Boatbuilding Blog

July 12, 2010

Grounding a Plastic Gas Tank

Filed under: Building - Electric & Plumbing — tomlarkin @ 1:50 pm

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:

  1. Run a green, 10 or 12 gauge stranded wire from the deck fill to the fill fitting on the tank.
  2. Run the same type wire from the fill fitting on the tank to the outboard (somewhere) as an ‘earth-to-ground’ connection.
  3. Connect the fuel tank sender to the gas gauge as in the diagrams below.
  4. 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:


Moeller Wiring Diagram


My notes:

  • 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.


183.572 – Grounding  (

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.

Grounding (Bonding): All metallic components of the fuel system that are in contact with fuel must be electrically grounded to prevent a static discharge from causing a fire or explosion.

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.



  1. Thank you for using my diagram to illustrate grounding of the fuel tank and metallic fuel fittings. I have three comments:

    It is unfortunate that I included the word “bonding”. Bonding is a completely different term related to a system for preventing galvanic corrosion and has nothing to do with grounding. Grounding is the correct term and is used for electrical systems. The two are often confused. Even I and the USCG did it. Mea culpa, since I helped write the USCG Boatbuilders Hand Book.

    All electrical systems should be grounded at only one point, usually the engine block on inboards, and on outboards it is also the engine. Some times though a ground buss is used, which is then connected to the engine block by a single large wire. So the wire in my diagram showing “to ground” is going to the grounding point. The wire from the fuel fill to the fuel level sender is attached to the metal plate at the top of the sender, and so is the ground wire, so they are both grounded. Of course it would probably be better to show it going to the same point where the ground wire is attached.

    I am gratified that you are showing this for outboards. Actually the USCG regulation you cite does not apply to boats with outboard motors, but the industry standard, published by the American Boat and Yacht Council, ABYC, does, and that is what the industry uses.

    Comment by Ike — July 13, 2010 @ 10:17 am

  2. I have ceaned up and updated that diagram.


    Comment by Ike — July 14, 2010 @ 7:17 am

    • Something I just ran across on the Old Tacoma Marine blog (

      To bond or Not to Bond

      To bond, or not to bond: that is the question:
      Whether less noble metals should suffer
      The slings and arrows of outrageous corrosion,
      Or to take arms against a sea of troubles,
      And insulate them. To dielectric: to isolate;
      No more; and by isolate to say we end
      The corrosion and the thousand natural shocks
      That hulls are heir to, ’tis a consummation
      Devoutly to be wish’d. To dielectric, to isolate;

      Comment by tomlarkin — July 17, 2010 @ 10:51 am

    • Ike, I updated the drawing – thanks!

      Comment by tomlarkin — July 17, 2010 @ 11:00 am

  3. That’s a great little poem. I’ll have to use it on my web site.

    Comment by Ike — July 17, 2010 @ 9:43 pm

    • I put the poem on the Wooden Boat Forum and got a response:

      To bond or not to bond, that is the question;
      Whether ’tis nobler in the hull to suffer
      The softening of the timbers as base ions
      Dissolve and melt away the noble timbers
      Of which our ships are made, or to cut cables
      And leave th’ignoble metals to dissolve?
      To bond, and whilst we sleep to leave our ships
      A prey to straying amperes whilst at rest
      Or under way – this is no means to keep
      Our vessels safe and whole – better to cut
      The bonding cables whilst we can and save
      Th’expensive timber, not the cheap base metal!

      Comment by tomlarkin — July 20, 2010 @ 12:22 am

  4. Very good article. Learned a lot.

    Comment by pantar — September 15, 2010 @ 4:08 am

  5. Hi

    I installed a new fuel tank which is made of fiberglass with special inner coating. The tank Gage sender is attached to the fuel tank, the fill and vent are metallic, but the filling hose is non-conductive. Do I need to ground this tank?

    Comment by Salem — December 4, 2010 @ 5:47 pm

  6. This is a subject which came up frequently when I was in the Office of Boating Safety. There is no way to effectively ground a plastic or fiberglass tank. The metal fittings must be grounded. A ground from the fill fitting to the fitting on the top of the tank, and then from there to the boats ground is all you need. Yes, static builds in the fill hose when fluid is running through it, but the ground wire from the fill to the sender discharges this static charge. Any static build up in the tank from sloshing is discharged though the sender ground. So you do not need to ground the tank itself.

