The wire that is to be used for this breaker 20 amp double pole, #12, correct? This is for a water heater.
Here is a pic of the wire. It appears to be, 12-2, shouldn’t it be 12-3 wire? The white is being used as a power feed?
12/2 can be used with 20a 240v breakers. It is a good idea to wrap a bit of black (or red or blue) electrical tape around the white conductor as it enters the breaker and the appliance to indicate it is a hot conductor.
12/3 is used in 20a 240v appliances which also use 120v (a light bulb, for example). That is why water heaters and central air conditioners often use 10/2, but dryers use 10/3 (the light bulb and sometimes electronic controls use 120v).
Daniel
But what about the neutral wire? Shouldn’t the neutral go back to the neutral/ground buss bar?
No Gary, that’s not how 240v electricity flows. With 120v, yes, current (amps) go out the hot leg and needs the neutral leg to complete the circuit. But with 240v, current does not flow through the neutral nor need it to close the circuit; the current flows out both legs of 120v, creating 240v electricity.
Again, the neutral conductor is only used in 240v appliances that also use 120v. And yes, NEC does allow two conductor cable (12/2 e.g.) to be used for strictly 240v appliances like well pumps and water heaters (usually 10/2 though).
BTW…Did you confirm the amp rating of the water heater? Most are 20-30 amps, requiring 30a DP breakers and 10/2 or 10/3 cable.
Daniel
Could not find the amp on the plate? Where else could you look for this info?
So Daniel, two hots and a ground is all you need for a 220v circuit? What then is there 12/3 and 10/3. I was allways taught to have two hots, 1 neutral and ground for 220v appliances.
Straight 220 appliances like water heaters and baseboard heat do not require a neutral and will commonly be wired with 2 conductor + ground cables.
Stoves, dryers and ovens have 120 volt loads like timers and bulbs, as well as the 220 load like the elements. These require a neutral and 3 wire + ground cables.
The formula for breaker size is -
total watts divided by 240, times 1.25 = min breaker size – or –
4,500 divided by 240 = 18.75 x 1.25 = 23.44 or min. 25 Amp breaker using 10 AWG conductors
Yes, it should be 10 AWG based on 4,500W and 240V…
Remember if you run into a water heater that has no nameplate you have a different set of MATH you may need to apply as the maximum OCPD…but basically it means the OCPD can be 150% of the rated current.
In other words…18.75 x 1.50 = 28.12 ( next size up = 30A OCPD )
**(E) Single Non–motor-Operated Appliance.
**
If the branch circuit supplies a single non–motor-operated appliance, therating 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.
So…if the nameplate says to use something you use it…if it only gives the wattage and voltage you size the conductors to 125% of the calculated ampacity ( 4500W / 240V ) and apply the percentage factors…the minimum 125% is on the OCPD and the Conductors…however if not nameplate giving actual breaker sizes which they usually dont you can use 150% of the calculated current ( 4500W / 240V ) and apply 150% and protect the circuit…which means this…
A 4500W water heater can have = 10 AWG to a 25A or 30A OCPD in most all cases…
just some educational moments…
here is a good picture to illustrate what Linda was saying…
http://www.mikeholt.com/onlinetraining/page_images/1100706775_2.jpg
Here is the other example I was refering to…just for knowledge only…
what if the ocpd is not specifically on the nameplate but the manual to the water heater states that a 4500w 240v water heater should have a 25a breaker and a 30a is installed? saw that a couple of times and put in the report to change it to a 25a based on the manufacturers manual. Seems though as if 30a is ok. what’s the consenus if the manual is attached to the water heater states one thing but another is installed but is apparently ok?
If in the manual I would say do as the manual says…remember a 10AWG protected with a 25A breaker is PERFECT…works fine.
We have to install the products as the manufacturer intends…so a manual would be fine…but remember 10 AWG is rated for 30A so a 30A breaker would be fine also…interesting enough the NEC says marked on the applicance…either way 25A or 30A would be fine in my view.