

 

[Andrew Shadow]

I agree - with poor connections left out of the equation, the frame is a good ground. Much of a car's electrical system uses the body/chassis with no issues.

Ah, c'mon Larry! Now you have brought me back to square one- why have an additional ground bar?

 

[Al Gully]

Well here's my take on grounding back to the fuse box.
A friend that I ride with often was thinking about getting a secondary fuse box to run all his assorries too, so I popped off my seat to show him my bluesea 6 port fuse box.
I was thinking I was going to be a bit ashamed on my cable routing that I was going to show him. Much to my surprise, I had 5 positive cables and 5 negative cables going to the fuse box and it looked really neat and tidy.
You'll be glad that you ran your positive and negative cables back to your fuse box for aesthetics and for the ease of troubleshooting when that arises.
Also all the connections are brass eyelets and or stainless steel buss bars, rather than to a mild steel bolt and a mild steel frame that can corrode.
How can you find time to do it a second time when you didn't have time to do it the first time.

 

[ST Gui]

otherwise wouldn't the entire electrical system as delivered by Honda be suspect?

But isn't it? Haven't there been several posts about more than a few members experiencing electrical problems that have been fix by augmenting existing convenient ground points used by Honda?

It would seem that somebody could establish a dedicated ground point by virtue of location and cable size that would be less susceptible to corrosion and subsequent damage. I think that's an advantage of having a dedicated ground in a aux fuse box.

Not doubt there more than Unks data point of not having one and avoiding a fiery death. In all fairness unlike radar detectors there's no 'the ground bar saved my bacon/payed for itself' data points.

And maybe the only point that anybody can't (well they can I guess) deny is that a ground bar makes a pretty convenient place for the ground side of a device/light/etc since you're already running some V+ to it. And as thekaz mention it would help stop ground loops (which can induce hum in audio gear).

 

[Andrew Shadow]

But isn't it? Haven't there been several posts about more than a few members experiencing electrical problems that have been fix by augmenting existing convenient ground points used by Honda?

No, I don't believe the electrical system to be suspect at all.
The overwhelming majority of vehicles wired in this manner have electrical systems that function just fine the overwhelming majority of the time for the overwhelming majority of people. Sure there are problems that arise with these ground connections. But in all the posts that I recall it is the connector terminal that is the fault and not that the engine/frame is a bad choice of ground path back to the battery. A bad connection can occur anywhere and is always the weak link in any circuit- power or ground. There have also been a few posts of failed fuse block terminals and melted fuse blocks as well. This does not support an argument that inline fuse holders or frangible links should be used in place of fuse blocks. All the posts that I recall the issue was resolved by repairing or bypassing the defective connector terminal. In either method the result is the same in that it restored the ground path through the engine/frame back to the battery.

I don't argue the convenience of the ground bar for ease of troubleshooting and repairing ground issues related to added on accessories. What I am trying to understand is if there is any benefit to the supply of electricity simply because a ground bar was used as opposed to using the engine/frame as a ground path if all other variables are equal.
i.e. The accessories were correctly wired up and there are no bad connections, no corrosion issues, no overloaded circuits, no undersized wires, etc.. Under these circumstances does the ground bar in and of itself provide superior distribution of the electricity? I don't see why as of yet. The only benefit I have read so far, apart from the above mentioned troubleshooting, is the ground loop issue for audio equipment and I know nothing about that whatsoever so I can't even offer a comment on it.

 

[st11ray]

I just run everything back to my EB PC8. Been that way for years. If it ain't broke, don't fix it.

 

[SMSW]

I don't argue the convenience of the ground bar for ease of troubleshooting and repairing ground issues related to added on accessories. What I am trying to understand is if there is any benefit to the supply of electricity simply because a ground bar was used as opposed to using the engine/frame as a ground path if all other variables are equal.

One quick and easy answer to your (bold) question is copper has lower resistance than a steel frame. Running a separate return path all the way back to a ground bar from the utilization equipment (aux. light, horn, what have you) gives you a low resistance path back to the battery. There is very little chance of corrosion/deterioration/increased resistance at connection points because there are only two - one at your horn and one at the ground bar at the panel. Simple is better, both for electricity and troubleshooting. That said, I don't deny umpteen quazillion cars and bikes function just fine, thank you, using frame/engine as a ground point. (Just because Ben flew a kite during a thunder storm does not mean you should try that too.) My best guess here is that economics comes into play - it is cheaper/easier to run a short wire from your horn to the frame than all the way back to the fuse box. Weight is also saved (read materials, and therefore money). More than a few practices today are a result of 'good enough' engineering that are hold overs from the days when we did not understand the systems as well as we do today.

Another point from standard wiring methods is a ground must have a dedicated connection - one is not permitted to use a screw whose function is holding say a cover in place as a ground point. Using the engine/frame as your ground path depends to a great extent on a myriad of connections and paths through dissimilar metals (steel to aluminum engine block to stainless steel bolt to frame to copper cable to battery) that can lead to galvanic corrosion, corrosion due to road chemicals, and plain old rust. Motor vehicles operate in a damp/wet environment and are subject to loosening vibrations and corrosive conditions not usually found inside a home or building.

And, Andrew, don't forget that trouble shooting and reliability are important aspects to what we are discussing. I would argue that a single ground bar/termination location makes troubleshooting easier and more reliable by eliminating potential corrosion/high resistance points throughout the vehicle. The National Electric Code (which is not applicable here but has some important lessons) requires all splices to be accessible. Under the NEC's definition for accessible, removing a fairing, removing the dash and digging deep into the bike to reach a melted connector does not qualify as 'accessible'. Yes I know this was not part of your bold question above.

A quick point. I am getting into some shaky territory here terminology wise. Line voltage wiring differentiates between a Grounded Conductor (the neutral or return path for AC) and a ground (a non current carrying conductor (well, that is until something goes wrong which is when the ground does something). We have a DC low voltage system on motor vehicles, and are using the frame/engine as a return path - or a non insulated current carrying conductor. This works because the voltage is low (ever hold a spark plug while cranking the engine? higher voltage can lead to = ouch) and direct.

 

[ReSTored]

Interesting discussion. I think in the grand scheme of things the incremental time/work effort/resources in running a second wire (the ground wire) when installing a fuse box or a new accessory to an existing fuse box is absolutely minimal and, as it eliminates any future issue of a poor local frame ground, is well worth it.