How to Install a Garbage Disposer—Part 6

Part 6: Mount New Disposer and Connect Plumbing Lines

Included in this video: Mounting the new disposer to disposer throat underneath sink and connect plumbing lines with P-trap.

Video Transcript: Part 6: Mount disposer and Connect P-trap  

Also Included:             Not in Video: Knock out dishwasher plug if using
                                       Not in Video: Connect Electrical if Hard Wired
                                       Not in Video: Cut down P-trap Parts as needed.
                                       Not in Video: Connect dishwasher if doing so.

NOT IN VIDEO SECTION: Knock Out Dishwasher Plug if using dishwasher connection

NOTE: Connecting your dishwasher via your disposer is a violation of plumbing code in my state, Illinois. The reason: if you were have a clog in the disposer drain line, the first place for the dirty water to go would then be your dishwasher. It’s a good point. But if you live in an area where it is legal, it is something to consider. The pro: every time you run the dishwasher, it then drains through your disposer, flushing it. Disposers can get kinda’ gooky and smelly over time, and it does help combat this.  Weigh the pros and cons in your particular situation, and make the call.


1.  Lay the disposer on a solid surface with the dishwasher knockout facing you.
2.  Read the instructions for knocking out the plug and follow them.
3.  I have rarely had to do this since using this connection is a violation of code here, but the few times I have, I used a flat blade screwdriver and hammer, and lightly “bonked” the knockout. It popped right out. You want to avoid the plug going into the disposer if possible–I don’t remember how I avoided this as it’s been a while. Comments with advice are welcome!


1. Mount the Disposer
    a.  Lay down in cabinet underneath disposer throat and get comfy. You’ll be here a while.

DIY Tech Tip

Tech Tip: If the cabinet is cutting into your back, try placing a folded towel or the like on the edge.
    b.  Line up retaining ring flanges on disposer with “ears” on disposer throat.
    c.  While holding disposer flush against throat, turn retaining ring counterclockwise as far as you can by hand. It should catch on rim of throat and disposer should now be hanging from this without your support.
    d.  Turn the disposer to position it in the general spot you will want it for connecting the piping (and electrical, if needed). It can still be turned after tightening, but it easier to do it now.
    e.  Using the wrench included with disposer, finish turning the retaining ring until it seats over the little “bumps” on the disposer throat. This can be easier said than done. If you are having trouble getting it all the way over:
         1.  Slide a screwdriver with a longer handle through the flanges and turn. The longer handle helps you get more leverage.
         2. If it still isn’t seated, tap on the screwdriver above with a hammer.
         3.  Some of these are harder than others to seat. Just keep at it as above until it is seated.
   f.  Check that the flanges are seated over the “bumps” on all three ears. This is important because it is what will prevent your disposer from vibrating off it’s mounting over time.


NOT IN VIDEO SECTION: Connect Electrical if hard wired

•  You will need to be able to see well to complete next steps. Set up work light if needed.
•  Feed the wires through the electrical inlet hole into the electrical well on the disposer. Try not to rip off your tape marking the wires.
•  Screw the electrical connector holding shielded cable to disposer using the opposite procedure you used to remove it. .
•  You will need to connect three wires.

 Note:  Inspect the exposed wires one last time for any cuts or cracks to the wire or sheath.  If you find any, cut the wire back to before the defect and use wire strippers to remove about ¾” of sheath from the end of wire.

