I have a permanently-mounted 25-gallon roto-molded plastic gas tank for my outboard motor. Gas sloshing in a tank (even a plastic one) can generate static electricity. Gas flowing from a fill hose into the tank can do the same. If the electricity sparks, it can ignite the gas fumes, causing an explosion. Grounding the parts should keep that from happening.
Here’s how I plan to ground the tank parts:
- Run a green, 10 or 12 gauge stranded wire from the deck fill to the fill fitting on the tank.
- Run the same type wire from the fill fitting on the tank to the outboard (somewhere) as an ‘earth-to-ground’ connection.
- Connect the fuel tank sender to the gas gauge as in the diagrams below.
- Unknown: connect the ‘earth-to-ground’ wire to the fuel tank sender negative wire?
My tank is made by Moeller Marine. Here are sections from their literature. They left a lot of questions unanswered:
- They don’t show the deck fill fitting being grounded at all. Everyone else says this is the most important thing to ground. See the Coast Guard regulations below.
- They don’t connect this ‘earth-to-ground’ wire to the sender. The Coast Guard regulations say to bond ‘each metallic component’ together.
- Others say to ground all the parts to the battery negative, not just the sender.
- They don’t suggest a wire gauge, for grounding, or for the sender wires.
Here are the Coast Guard regulations.
USCG BOATBUILDER’S HANDBOOK
183.572 – Grounding (http://www.uscgboating.org/regulations/boatbuilder_s_handbook/fuel_standards_partp.aspx)
Each metallic component of the fuel fill system and fuel tank which is in contact with fuel must be statically grounded so that the resistance between the ground and each metallic component of the fuel fill system and fuel tank is less than 100 ohms.
Fuel flowing from the dispensing nozzle into a fuel tank is a potential source of a static electric charge which could cause a spark between the dispensing nozzle and metal component of the fuel tank fill system. To prevent such a spark from occurring, metallic components of the fuel tank fill system and metallic fuel tanks must be grounded.
Grounding or bonding may be accomplished by connecting the metallic components electrically by running a wire from one component to the next, and so forth to the boat’s ground. Grounding can usually be accomplished by a connection to the common bonding conductor or the engine negative terminal.
If the fuel tank deck fill fitting is nonmetallic, and nonconductive hose is used as a fill pipe, there is no need for grounding the fill fitting. Chrome-plated plastic fill fittings are treated the same as metallic fittings.
1. If a metal hose attachment fitting is used, it must be grounded.
2. Fill cap retaining chains need not be grounded.
And here are some of the most reasonable-sounding snippets on the topic I could find:
Here’s why it’s a good idea to connect it to the negative terminal on the battery. You would do this to create an inductor that will absorb the energy of a static discharge and thereby create a path of lower resistance. This is called a floating ground and is the most common type of grounding found in marine vessels.
You are putting a plastic tank in an aluminum boat. If a metal fill and metal vent are used, they need to be grounded, as well as any lengths of metal tube in the fill or vent hoses. The plastic does not need to be grounded. Grounding (according to the Coast Guard regs) should be done with marine grade 14 AWG green jacketed STRANDED wire.
On boats without a grounding plate, the chassis of the outboard motor is considered the ground to the sea. Usually the outboard motor will have some bare metal anodes in contact with the sea. Even if the motor is tilted up, usually there are anodes on the mounting bracket that are still in contact with the sea.
All metallic components of the fuel system should be bonded together with a 10-AWG wire with green insulation. The fuel system ground should be connected to either the vessel ground plate on the hull or, if the vessel does not have a grounding plate on the hull, to the battery negative terminal. It is assumed the battery negative terminal is bonded to the chassis of the outboard motor.
If you have a fuel gauge for the tank the sending unit must be grounded or the gauge won’t work unless it is a gauge in the cap of a plastic tank that is mechanical. If this is a aluminum boat make sure to ground to the ground bus or negative terminal do not use the boat as the ground source. You probably should bond the SS filler neck to ground also use a hose clamp if you can’t bolt it somehow.
If you look at factory tank installations, this is what you will probably see:1. The tank is grounded itself to the battery either via a fuel gage or a separate wire.2. The deck fuel filler receptacle is grounded to the boat battery and/or the tank.
The most important ground is between the metal deck fill and the fuel/tank so that there won’t be a spark between the nozzle and deck plate when fueling. All tanks, but perhaps the plastic ones more than others, tend to build a static charge as the fuel sloshes.
Edit: July 16 – updated image with new version – see comments by Ike below
This seems useful:
Installing a new fuel gauge
Installing a new fuel gauge may not seem like a cause for celebration, but if you install and wire it yourself, you’ve saved enough money to throw a couple of very good steaks on the grill. Wiring a fuel gauge is much the same as wiring any other gauge on your boat: one wire comes from the ignition to the instrument, one wire comes from the sensor to the instrument, one wire comes to the instrument light and one wire from the instrument goes to the boat’s common ground. The wires may follow a seemingly unusual path, but the system works.
Step 1 Disconnect your boat’s battery. Look at the back of the gauge. There are four male blade terminals, one for the lights (marked "L"), one for the power (marked "I"), one for the ground (marked "-") and one for the wire from the fuel tank sender (marked "S")
Step 2 Slip the female blade terminal of the stranded copper wire onto the "L" blade terminal of the fuel gauge. Remove the blade terminal for the instrument light of the instrument nearest the fuel gauge. Slip the stripped end of the stranded copper wire from the "L" terminal of the new fuel gauge into the female blade terminal for the instrument light for that existing instrument. Replace the female blade terminal on the blade terminal for the instrument light of the instrument next to the fuel gauge. This is called "daisy-chaining" the instruments.
Step 3 Daisy-chain the "I" wire of the fuel gauge to the power terminal of the instrument next to the fuel gauge. Daisy-chain the "-" wire to the ground wire–which may be also be marked "-" or "GND"–of the instrument next to the fuel gauge.
Step 4 Pull the wire from the fuel sender up through your boat’s dash. Slip the female blade connector on the wire from the fuel sender onto the "S" terminal of the fuel tank gauge. Install the newly wired fuel gauge in your boat’s dashboard.