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Wire size for tankless water heater
50-60 amp breaker use 6 gauge / 
6-2 wire
Buy:
6-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
40-50 amp breaker use 8 gauge / 
6-2 wire
Buy:
8-2 wire
Southwire electric tools
NMB is house wiring
UF is underground
30 amp breaker use 10 gauge / 


Buy:
10-2 gauge/ 30 amp
10-3/ 30 amp
Southwire electric tools
Main breaker warning
Screws on main breaker are HOT at all times
Even when main breaker is OFF, both screws are HOT
Only if meter is PULLED are these screws safe to touch
What size breaker and wire for tankless?
240Volt ratings:
greater than 7.7 Kw= 7600 watts / 40 amp breaker/ 8 gauge wire
greater than 9.6 Kw = 9600 watts/ 50 amp breaker/ 6 gauge wire
greater than 11.5 Kw = 11500 watts/ 60 amp breaker/ 6 gauge wire
greater than 13.4 Kw = 13400 watts/ 70 amp breaker/ 4 gauge wire

This example shows Tankless with terminal connections for 3 wires
Larger image
Electricity supplied by 3 separate wires from 3 breakers
Single wire cannot be split apart to connect to multiple terminals.

If main breaker is 150 amps, then it is max-ed out carrying this tankless load.
Breakers are rated safe maximum at 80%
Main breaker 150 amps x 80% = 120amps safe maximum

Tankless electric water heaters consume large amount of electricity
Electric elements cannot heat water as fast as natural gas or propane gas.

To reduce energy consumption, add a tempering tank to preheat incoming cold water, keep thermostat set low, reduce consumption, add low-flow shower heads. 
If water moves too slowly through the pipe, or water pressure is low, or incoming water is too cold for amount of heating required, tankless may not activate.
wire size chart
GE-Breakers

Buy circuit breakers
Eaton CL series circuit breaker
CH circuit breakers at Amazon
Federal Pacific circuit breaker
QO circuit breakers at Amazon
double-pole breaker
Buy circuit breakers
Choose from 15 to 125 amp
Siemens circuit breaker
GE circuit breakers at Amazon
Double check wire size and distance
100'-150' etc requires larger diameter wire

240 volt/ each breaker
greater than 7.7 Kw= 7600 watts / 40 amp breaker/ 8 gauge wire
greater than 9.6 Kw = 9600 watts/ 50 amp breaker/ 8-6 gauge wire
greater than 11.5 Kw = 11500 watts/ 60 amp breaker/ 6 gauge wire
13 Kw = 13400 watts/ 2 60 amp breakers/ 6 gauge wire
Resource:
Select correct wire and breaker size
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