1. Connect the ground wire:
    IMPORTANT: Your disposer MUST be grounded to avoid creating an electrical hazard should it get wet (a reasonable possibility at some point considering it’s location.) If the wiring doesn’t seem to match up to what I’m describing here, do whatever you need to do to make CERTAIN which wire is the ground, and where it is supposed to be connected to the disposer. Proper grounding can prevent future electrocution, seriously. This isn’t the place to take an educated guess.
    a.  The ground on your incoming electrical is usually the green wire, and hopefully you marked it as the ground wire when you removed it.
DIY Tech TipTech Tip: On some older electrical, there isn’t a separate ground wire; the shielded cable itself is the ground. In this case, make sure your electrical connector at the inlet hole is good and tight to both parts so a good ground connection is made. Often the cable also has an unshielded wire (not copper–zinc?) running through it to help the ground somehow. I usually connect this to the ground screw also, but I don’t know if it actually helps. It figure it can’t hurt.
b.  The ground on the disposer will be a screw, also usually kind of greenish colored, which threads into the housing somewhere, again usually inside the electrical recess panel.
c.  Remove the ground screw.
DIY Tech TipTech Tip: use your bare hands, and hold onto it. You don’t want to drop it inside the disposer.
DIY Tech TipTech Tip: If you do drop it inside the disposer, like Girl With Wrench did even after cautioning you, a magnet is the easiest way to retrieve it. I have one on little telescoping rod that looks like an old radio antenna.
d. Make a loop on the end of the ground wire by grabbing the end with needle nose pliers and turning. Put the screw through this loop on the ground wire, replace it and tighten.
DIY Tech TipTech Tip: Start the screw by hand the first couple of turns. It should turn easily. If it doesn’t, back it off and try again until it turns easily. It can take a few tries to catch the threads properly. If you force it, you are likely to ruin the threads on the ground screw and will have to go buy a new one before proceeding. :)
DIY Tech TipTech Tip: You want this good and tight, but don’t give it everything you have or you are likely to strip something out.
2. Connect the hot and the neutral wires. Note: If you do this incorrectly, you may get some fireworks :)
a. Determine which wires connect to which. In a perfect world, you had a black and white in the old disposer, and these are marked on your incoming electrical. If not, you may have to do a little homework to work this out. The instructions will tell you which wire is hot on your new disposer. Hopefully you can then use process of elimination and the internet to determine which wire is hot on your incoming whip. The other will then be neutral.
b.  Start with whichever one is shortest, if applicable.
c.  Simply hold the two wires so the ends are even. Each should have about ⅜” of bare wire on the end.
d.  Put the wire nut over the two until it is all the way down, and turn clockwise to tighten.  
DIY Tech TipTech Tip:If you haven’t done this before, it is surprising how nicely they tighten up. Keep turning until the nut feels tight, and doesn’t come off when tugged a bit. It will almost always come off if you give it a good yank, so not too much tugging, ok?
DIY Tech TipTech Tip: If you’re having trouble getting the wire nut to “grab”, try starting the two wires together with a pair of lineman’s (flat nose) pliers, and then put the wire nut on after you’ve made a couple of turns with the pliers. You can also use the pliers to hold the wires in place for the nut if they are short and hard to grab.
e.  Now wrap electrical tape over the whole thing, from the top of the nut to about an inch below it, wrapping continuously in a circle like a candy cane. This will keep the nut in place over time despite all the vibration from the disposer when in use, and it will also help keep the wiring dry if there should be a leak at any point.
DIY Tech TipTech Tip: Stretch the electrical tape a bit as you wrap to get a good, tight seal.
f. Repeat the procedure for the other wire.


2.  Connect the Drain Piping
    a.  Inspect the disposer outlet, drain inlet and pieces you have to insure the everything will reach and connect, and that your disposer will drain properly. There is a lengthy discussion about drain heights in the page text from Part 3.
    b.  Hold the pieces in place and check visually:
         1) Do they reach each other? In my case, I needed an extension tailpiece because the P-trap didn’t come up high enough to seat well on the disposer tailpiece. You should have at least an inch of pipe going into the next piece in all places.
         2) Are any too long? You will need to cut them down. 

NOT IN VIDEO SECTION: Cutting down Trap Parts.

