Electric water heater question

Status
Not open for further replies.

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
A 20A breaker will carry 20A all day and all night. 80% is just a rule used in certain areas of the code.

OKKKK. I am way confused.

If a 20A breaker can hold a 20Amp load continuously then why are we doing the 1.25 multiplier (the 125% rule)

Another words, a 25A continuous load requires a 35Amp breaker. You guys are saying we can install 25Amp breaker and it will hold continuously???
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
Because the code says so. I'm sure it adds a layer of safety and reliability, but it's about the code not the breaker.
If am understanding this correctly,
ALL breakers are 100% rated. However, we can not use it at 100% rating because the code says so.
 

kwired

Electron manager
Location
NE Nebraska
20 amp breaker should hold a 20 amp load indefinitely - if the ambient temp remains at 40C (I think it is 40C that all the ratings are designed around).

Put a 20 amp continuous load on a standard 20 A breaker for more then three hours along with some adjacent breakers that may contribute some heat and see if you can maintain 40C or less. If you put the panelboard in a cool ambient area you may be fine, put in a control room with other heat producing items it may trip that 20 amp breaker before 3 hours has passed.

I think NEC has selected what they have to give reasonable protection against nuisance tripping yet provide effective overcurrent protection.

These rules have been around longer then I have been in the trade, some even longer then before I was born, I don't know exactly how most of them came about.
 

iwire

Moderator
Staff member
Location
Massachusetts
If am understanding this correctly,
ALL breakers are 100% rated. However, we can not use it at 100% rating because the code says so.

Kind of, but there is more to it than that.

You can't just use a 100% rated in say a load center in between other breakers.

If you need to use it at 100% you will use a 100% rated assembly which basically takes a standard breaker and places it in its own enclosure. The issue is heat from the other breakers.

https://www.cumminspower.com/www/literature/technicalpapers/NAPT-5563-EN.pdf
 

kwired

Electron manager
Location
NE Nebraska
Kind of, but there is more to it than that.

You can't just use a 100% rated in say a load center in between other breakers.

If you need to use it at 100% you will use a 100% rated assembly which basically takes a standard breaker and places it in its own enclosure. The issue is heat from the other breakers.

https://www.cumminspower.com/www/literature/technicalpapers/NAPT-5563-EN.pdf

But a standard breaker in an individual enclosure is not rated 100%. So there is more to it then just putting it in it's own enclosure as well.

I was once taught that standard breakers "sink" some heat into the conductors and that 100% breakers do not rely as much on this heat sinking function. May be more then that to the differences as well but I believe this is at least a part of what is different in many cases - especially if the breaker uses a thermal element of any kind, electronic units may be totally different ballgame.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
WOW, very cool.

Thanks guys.

Learned something.

As my signature:

Education is a progressive discovery of our ignorance.
 

readydave8

re member
Location
Clarkesville, Georgia
Occupation
electrician
Ok, so a 20 amp breaker will hold 20 amps forever, so there is no need for 125% rule (or maybe it's called 80% rule), and there is no good reason for NEC to limit 20 amp breaker to 16 continuous.

Except that it may not hold 20 amps forever, according to what is near it in panel, ambient temp, etc.

Does that sum it up?
 

JFletcher

Senior Member
Location
Williamsburg, VA
I see a lot of water heaters on 12 gauge-20 amp circuits.

Ive seen a few wired with 14/2 and a 20A breaker never have electrical problems. ofc the heaters were located next to the panel and there was all of 4' of wire between the 2, in free air, and the panel lightly loaded. ofc the 4' of wire was yanked and replaced with 10/2 and a 30A breaker installed.

someone asked whether a 25A breaker could be installed with OP's continuous load of 18.75A. Yes, as 18.75/25 < 80%, but it still has to be 10ga wire.
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
Ok, so a 20 amp breaker will hold 20 amps forever, so there is no need for 125% rule (or maybe it's called 80% rule), and there is no good reason for NEC to limit 20 amp breaker to 16 continuous.

Except that it may not hold 20 amps forever, according to what is near it in panel, ambient temp, etc.

Does that sum it up?
It's pretty much all about heat. Temperature rises as amperage rises. It's the same with wire, too.

Look at an amperage chart, capacity is going to be relative to ambient temperature as well as amps, which produce heat.

That's why you can have a 20amp breaker running at max, in between other breakers with no load, and it's fine. But load 3 or 4 breakers in a row at max and there will likely be problems in the long run.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Ok, so a 20 amp breaker will hold 20 amps forever, so there is no need for 125% rule (or maybe it's called 80% rule), and there is no good reason for NEC to limit 20 amp breaker to 16 continuous.

Except that it may not hold 20 amps forever, according to what is near it in panel, ambient temp, etc.

Does that sum it up?
A 20A breaker may hold more than 20A forever. There is a range that breakers are allowed to operate in according to UL rules. I think it's five percent.

The eighty percent restriction is anybody's guess. I think the NEC likes to keep things safe and reliable with rules that blanket a lot of variables.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Ok, so a 20 amp breaker will hold 20 amps forever, so there is no need for 125% rule (or maybe it's called 80% rule), and there is no good reason for NEC to limit 20 amp breaker to 16 continuous.

Except that it may not hold 20 amps forever, according to what is near it in panel, ambient temp, etc.

Does that sum it up?

In a resi panel with transient heat loads then yes, I agree.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
What is a transient heat load?

Residential loads are sporadic in nature. I cant see all 40 spaces in a resi panel at full load 24/7.


And oh, forget heat. That was my mistake. Inadvertent words sometime find their way into posts after I hit submit :lol:
 
Status
Not open for further replies.
Top