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80% derating rule on breakers


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8 times out of 10, I will find 20 amps breakers on water heaters that mention 18.75 amps on their labels such as this one here below.

18.75 being 80% of the minimal capacity of the required breaker (18.75/.8= 23.4 amps), would the 80% rule be applied, those 20 amps breakers are inadequate. Do you guys call out breakers (or fuses) that do not meet this 80% derating rule?

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tn_201332410534_wh%20label.jpg

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I'm in Quebec and I rarely see 30 amps breakers on water heaters. Most are as descibed above. Installation guides state that WH should be installed according to the Canadian Electrical code CSA C22.1.

I guess where I get comfused is with reading the labels. A 3800 watts single element will be fine with a 20 amps breaker but the 4500 watts max rating will require 25 amps.

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Strange label. Two elements, both 3,800 watts but the maximum rating is 4,500? Why the change to 4,500?

Marc

Don't know, but the heater is 240 volts according to the label. With the following calculation; 4500 watts/240 volts= 18.75 amps. therefore a 25 or 30 amps breaker should be used.

Now the problem lies with the wiring. If an electrician used a 12 g wire thinking a 20 amp breaker was fine and we call it out as being inadequate, that might cause some problems wouldn't you think?

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I'd call it out regardless. The guy who wired it up may have thought the elements were 3,800 watts and he could have been right at that time. The element amperage on the label is for 4,500 watts which crosses the threshold from #12 to #10 wire. I'd rather be safe and say a #10 is needed. Re-wire it.

Label triumphs the sparky.

Marc

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Hold on. If the 20 amp breaker is adequately supplying the load and protecting the feeder, is there a problem here?

That is a standard installation and you are not going to easily convince an electrician that it needs to be repaired.

Only one element comes on at a time. My guidebook says 3000W to 3800W gets a 20 amp breaker. Yes, the CEC calculation is different than the NEC. CEC says the load is not to exceed 80% of the breaker rating.

20 amp is standard for 40 gallon electric water heaters at least in my area as well. We will see 30 amps only on the larger 60 gallon units. So you can report what you think is best, but I would discuss this with a local electrician first.

FYI, my water heater is on a 15 amp breaker and I see no reason to fear.

It is John Wood Space Saver 130 litre unit (34 US gallons). The elements are 2255/3000W.

My tank has a Max Allowable rating of 4500W. I think that means the max size element that can be installed in this tank is 4500W(?)

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AO Smith (for example, others similar) water heaters can be wired for simultaneous element operation or not. This is done at installation with internal wiring.

So, before you decide whether the circuit is correct or not you have to determine how the water heater is wired to operate.

Beyond the scope of a home inspection. Note what you will in the report, but don't expect your call to be the final word.

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found this interresting site.

http://waterheatertimer.org/Figure-Volt ... eater.html

here's an excerpt;

Check label on side of tank for volts and watts

Every appliance, switch, outlet and bulb has a label

Residential water heaters with 2 elements are 240Volts

Residential water heaters are non-simultaneous

This means both elements are never ON at same time.

Element wattage is tank wattage

If tank has 4500 watt elements, then tank wattage is 4500W

Recommend 30 amp breaker and 10 gauge wire for all 240Volt water heaters

What size circuit breaker? 30 Amp

For safety > a circuit breaker is allowed to handle 80% of Amps shown

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found this interesting site.

http://waterheatertimer.org/Figure-Volt ... eater.html

here's an excerpt;

Check label on side of tank for volts and watts

Every appliance, switch, outlet and bulb has a label

Residential water heaters with 2 elements are 240Volts

Residential water heaters are non-simultaneous

This means both elements are never ON at same time.

Element wattage is tank wattage

If tank has 4500 watt elements, then tank wattage is 4500W

But the tank label you posted has 3800 watt elements, no?

Recommend 30 amp breaker and 10 gauge wire for all 240Volt water heaters

Sure, but I think it is a waste of time and could get you into a pissing match with an electrician who says #12 and 20 amps is correct and meets the code requirement.

BTW, I bookmarked that site. It has info on there even Billy Bob can understand, which could make our job even crazier. How to install service and a breaker panel, etc. That's too much info for some people. [:)]

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