BREAEKER SIZING

[shocker3218]

At a final inspection today the inspector told me I had wrong breaker size on hot water heaters. I have 4500 watt water heaters on 30 amp breakers.

It is a 208 volt single phase panel.

he says 125% of full load is 20.6 amps and wants 25 amp breakers.

What am I missing? I cannot figure out how he arrived at that number. (I will post how he did tommorrow after he tells me)

 

[templdl]

Re: BREAEKER SIZING

4500w/208v(single phase)=21.6a or am I missing something here? And, X 1.25%=27a.
However, you may just as well pop in a 25a breaker to make the inpector happy not have any problems anyway.

 

[shocker3218]

Re: BREAEKER SIZING

replacement breakers are on the way (43 of them)

I calculated same way you did.

This inspector has been inspecting for quite a while, but this one really throws me off.

 

[bob]

Re: BREAEKER SIZING

If the water heater is rated 4500 watts at 240 volts, then the amperage at 208 is about 16 amps.
1.25 x 16 = 20 amps. If it is 4500 watts at 208 then the amperage is 21.6 x 1.25 = 27 amps.
What is the rating?

 

[ryan_618]

Re: BREAEKER SIZING

See section 422.13 and 422.10 of the 2002.

 

[shocker3218]

Re: BREAEKER SIZING

rated 4500 watts at 240 volts

But isn't a watt a watt? Isn't the resistance of the element a constant?

 

[al hildenbrand]

Re: BREAEKER SIZING

Yes the resistance of the element is constant.

4500W ? 240V = 18.75A

240V ? 18.75A = 12.8 Ohms

208V ? 12.8 Ohms = 16.25A

16.25A x 208V = 3380 Watts.

The 240 volt 4500 watt water heater will run a lot longer on 208 volt in order to warm the same amount of water.

 

[apauling]

Re: BREAEKER SIZING

we don't install 2 amp breakers to protect light bulbs, we install 20 amp breakers to protect the 12 guage wire that feeds the bulb, even if it is the only piece of equipment on the circuit.

paul

 

[iwire]

Re: BREAEKER SIZING

It does not matter if we run 6 AWG to this heater we can not protect it with a 60 amp breaker.

The circuit breaker protects the wire and the appliance.

Check out 422.11(E)(3) if the overcurrent device size is not marked on the appliance you can not exceed 150% of the load current. If no standard size breaker matches you can move up.

As long as the load current is higher than 17 amps the 30 amp breaker would be acceptable.

17 amps + 150% = 25.5 amps round up to a 30.

[ May 05, 2004, 04:21 AM: Message edited by: iwire ]

 

[al hildenbrand]

Re: BREAEKER SIZING

For the water heater that Shocker3218 is describing, however, it appears that the run current doesn't exceed the threshhold for the 30 Amp breaker.

Unless the water heater nameplate says differently, the heater draws 16.25 Amps @ 208 Volts.

16.25A x 1.5 = 24.375A

The next size breaker is 25 Amps.

It appears to me that Shocker's inspector nailed it.

 

[spsnyder]

Re: BREAEKER SIZING

Isn't it fair to say that for a purely resistive load (like a heating element) the resistance is constant. I think this is Shocker's misunderstanding. In a motor load the power is constant. In other words, for a motor the current and voltage are inversely proportional and will vary such that the power of the motor is kept constant. So that a motor fed with lower voltage will draw more current to obtain the power rating of the motor. In a resistive load power will vary depending on voltage as shown by Bob and Al (great explaination Al). Scott

 

[bphgravity]

Re: BREAEKER SIZING

Motors have fixed resistance as well. The effects of inductance adds reactance to the cirucit which changes the power. For motor and other inductive loads, "R" cannot be used for calculations unless you figure in the power factor. Otherwise you must use "Z", the total impedance.

 

[steve66]

Re: BREAEKER SIZING

422.11(E)(3) says overcurrent protection cannot exceed 150% of the rated current which is 4500W/240V = 18.75 amps. 150% of that is 28.125 amps. This can be rounded up to 30A.

I think the 30A is OK. It doesn't make any difference if the heater is actually connected to 208V. That doesn't affect its rated current.

Steve

[ May 05, 2004, 11:58 AM: Message edited by: steve66 ]

 

[jschultz]

Re: BREAEKER SIZING

This is similar to electric ovens. We do multifamily high rises. The ovens have less KW output at 208v than 240V. Someone calculated it out above. Does the water heater have a rating for the 208V value? I wonder if the plumbing engineer understands this. He may have undersized the electric on his water heater.

 

[bphgravity]

Re: BREAEKER SIZING

Originally posted by steve66:
It doesn't make any difference if the heater is actually connected to 208V. That doesn't affect its rated current.

Of course it matters what voltage the load is connected to. The water heater has a fixed resistance by design and manufacturing. Ohm's Law clearly shows that if resistance is constant, any change to voltage will have a change to current. See "al hildenbrands" first post.

A 4500-watt water heater can only consume that power at 240-volts. As the voltage drops, the power drops. In fact, in order to achieve 4500-watts at 208-volts, the current would have to increase.

[ May 05, 2004, 02:22 PM: Message edited by: bphgravity ]

 

[iwire]

Re: BREAEKER SIZING

Bryan I agree with Steve if what he means it will not affect the maximum breaker.

If this unit has a tag that says 4500W @ 240V that is the "rated" current even if it is not the current the unit will draw on 208V.

The wiring inside the unit was tested at that higher current level so basing the maximum OCPD on the tagged current makes sense.

 

[steve66]

Re: BREAEKER SIZING

I did mean that the actual voltage won't affect what size breaker can be used. I do realize that at 208V, the heater will draw less current and produce less heat. Bob said it well.

Steve

 

[jschultz]

Re: BREAEKER SIZING

Originally posted by iwire:
Bryan I agree with Steve if what he means it will not affect the maximum breaker.

If this unit has a tag that says 4500W @ 240V that is the "rated" current even if it is not the current the unit will draw on 208V.

The wiring inside the unit was tested at that higher current level so basing the maximum OCPD on the tagged current makes sense.

current is amps not watts.

 

[iwire]

Re: BREAEKER SIZING

Originally posted by jschultz:
current is amps not watts.

Yes it is.

Here is how the code article is written.

422.11(E)(3)If the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes, not exceed 150 percent of the appliance rated current. 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.

I have never seen a water heater marked in Amps only Watts.

I imagine they expect we can find the rated current from the rated watts and rated voltage.

 

[bphgravity]

Re: BREAEKER SIZING

I would be willing to bet the unit nameplate indicates the wattage at both 240-volts and 208-volts. 4500-watts and 3380-watts respectively.

The nominal voltage of the system must be used when calculating the ampere rating. I understand that the rating is increased on the higher nominal system, but that is not how the appliance is being installed. The logic that we can use the rating at which something is tested is ridiculous. Its like using 600-volts for calculations because my conductor insulation is rated for it, or using the rating of a peice of equipment at 240-volts even though I intend to use it at 120-volts.

[ May 05, 2004, 09:27 PM: Message edited by: bphgravity ]