1. Loosely connect the parts of the P-trap together.
2. Hold the top of the P-trap against the disposer tailpiece at roughly the spot where it will connect and adjust the pieces to where they will generally sit to slide into the trap adapter on the drain. it’s ok if it’s too high or low right now. Look the last piece of the trap. Is is too long? If yes, mark the spot where you need to cut it. It should be at least 2” back from the trap adapter (to give us play since the trap is not actually in place) but before the first turn of the drain line. You want to trap to end before the turn starts once in place because otherwise it will restrict the flow of water from the drain.
3.  Cut this piece as marked. You can use a hacksaw, sawzall or even a tubing cutter. After cutting, make sure to rub around the edges with your glove or a rag to remove the little bits dangling. If you have burrs on the inside, scrape the off with a razor knife.
4.  Now again loosely connect your trap parts, and this time slide the assembly into the drain at the trap adapter. Insure it goes into the pipe at least an inch.
5.  Now turn the pieces as needed, and line up the beginning of the trap with the disposer tailpiece. The trap assembly going into the drain should be resting at a slight angle, pointing downward into the drain.
6.  Is the tailpiece too short? Is the tailpiece too long (does it not fit at all, or does it push the trap down too far so it is back-pitched and angling toward the disposer instead of the drain)?
    a. Too short: You will need an extension tailpiece, which will need to be cut down. Get the extension, put it on the tailpiece, and proceed as below.
    b. Too long: Set the top of trap next to the tailpiece in place, and mark your cut on the tailpiece (or extension,) at least an inch down from the top but still short enough that it won’t push the trap down out of position.
7. Cut the tailpiece (or extension) as marked with a hacksaw. You may have to remove it from disposer if you can’t get a hacksaw in there.
8.  Loosely assemble all pieces, sliding end into drain and top onto tailpiece. Insure that the pipe going into drain isn’t too far in as noted above. If you can’t tell, mark the pipe where it goes into the drain, remove it, and measure from the mark to the end to see how much is in the drain.


You should now have a P-trap assembly that fits correctly and is ready for connection.


  c.  Loosely connect all parts of the P-trap. The pieces should turn easily. Do not use pipe dope on plastic traps. Both are petroleum based, and it will degrade the plastic over time. If using a metal trap, use pipe dope on the metal threads before connecting.
DIY Tech TipTech Tip: Several of the parts need to have the clamping ring and gasket on the pipe before connection. See your instructions.
DIY Tech TipTech Tip: The gaskets are triangular. The flat part goes toward the clamping ring; pointy part toward the pipe.
         d.  Position the trap assembly in place. Slide the end into the trap adapter on the drain, and the top onto the tailpiece.
DIY Tech TipTech Tip: You may need to turn the parts different directions to get it to fit. This will not affect function.
e.  Take good look at the trap from different directions. All the verticals should be straight up and down, and the horizontals should be in line with each other, not cocked at at angle. The only thing that should not be completely straight is the end of the assembly entering the drain. It should be at a slight downward slope toward the drain. The ideal amount is a ¼” per foot. If the drain is properly pitched, it will still enter the drain in line with it; both will be pitched slightly.
f. Once it is nice and straight, tighten everything down by hand. Check that nothing cocked during tightening.
g.  Do your final tightening. Tighten as much you as you can by hand.
DIY Tech TipTech Tip: If this hurts your hands, wear gloves to get a better grip.
h.  Then give it a LITTLE more (about ⅛ of a turn) with channel locks.  Do NOT overtighten.
DIY Tech TipTech Tip: If you overtighten, plastic rings can split. Even worse, not always right then; often later–leaving you with a leak you are not aware of.  If you overtighten the metal rings, it can mash and mangle the rubber gaskets, and they will leak. 

NOT IN VIDEO SECTION: Connect dishwasher if using
1. You should have plug knocked out already from above. If not, return to top of this page and do it now.
2.  Slip dishwasher drain hose over inlet on disposer. They are usually barbed and will need some force. Get it on all the way.
3.  Position hose clamp in roughly center of inlet stem, and tighten to hold in place. Tightening with screwdriver by hand is fine. Do not overtighten.
4.  IMPORTANT:  Now insure that the hose goes up to top of the cabinet just underneath counter somewhere between the dishwasher and the disposer and secure it in place. Often you can find something to run a cable tie through to make life easier–a bracket, sink clip or whatever. If not, use a pipe strap and screw it to the cabinet or underside of counter. The reason this is important is your dishwasher is the lowest point in the system. Water flows downhill right? So if your disposer fills up and is running slow, waste water will backfeed through the dishwasher hose into your dishwasher. If the hose runs up underneath the counter, your drain would have to be clogged and your sink filled with standing waste water almost to the rim before water will start to backfeed into your dishwasher.


Almost done!

Next: Part 7: Testing Disposer and Plumbing Lines & Cleanup


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    How to Install Garbage Disposer for Mechanically Challenged—Part 6—Mount Disposer &Connect Plumbing | GirlWithWrench DIY for the Mechanically Challenged

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