Another Water Heater question

[brother]

Another water heater went out. I am curious as to how the manufacturer wired the heating elements, this is assuming nothing was changed on it over the years, and the name plate is right. The fuses feeding it was too small in my opinion even for the name plate.

But I am not sure how they came up with the 43 amps. on a 30 kw. I can only assume it only operates 3 heating elements, or 4 at most. Since they are 5kw each. What do you think?

5k X 3= 15000 watts. 15000/480= 31.25
31.25 x 1.25= 39.0625

or 4 x 5k= 20,000 watts 20000/480 = 41.66

maybe they took the 41.66 amps and round up to 42 amps. So the name plate says 43. No paper work on it as it has been there for years they tell me. They used onlly 40 amp fuses to supply it. I wouldve at least went with the 50amp, and thats without the 125%. added.

Am I correct in thinking that only 4 heating elements are going at 1 time?

 

[don_resqcapt19]

You have 30 kw at 3 phase, 480 volts for a total load of 36.1 amps.
30000/480/√3
Not sure how they got to 43 amps on the nameplate.

 

[dm9289]

You have 30 kw at 3 phase, 480 volts for a total load of 36.1 amps.
30000/480/√3
Not sure how they got to 43 amps on the nameplate.

Sort of odd their document agrees with your 36 amp. Maybe the sticker guy was 1 column off.

 

[infinity]

But I am not sure how they came up with the 43 amps. on a 30 kw. I can only assume it only operates 3 heating elements, or 4 at most. Since they are 5kw each. What do you think?

5k X 3= 15000 watts. 15000/480= 31.25
31.25 x 1.25= 39.0625

or 4 x 5k= 20,000 watts 20000/480 = 41.66

Don's answer is correct for the total load of 30 kw. If you're calculating for 3Ø your math left out the 1.73 factor (√3) for 3Ø voltage,

 

[busman]

The correct FLA is 36. I thought maybe the 43 was 1.25 for continuous load, but that would be 45A. However, 43A would be 1.2 times. I wonder if someone thought 1.2 was the reciprocal of 80%?

Mark

 

[brother]

Sort of odd their document agrees with your 36 amp. Maybe the sticker guy was 1 column off.

Thanks for info. Where did you find the document or manual? Do you have a link? Does it have a recommendation on breaker and wire size?

 

[brother]

You have 30 kw at 3 phase, 480 volts for a total load of 36.1 amps.
30000/480/√3
Not sure how they got to 43 amps on the nameplate.

Thanks don. Brain spasm left out 1.73 trying to get the same answer the manufacturer had.