water heater breaker size

Merry Christmas
My customer just installed a 36 gal water heater. The label says upper and lower elements are both 4500 w. But it says "total watts connected : 4500 watts" I've always wondered how two 4500 w elements can be only 4500 w connected, I obviously don't understand how they work. ( I always install a 30 amp breaker and #10 wire. ) The existing circuit was #12 wire on a 20 amp breaker. I replaced the #12 with #10. But when I divide 4500 by 240 v, I get 18.75 amps. So is the 20 amp breaker ok ? Thank you !!
 

infinity

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New Jersey
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Journeyman Electrician
There are two elements but only one will be on at a time so the maximum load will be 4500 watts. This is considered a continuous load so:
4500/240*125%=23.4 amps. With NM cable #10 conductors on a 25 or 30 amp OCPD will work. With MC #12 on a 25 amp will also work.*

*
Disregard this part as it's incorrect. :oops:
 
Last edited:

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
There are two elements but only one will be on at a time so the maximum load will be 4500 watts. This is considered a continuous load so:
4500/240*125%=23.4 amps. With NM cable #10 conductors on a 25 or 30 amp OCPD will work. With MC #12 on a 25 amp will also work.

How does #12 work on a 25 amp breaker?
 

Dennis Alwon

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Chapel Hill, NC
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You still can't use 75C #12 on 25 overcurrent protective device unless it is mentioned in 240.4(G). Did I miss it
 

infinity

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New Jersey
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How does #12 work on a 25 amp breaker?

You still can't use 75C #12 on 25 overcurrent protective device unless it is mentioned in 240.4(G). Did I miss it

You're right it doesn't, I was thinking of a motor load with 75° C terminals on both ends. I added a disclaimer as not to confuse someone with my stupidity. :oops:
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
Thank you !


Here is a diagram. You can see the top t-stat is a spdt t-stat. Once the upper part of the tank is satisfied then the lower half can come on if it needs it. If the tank water gets cold at the top it will shut off the bottom power and turn the top element back on

1596149492299.jpeg
 

Dennis Alwon

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Retired Electrical Contractor
You're right it doesn't, I was thinking of a motor load with 75° C terminals on both ends. I added a disclaimer as not to confuse someone with my stupidity. :oops:


No problem... I have done the same many times. I had to double check because I know you are usually correct so I thought I missed something
 

Stephen111

Member
Location
United State
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Worker
Most electric heaters need a 240-volt dedicated circuit; This circuit includes a 30-amp two-pole breaker with a 10-2 non-metallic cable or metal-clad cable. A perfect breaker size should be proportionate to the electrical specifications of your water heater.
 

robertd

Senior Member
Location
Maryland
Occupation
electrical contractor
Here is a diagram. You can see the top t-stat is a spdt t-stat. Once the upper part of the tank is satisfied then the lower half can come on if it needs it. If the tank water gets cold at the top it will shut off the bottom power and turn the top element back on

View attachment 2553099
What happens if you disconnect the lower element from the upper one and feed it with it's own circuit? Does it heat up faster or cause problems?
 

ramsy

Owner/Operator
Location
LA basin, CA
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Service Electrician 2017 NEC
If you do the calcs a water heater would have a 25 Amp breaker. Our state electrical rules allow a 30 amp for a water heater.
Table 220.3 calls 422.11(E) for water heater OCP

20A, 25A, or 30A OCP on #10 wire meets that requirement.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
What happens if you disconnect the lower element from the upper one and feed it with it's own circuit? Does it heat up faster or cause problems?
Sure it would heat up faster if both elements came on together but you will be drawing twice the load and it would be illegal . Can you image the demand on the power company if all water heaters used 9000 watts instead of 4500 watts.
 

kwired

Electron manager
Location
NE Nebraska
Sure it would heat up faster if both elements came on together but you will be drawing twice the load and it would be illegal . Can you image the demand on the power company if all water heaters used 9000 watts instead of 4500 watts.
Would run less time though.

On demand water heaters can give POCO higher instantaneous demands as well, though would be somewhat less kWhrs over longer time periods.
 

kwired

Electron manager
Location
NE Nebraska
Conventional WH heats upper portion of tank when it is below upper stat set point. This partly because water in top of tank is first water to be used.

Once upper stat is satisfied lower element takes over. Still going to be some additional rise in temp at top of tank though.

Lower element does the majority of the heating, upper one only comes on if you using larger volume of water and water in top of tank cools enough for it to turn on again.

Malfunction can disrupt this described operation, often causing less hot water reserve or if upper element fails and top of tank cools enough, you will have no hot water at all even though the lower element is in working condition.
 

kwired

Electron manager
Location
NE Nebraska
If you do the calcs a water heater would have a 25 Amp breaker. Our state electrical rules allow a 30 amp for a water heater.
No need for a state rule of 30 amps, 422.11(E)(3) allows a 4500 watt 240 volt water heater to be protected by a 30 amp breaker.

Jump to 6000 watts and 30 amp breaker generally does hold, but since this is supposed to be considered a continuous load you need more than 10 AWG and min of 35 amp breaker - though it can also be on as much as 40 amp breaker.
 

Dennis Alwon

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Location
Chapel Hill, NC
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Retired Electrical Contractor
No need for a state rule of 30 amps, 422.11(E)(3) allows a 4500 watt 240 volt water heater to be protected by a 30 amp breaker.

Absolutely. We used to be required to have a 25 amp breaker but I am wondering if there was a rule change to allow overcurrent protective device at 150% and allowing next size up rule.
 
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