Grounding wire# 8 cooper

You did not post the make and model of the unit. However you should have three wires on a 120 volt heater. Black is HOT. White is neutral and green or bare copper is ground. Anything that you check with a meter connected to black should show 120. white connected to ground across the meter would show 0.

I assume by main power feed your talking about the breaker box? If so make sure the breaker box is actually grounded. Also check inside the heater and make sure the ground wire in there is connected and has a solid connection.

Most Larger water heaters are 220 volt ac single phase. This means you need 3 wires, two hot wires one neutral or ground. You will need to come from the breaker box off of a dual breaker of 30 amps or better. See the manual with the new heater. In your breaker box you have two sides or in coming service. The right side and the left each side is hot 110volts ac. you need to pull one leg (hot) from each side into the dual breaker out to the heater connections. The ground or neutral wire is un-insulated copper. Run these wires out to the heater. One black hot at 110 vac, one white hot at 110 vac and one plain copper ground.

You flooded water heater wiring compartment with detergent, and the wires were shorting so you turned off power and cleaned detergent. Now water heater doesn't work.

1) The heat from shorting might have tripped ECO reset button.
Open top cover on water heater, and push in red reset ECO button.
http://waterheatertimer.org/images/Jly-022-thermostat2-175.jpg
Be sure to re-install insulation and cover.

Other steps:
2) Turn off power. Remove wire nuts. Cut off copper ends of wire, and re-strip each wire so fresh copper shows. Make sure no detergent is on the copper. Use new wire nuts and twist very tight against copper wire.
3) Cut back burned ground wire. Remove ground screw and clean thoroughly. And then reattach clean copper to ground screw.
4) Turn power back on.

5) If water heater still does not work, then test across top two screws on upper thermostat for correct voltage.
http://waterheatertimer.org/images/Jly-022-thermostat3-175.jpg
http://waterheatertimer.org/Test-electricity-to-water-heater.html

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If tank is rusted out, then that is caused by depleted anode rod, or tank that is not electrically grounded on hot side pipe.
If pipe nipple is rusted out, then that can be caused by improper connection between copper and galvanized pipes, and caused by non-grounded hot water pipes.

If tank is leaking, replace it.
If nipple is leaking, remove with pipe wrench, apply teflon tape or joint compound on threads, and install new nipple. Replace anode rod if possible. Correct copper-galvanized connections. Add ground wire between hot and cold pipes.

http://waterheatertimer.org/Leaking-water-heater.html
http://waterheatertimer.org/Replace-anode-rod.html

The #6 bare copper jumper wire or ground wire between hot and cold pipes is to prevent stray household current from degrading metal parts of water heater and causing early failure of tank.
It's for your benefit and doesn't cost much, and may in fact be unnecessary .... except in cases where it is necessary which you wouldn't know until tank rusts out prematurely.
Here's some exciting additional reading:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#Ground-tank

look at your voltage 120 volts, or 240 volts, then amp rating. #8 THHN wire, or #8 romax is only rated for about 40 to 50 amps, your ground wire would be a # 12.

I would use # 8-2-with ground to have positive grounding at device , not 8-3 witch is more costly , also copper only no aluminum

You have clearly lost your ground due to PVC. One solution would be to run copper ground wire from the hot water tank to the metal splice on each end of the PVC. It is not necessary to go all the way back to the breaker box.

Shut off breaker to you water heater. STILL TEST FOR VOLTAGE BEFORE YOU TOUCH ANY WIRING. The 3 wire that go to you water heater disconnect them. You are going to install them to your WH 40 timer.
The wires that you disconnected will now go to terminals: Black to Terminal 1 & White To Terminal 3, The Ground copper wire will go to the ground terminal should be a green screw.
You will have to buy some 10/2 wiring to go from your timer to your water heater. You can buy it by the foot at Lowes. Figure how much you will need.
With the new 10/2 wire install the Black to terminal 2 & White To Terminal 4, the ground wire will go to the ground screw with the other ground wire.
Wiring that went to terminals 2 & 4 will be installed to you water heater, the same way when you disconnected them. Set your timer clock and your done. Turn breaker back on!