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How to troubleshoot Intermatic WH21 timer
WH21 timer
WH21 at Amazon
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Manual/ Resources:
WH21 / 25 amp water heater timer

WH21 manual

Avoid electrocution/
Tape multimeter leads to wood
Stand on dry boards
Do not touch metal
Wear dry clothes
Do not touch wet water heater
Do not touch wet timer box
Wear insulated gloves
Do not hold metal object in mouth
Do not lean into anything made of metal
Buy cheap multi-meter at box store. Set to 250Vac for tests.

240Volt wires can be white-and-black or red-and-black. Recommend 30 amp breaker and 10 gauge wire.
See Basic water heater circuit

WH21 troubleshoot
If timer dial is rotating, and keeping good time, then clock motor is good.
If the dial is not rotating, then clock motor might be bad.
WH21 clock motor WG233-18
Buy WH233-18 clock motor at Intermatic
Buy new WH21 timer at Amazon
Buy replacement trippers at Amazon

If dial is not rotating, pull off the timer dial and then reset to make sure it is properly seated.
Timer dial must be pushed inward for dial to turn water heater on-and-off.

Manual/ Resources:
WH21 manual
WH21 / 25 amp water heater timer

If you cannot get dial to rotate correctly, then check wiring.
Check that only copper wire and no insulation is inserted under pressure plate.
Check that screws are very tight against copper wire, and there is no evidence of melting or burning. Use nose to smell for burning, and ears to listen for crackling.
Do not mix 2 different gauge wires under same pressure plate, Do not put solid copper wire and stranded copper wire under same pressure plate.

Manually rotate dial and see if water heater turns ON-OFF. Listen for bubbly-fizzing sound by putting ear against water heater. If tank is fully cold, then upper element should turn on. If tank is fully heated, then no element should turn on. If lower part of tank has cooled, then lower element should turn ON. How electric water heater works  Troubleshoot electric water heater

Check for 240Volts across Line1 and Line2 on timer. Electrocution danger. Observe safety precautions shown on left.
Check that each Hot Line1 and Hot Line2 test 120Volts to green ground screw.
If you do not have 240Volts across Line1 and Line2, then circuit breaker might be tripped, or circuit breaker is loose on busbar. Action: Move wires to another same-amp circuit breaker. Test electricity to water heater

Terminal block has 3 screws: 1-2-3. Far right screw #3 is turned ON-OFF by timer. Other two screws #1-2 remain hot at all times. Test far right #3 screw to ground screw. When timer is ON, test should read 120Volts. Manually rotate timer dial. When timer is OFF, test should read NO volts. If test shows that timer is not turning ON-and-OFF, then timer is bad or trippers are worn and need replacement.

If dial is rotating correctly, but water heater never turns on-off, then trippers might be bad.
WH21 uses 156EB1945A trippers (set includes 2 silver and 2 brass trippers)
Intermatic trippers and parts     Order trippers from Intermatic

To stop timer from turning water heater ON-OFF, pull time dial outwards.
Timer must be reset for current time after power outage.
WH21 manual
Intermatic WH40 timer
Upgrade to WH40
Same as T104 except with convenient override lever
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If WH21 has stopped functioning,
Upgrade to better timer.
Buy heavy-duty 40-amp WH40 or T104 timer as replacement.
Only 1 difference between WH40 and T104: WH40 has manual override that sticks out of enclosure.
Both WH40 and T104 are identical otherwise: Both are heavy-duty and have replaceable 156T1978A trippers and WG1573 clock motor.
Both WH40 and T104 run cooler with less amp loss from heat, than 25-amp WH21 timer

WH40      208-240-277Volt 10000 watt 40 amp at Amazon
T104-20 (WH40)     208-240-277V (no external switch) at Amazon
156T1978 trippers at Amazon
Replacement trippers at Amazon
Clock motor WG1573 at Amazon
10-2 wire at Amazon
Wiring diagrams: How to wire WH40
How to wire T104
WH40 manual
WH40 sell sheet
How to operate and set timer
List of replacement parts
Photographs of timer and timer parts
Troubleshoot Intermatic timer
See all Intermatic water heater timers
See more water heater timers

GE 15070 in-wall timer to control water heater.
Installation requires contactor. Timer turns contactor ON-OFF.
See timer installation
See GE 15070 timer and similar timers
WH21 timer is 25 amp timer.
Volts x Amps = Watts
Watts divided by Volts = Amps
Most residential water heaters are non-simultaneous. This means both elements are not ON at same time.
How electric water heater works
How to wire thermostats

Calculate safe maximum for timers/ electrical circuits
Read label on side of water heater for element wattage. Element wattage is tank wattage.
For example: If water heater has 5500 watt elements: 5500 watts divided by 240Volts = 22.91 amps
4500 watt water heater divided by 240Volts = 18.75 amps.
Electricians use 80% rule for safe maximum. WH21 is rated for 25 amps x .8 = 20 amps.
Calculation shows that WH21 is ok for 4500 watt water heater, but beyond safe-maximum for 5500 watt water heater.
Exceeding safe maximum means WH21 will run hot and not last as long.