    Comment by Ike — December 4, 2010 @ 11:37 pm

  7. I might add that there are two reasons for grounding the fill to the tank and the boats ground. One is the static issue, but the other far more critical one is your boat needs to be in gound to ground contact with the fuel pump nozzle when filling the tank. The marine fuel pump ashore or on the dock, or at an auto gas station, is grounded to earth and there is a ground wire built into the hose that connects the nozzle to ground. All gas stations have warning signs telling you to make sure the nozzle is fully inserted before you started pumping gas. This is so any static in the system gets discharged to ground ( and in this case I litterally mean ground, or earth, as they say in the rest of the world)

    So it is vital to make sure the system on the boat is connected, Otherwise you may get a spark jumping from the fill to the pump nozzle. The US Air Force has a great video of this happening at a filling station. The person was filling a small gas can, and did not remove it from the bed of his pickup, and place it on the ground. The fire ball was very dramatic. Fortunately no one was injured and the fire department was very swift and put out any resulting fire before it spread.

    So make sure your boat’s fuel system is grounded.

    Comment by Ike — December 4, 2010 @ 11:55 pm

  8. Thank you very much for the valuable information. This would save a lot of time for me as I was waiting for 3 days trying to solve this issue.

    Can the ground share the same screwed terminal with the wire that connects the gauge and the sender?


    Comment by Salem — December 5, 2010 @ 1:58 am

  9. Yes, if the screw is long enough to hold several ring type connectors. In any case I wouldn’t put any more than 2 connectors on the screw (maybe 3). Any more than that I would install a buss bar that each ground wire goes to and then a single wire from the ground buss to the boat ground

    Comment by Ike — December 5, 2010 @ 3:40 am

  10. One last comment. About four years ago the USCG and ABYC studied the issue of plastic fuel fill fittings and determined that if you do fill up at the gas station, the plastic fitting should not be grounded. Here is the e-mail they sent me.

    “Recent events have caused the boating industry to examine the policy regarding the bonding of plastic body fuel fills with metallic caps and retaining chains. Existing USCG & ABYC policy states that the bonding of these components is voluntary. A study by IMANNA Laboratories has shown that connecting the metallic retaining chain and cap of a plastic body fuel fill assembly to a boats bonding system may result in electrostatic discharge from a land-based fuel pump nozzle to the metallic components of the assembly when the boat is not in the water. This condition does not exist when the boat is in the water due to the equalizations of the ground potentials between the fuel pump nozzle and the boats bonding system.

    It is recommended by ABYC and the USCG that new and existing installations of this type of fuel fill assembly DO NOT INCLUDE any attachment to the boats bonding system. Existing connections should be removed from the point of connection to the boats bonding system to the fuel fill assembly. Removal of the metallic components of the assembly is not necessary; however, the U.S. Coast Guard and ABYC still require that METALLIC body fuel fills be bonded.

    For further information contact:

    John Adey, ABYC (410) 956-1050 ex, 29

    Comment by Ike — December 9, 2010 @ 9:21 am

  11. […] Grounding a Plastic Gas Tank […]

    Pingback by Boat Gas Tank | Boat Trader News — February 18, 2012 @ 1:35 pm

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    Comment by a new plumber — October 27, 2012 @ 11:08 am

  13. The Aero Tanks – The Company Aero Enterprises is Fuel Tank Leader Since 1969. Aero Tanks specializes in design and building replacement and auxiliary gas and diesel tanks. little more info about the manufacturers:

    Comment by Daniel R. — December 10, 2015 @ 4:46 pm

  14. Would like to order gas gauge for alum gas tank — like figure 4 on this page. I have a aluminum boat 1985 18 foot. How and where can I get one?

    Comment by Gene Evenson — June 5, 2017 @ 7:54 pm

    • If you are talking about the tank sensor, that is, the part that fits inside the tank and measures the fuel level, you can get them through almost any online vendor of parts. However you can also get them direct from the manufacturer. The largest maker of these is WEMA. Or you can just buy one off Amazon. You will need to know the dimensions and capacity of your tank. The most critical measurement is the tank depth. They come in different styles. The cheapest and least accurate is the lever arm and float type. Better is the inductive type. Then there are electronic ones, no moving parts, that are the most accurate but the most ex[pensive. I bought and inductive. It has a float but the float just moves up and down on a column. No electrical connection inside the tank.

      Comment by Peter Eikenberry — June 5, 2017 @ 10:37 pm

  15. what if i bypass ignition switch and connect all wires directly to the battery? will it still work or!

    Comment by tse — June 18, 2017 @ 10:21 pm

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