5kW water heater - 30.04A - can I rounding down to a 30A breaker?

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malachi constant

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Title kind of says it all.
On a project I am engineering we are replacing an existing electrical water heater of unknown nameplate value (but we do know it has a 30A/2P breaker) with a new 5kW model. 5000VA / 208V / sqrt(1) x 1.25 = 30.048 amps. Man, would I would love to reuse that breaker and wire... But do I have to go up to a 35A breaker?

(And of course I would have the contractor field verify the wire is in good shape.)
 
Dennis Alwon said:
Article 220.5(B) states we can round down if the fraction is less than .5
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We have looked at this before and I still don't agree that the rule in Article 220 applies here. The rule that requires 30.048 amps is a conductor ampacity rule, not a load calculation rule.
 
don_resqcapt19 said:
We have looked at this before and I still don't agree that the rule in Article 220 applies here. The rule that requires 30.048 amps is a conductor ampacity rule, not a load calculation rule.
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You may be correct. However there is no way this will have a negative affect on the conductor, IMO esp with 125% added in already
 
It is interesting as T. 220.55 is often used for determining ampacity of the conductor. How many 240V 12 kw ranges have you seen that use a #6 with a 50 amp breaker.
 
I am with Don on this one (a rare event, indeed). It seems superficial, and if I were an inspector I might accept it. But by applying Charile's Rule and reading the text as it is written, I would say you need a 35 amp breaker and #8 wire.
 
Following the letter of the text, I think I'm with Dennis on this one.

210.11 says branch circuits...shall be provided to supply the loads calculated in accordance with 220.10.

220.14 (via 220.10) says that appliances loads should be calculated based on the ampere rating of the appliance served. 5kw/208v=24.03. 220.5(B) says that where calculations result in a fraction of an ampere of less than 0.5, the fraction shall
be permitted to be dropped. So the load calculated by 220.10 is 24A.

210.19(A) says the branch circuit conductors shall have an ampacity not less than the maximum load to be served (24A), and that the minimum branch-circuit conductor size shall have an allowable ampacity not less than 125% of the continuous load (24*1.25=30 => #10awg.)

210.20 say the OCPD shall not be less than 125% of the continuous load (24*1.25=30A.) I'd say a #10 wire and 30A OCPD would be acceptable per 210.11, 210.19, 210.20, 220.5 and 220.10.
 
David, that is my take but there is a lot of thinking that 220 is for load calculation of services and feeders. I am not sure why that would not also apply to branch circuits.

My post about the range earlier is also based on this art. I have rarely seen a 12kw range wired with 50 amp wire on a 50 amp breaker simply because of 220.55. This being the case 220.5(B) should apply to branch circuits.
 
Dennis Alwon said:
David, that is my take but there is a lot of thinking that 220 is for load calculation of services and feeders. I am not sure why that would not also apply to branch circuits.

My post about the range earlier is also based on this art. I have rarely seen a 12kw range wired with 50 amp wire on a 50 amp breaker simply because of 220.55. This being the case 220.5(B) should apply to branch circuits.
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I don't know why 220 wouldn't apply for branch circuits, either. Especially since it is titled "Branch-Circuit, Feeder and Service Calculations" and Part II of 220 is specifically for "Branch-Circuit Load Calculations."

210.11 is explicit that the branch-circuits shall be provided for the loads calculated in Article 220.

I agree about the range, a 12kW range would have a load of 33A (@240V) when calculated in accordance Art. 220. Of course, 210.19(A)(3) requires a min 40A branch circuit for the range, so you cant use a 35A OCPD.
 
It sure is a fine line to try and stay on,,,,hmmmmm,,,,,,,,,,,,,,per the book and what side of the bed you get up on may be the answer,,,,,I say 30 today, maybe 35 tomorrow.

dick
 
curious.. storage type or instant water heater...
5kw is unusual here for a storage type water heater.. most are 4500 watt/240 volt
 
Just buy the 240 V version and apply it at 208 V. The wattage is proportional to the square of the voltage. A heater rated 5 kW at 240V will only draw 3.76 kW at 208V (5000 x 208^2/240^2 = 3756). At 208V that's 18 amps and you can use #10 and a 30/2.
 
JoeStillman said:
Just buy the 240 V version and apply it at 208 V. The wattage is proportional to the square of the voltage. A heater rated 5 kW at 240V will only draw 3.76 kW at 208V (5000 x 208^2/240^2 = 3756). At 208V that's 18 amps and you can use #10 and a 30/2.
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We are not sure that is the case. The heater may be a 5kw rated 208V in which case it would draw more at 240V
 
Hm, my understanding of voltage/power/current is that a 5 kw heater is going to draw the amps required for 5 kw regardless of the voltage. That is, it would draw 20.83A if wired at 240V and 24.04A if wired at 208.

If you have a 240V rated water heater and power it with a 208V system, it's going to run at 208V. I don't see how getting a 240V heater is going to help anything in this situation.
 
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