If electrical device is run at-or-near maximum amp rating, the device will not last as long because of heat.
Same as running a car at full throttle. Car will not last as long.
In contrast, 40-amp WH40 and T104 timers are better long term investment for water heater timer.
WH40 and T104 are rated for 40 amps, with safe maximum of 32 amps, so they withstand more amperage without heat loss, and will last longer.
How to wire WH40
How to wire T104
Intermatic T174 timer is identical to T104, except T174 has day-skipper feature

Calculate safe maximum for water heater circuit
If residential water heater is connected to 20 amp breaker and 12 gauge wire, then the same overheating problem occurs over time.
Circuit breaker will not last as long, and damage can occur to busbar.
See basic water heater circuit
20 amp breaker is subject to 80% safe-maximum rule.
20 amp circuit breaker x .8 = 16 amps. So safe maximum for 20 amp breaker is 16 amps.
As shown above, 4500 watt water heater draws 18.75 amps. And 5500 watt water heater draws 22.91 amps.
So 20 amp breaker with 12 gauge wire are rated too small for most water heaters. Over time the breaker can wear out sooner due to overheating.

Recommended that all water heaters be connected to 30 amp breaker and 10 gauge wire.
Safe maximum for 30 amp breaker x .8 = 24 amps.
30 amp breaker and 10 gauge wire run cooler, with less heat loss and more efficiency. Circuit breaker and main box last longer with less chance for heat damage.
Figure Volts Amps Watts for water heater

Solid copper wire only
Insdulation under screw plate
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Both wires MUST be same gauge
Straighten each wire
Tighten screw very tight against copper wire

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Connect stranded and solid with wire connector and run jumper to screw plate.
See example image

Use wire connector    
Push on wire connectors
Use push-on wire connectors
3 wires/ or different gauge wires
Join wires with wire connector or push-on connector
Run jumper to screw terminal.
Push on wire connectors
Red 3P connector (shown) for 10-14 gauge wire
How to select and install wire connector
10-3-wire  12 Ga
10 gauge or 12 gauge?
10 gauge is larger/ for 30 amp breaker. 12 gauge is smaller/ for 20 amp breaker
10 gauge wire can have black-white or black-red
12 gauge will have black and white wires

Buy short pieces 10 and 12 gauge wire at local box store
Compare with your wire.
Match color code and amp rating for wire
With 120 and 277 Volt circuits, white wire is always Neutral, and black is Hot
Hot wire is always switched on-off. Neutral wire remains connected without switching.
With 208 and 240 circuits, both wires are Hot, and both wires carry voltage.
Either Hot wire can be switched on-off. Or both Hot wires can be switched on-off.
Add black tape when white wire carries voltage (applies to 208 and 240V circuits)
CA3750 has 5 knock-outs
1 on back, 1 on each side and 2 on bottom

Keep enclosure door closed
Fire safety
Always keep enclosure (box) cover closed
Romex connector Use Wire connectors, or conduit clamps to secure wire to enclosure
By code, all wire must be securely attached to enclosure to prevent wire pull-out.
Wires or conduit must be secured to structure, so there are no loose wires.
Wires inside the enclosure cannot lay over top of connection terminals.
Join all ground wires together and cover with wire connector.
Push ground wires down and away from connection terminals.
2T2500GA plastic outdoor enclosure has 5 knock outs for 1/2" and 3/4" Wire connectors 1 on back, 1 on each side and 2 on bottom
More Intermatic enclosures
30 amp breaker use 10 gauge /

10-2 gauge/ 30 amp
10-3/ 30 amp
Southwire electric tools
Yellow 12 gauge 20 amp

Yellow/ #12 gauge wire for 20 amp
with ground
12-2 gauge/ 20 amp
12-3/ 20 amp

NMB is house wiring
UF is underground
Rolls of stranded wire
White 14 gauge 15 amp

White/ #14 gauge wire for 15 amp
with ground
14-2 gauge/ 15 amp
14-3/ 15 amp

NMB is house wiring
UF is underground
armored cable
Armored cable
Southwire armored cable cutter
Southwire armored cable cutter
Rolls of stranded wire

Stranded wire
stranded wire
Rolls of stranded wire
50-60 amp breaker use 6 gauge /
6-2 wire
6-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
40-50 amp breaker use 8 gauge /
6-2 wire
8-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
Copper ground wire
Use same size as other wire in circuit

12 gauge copper ground wire
Ground wire
Green ground wire
Ground pigtails
Ground rods/ ground clamps at Amazon

End cutting nippers at Amazon

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