Water Heater Amperage? Maybe this is elec.

I was looking at my brother in laws 50 gallon AO Smith electric water heater last night and noticed that the unit was on a 60 amp breaker. Is that correct or is it over amped? I could not find a max amp on the unit similar to the AC compressor nameplate.

Sounds like it is overfused. What gauge wire was used?

Yes, it is over sized…I have never seen a 60A water heater and most certainly not any made by A O Smith.

Hint…always take a picture of the nameplate for future reference. Now I know the one you had did not have a nameplate…this is just a general reminder as it is nice for reference later and do it on all appliance nameplates.

Chances are the 50 Gallon water heater is a 4,500W unit, so figuring this for your friend

4,500W/240V = 18.75 x 1.5 = 28A ( next size up rule per 422.11(E) ) 30A breaker is the proper size

Here is the section if you would like to see it…:

**(E) Single Non–motor-Operated Appliance.
**If the branch
circuit supplies a single non–motor-operated appliance, the
rating of overcurrent protection shall:
(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.


Another point along this line is; both heating elements are NEVER on simultaneously, hence no need for a huge breaker to handle the load. I found that many people were unaware of this little tidbit of info.

Yeppers…thats why you only use 4,500W in the equation…not the sum of BOTH elements…:slight_smile: nice clarrification point as well…:slight_smile:

Another good point is the NEC defines this as a Cont. load so the conductors have to also be sized at 125% of the non- continuous load.

**[FONT=Times New Roman]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.

Ironically enough…422.11(E) says it needs to be 150% on the OCPD and in all Mr. Holts books it desribes it as such…yet 422.13 says it is considered continuous and we know that means 125%…

*Note- if the unit is rated for 100% contin. then that is fine also but I dont see that in standard water heaters…just threw that in.


Part II. Branch-Circuit Requirements

Text was revised to specify that a fixed storage water heater of 120 gallons or less is to be considered a continuous load. This simple change causes feeder/service conductors and overcurrent protection devices to be increased in size.

An electric water heater having a capacity of no more than 120 gallons is considered a continuous load.

Author’s Comment: Circuit conductors and overcurrent protection devices must have an ampacity of not less than 125 percent of the ampere rating of the appliance. Figure 422-1

  • Branch circuit conductors [210.19(A)(1)], overcurrent protection devices [210.20(A)]
  • Feeder conductors [215.2(A)(1)], overcurrent protection devices [215.3]
  • Service conductors [230.42(A)(1)]

Question: What is the calculated load for a 4,500W, 230V water heater?

(a) 15A (b) 20A © 25A (d) 30A

Answer: © 25A
I = P/E
P = 4,500W
E = 230V

I = 4,500W/230V
I = 20A

Calculated continuous load for conductor sizing and protection = 20A x 1.25 = 25A

[FONT=Times New Roman]The revision to 422.13 ( in 2005 NEC ) specifically identifies an electric water heater with a storage
capacity of 120 gal or less as a continuous load. This revision requires the branch
circuit overcurrent device and conductors to be sized based on 125 percent of the water
heater nameplate rating unless the overcurrent device and the assembly it is installed in
are listed to be used at 100 percent of its continuous current rating. In addition, feeders
and services that supply water heater branch circuits are also impacted by the fact that
this type of equipment is considered to be a continuous load.

lol…The point is I think mike needed to make the change in his newest book and he probably did already…:slight_smile:

Wow. I need to reread this thread again. Holy cow! 30amps max. :shock: :shock: Thanks Paul

Does anyone have good verbage in laymen terms, that I could use to explain this issue to clients?

Most clients will understand this about electric water heaters:

Your water heater is wired to a 30 amp circuit and should give good service for about 10 years. The most common problems are burned out elements. A bad top element will cause the tank to have no hot water and a bad lower element will cause it to have reduced hot water capability. The top thermostat must be hot before it lets the lower one come on. The water is drawn from the top and the cold water comes in at the lower part of the tank interior.


Our 40 Imperial (Canadian) gallon electric hot water heaters (roughly equivalent to the US 50 gal units) have 3000 watt elements only and are breakered at 20 amps while using #12/2 wire. Why the 4500 watt elements in a similarly sized unit…faster recovery times??

Our 60 Imp gal tanks have 4500 watt elements with #10/2 breakered at 30 amps.

It is true that with residential water heaters both elements are never allowed “on” at the same time. However, I have seen numerous comercial heaters that do allow both elements (4500 to 5000 watt) “on” at the same time. Personally I do not know if they “within code” or not. Of course, these heaters are wired for the increase amperage (#6-wire) and are breakered accordingly (60 amp) as well. Are we sure that this residential heater was not “salvaged” from a defunct restaurant. If it was, I am not saying it would be a correct usage… just explaining why it might be there.