I am needing to add a 30 amp circuit to an addition on my house. From the breaker box to the appliance (small ele water heater, 2400 watt, 120 volt), it will be about 160 feet. What size wire should I use on this long run. Thanks.

Are you sure of the 120v number? This is a very large load to be used as a 120v circuit?

If so then yes, you will need to upside the wire to at least #8cu.

Voltage drop on 120v circuits is nasty.

Thanks, Yes I am sure about the 120V. Because I asked the supply house that question specifically. Thanks for the answer.

Any chance that 240 volt is available as an option for this appliance?

My calcs:
Vd= 2*L*I*R
Vd=2 * 160 ft * 20 A * .000491 ohms/ft for #6 copper wire
Vd= 3.14 volts dropped

Using 3% of 120 volts as a max Vd = 3.6 Volts, seems #6 copper wire is in order.

Using 5% as a max, then #8 is fine, as Stan mentions.

Resistance heat works fine on reduced voltage, just does not get as hot as designed or intended.

If 240 volt option is available, then #10 wire will work.

Perhaps, a larger feeder for possible future load may be advisable?

160 feet is a long run to need to do it again.

I recommend #6 cu
#8 cu Will work

Thanks, exactly what I needed to know!