5000/240 x125%= 26 amps
#10 wire with a 30 amp ocpd
Overcurrent protective device can be 150% of the rating however the wire size would be larger than a #10. Thus you could use a 35 amp breaker but the wire would need to be #8. No reason to do this..
422.11 (E) Single Non–Motor-Operated Appliance. If the branch
circuit supplies a single non–motor-operated appliance, the
rating of overcurrent protection shall comply with the following:
(1) Not exceed that marked on the appliance.
(2) Not exceed 20 amperes if the overcurrent protection
rating is not marked and the appliance is rated
13.3 amperes or less; or
(3) Not exceed 150 percent of the appliance rated current if
the overcurrent protection rating is not marked and the
appliance is rated over 13.3 amperes. Where 150 percent
of the appliance rating does not correspond to a standard
overcurrent device ampere rating, the next higher standard
rating shall be permitted.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 for the purposes of sizing
branch circuits.
422.10 Branch-Circuit Rating. This section specifies the
ratings of branch circuits capable of carrying appliance current
without overheating under the conditions specified.
(A) Individual Circuits. The rating of an individual branch
circuit shall not be less than the marked rating of the appliance
or the marked rating of an appliance having combined loads as
provided in 422.62.
The rating of an individual branch circuit for motor operated
appliances not having a marked rating shall be in
accordance with Part II of Article 430.
The branch-circuit rating for an appliance that is a continuous
load, other than a motor-operated appliance, shall not be
less than 125 percent of the marked rating, or not less than
100 percent of the marked rating if the branch-circuit device
and its assembly are listed for continuous loading at
100 percent of its rating.
Branch circuits and branch-circuit conductors for household
ranges and cooking appliances shall be permitted to be in
accordance with Table 220.55 and shall be sized in accordance
with 210.19(A)(3).
Edit to add: I'm way late. Dennis has all this covered -
2017 NEC
Reference:
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 for the purposes of sizing
branch circuits.Informational Note: For branch-circuit rating, see 422.10.
5KW at 240V = 21A (20.833 ...A)
Rated continuous >> 20.8 X 1.25 = 26A, which would require a #10CU.
That being said, recommend checking the water heater nameplate.
carl
Here is something to be careful with these water heaters. If it is a residential one it is usually not an issue but if the water heater is 5kw rated at 240 volts and you connect to a 208 volt system you would then have about 75% less wattage at that voltage and hence lower ampacity.
5000 watt heater at 240 will be 3750 watts at 208
AMps= 21 amps without the 125%
R = V^2 / P
R= 240 x 240 / 5000= 11.52
P= V^2/ R
P = 208x 208 / 11.52 = 3755
or 5000 x 75%= 3750--- pretty close
