Water heater electric ?

[nbhi]

Have a 3500watt water heater on a 20 amp double pole, 12 awg.
If my math is correct 3500watt/220v = 15.90 amp.
This shouldn't be an issue?

[Speedy Petey]

Nope, pretty common actually.

Thing is, your calc is off a bit. "220" is dead. Unless you know for fact that the nominal voltage is 220v, we use a more accurate 240v.

120/240v is nominal these days.

3500w / 240v = 14.58

[nbhi]

So Speedy, this setup should be ok?

[Mike Whitt]

Also a fixed storage-type water heater that has a capacity of 120 gallons or less shall be considered a continuous load so that would be 14.58 times 125% or a load of 18 amps

[Mike Whitt]

Originally Posted by nbhi

So Speedy, this setup should be ok?

yes

[nbhi]

Speedy
Should I also calculate the rating a 125%, which would be at 18.22 amps?

[Mike Whitt]

Per the NEC

422.13 Storage-Type Water Heaters.
A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load.

210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Yes as it is required to be done.

[Speedy Petey]

Yup, Mike gave all the pertinent info.

[Greg Fretwell]

BTW these elements are rated at 240v, so if you really have 220v the watts will just be lower, by a factor of the square of the difference.