Heater Calculations Continued

[Krim]

This is the original post that I would like to continue :
http://forums.mikeholt.com/showthread.php?t=52313&highlight=Heater+Calculations

The following is the message that I'd PM'ed to Ed MacLaren in hope of resolving a rather simple question :

Ed Maclaren,
Through a search I came upon your comments posted in a "Heater Calculations" post . I have a similar problem and by using your information in performing my calculations, I end up with 20.84 Amps OCP required. My question is " Would I be OK using a 20 Amp CB , or do I have to go with a 25 - 30 Amp ?".
The details : 6kw immersible (over the side) tank heating elements, 208-three phase, that will be outdoors maintaining water temperature for oyster spat to grow in . This is all the info that I've been provided with so I guess the heaters will have to be based on 100% initial power factor .
Thank you,
Carl

I'd appreciate it if you'd post your expert opinions after reading .

Carl

 

[rattus]

This is the original post that I would like to continue :
http://forums.mikeholt.com/showthread.php?t=52313&highlight=Heater+Calculations

The following is the message that I'd PM'ed to Ed MacLaren in hope of resolving a rather simple question :

Ed Maclaren,
Through a search I came upon your comments posted in a "Heater Calculations" post . I have a similar problem and by using your information in performing my calculations, I end up with 20.84 Amps OCP required. My question is " Would I be OK using a 20 Amp CB , or do I have to go with a 25 - 30 Amp ?".
The details : 6kw immersible (over the side) tank heating elements, 208-three phase, that will be outdoors maintaining water temperature for oyster spat to grow in . This is all the info that I've been provided with so I guess the heaters will have to be based on 100% initial power factor .
Thank you,
Carl

I'd appreciate it if you'd post your expert opinions after reading .

Carl

Krim,

Your post implies three 6KW heaters, but your numbers imply three 2KW heaters. Which is it?

 

[Dennis Alwon]

No disrespect for anyone but why do you think no one on this forum can answer that question? It appears pretty staright forward.

6000/360= 16.6
16.6*125%= 20.8

Your wire has to be sized for 30 amps since #12 is only good for 20. I am sure a 20 amp breaker will probably hold if it is rated for continuous load.
25 or a 30 would be my choice.

Think of a standard water heater. 4500/ 240 = 18.75 amps.

We always use 10 wire with either a 25 or 30 amp breaker.

18.75 * 125%= 23.4

Am I missing something?

 

[Krim]

Krim,

Your post implies three 6KW heaters, but your numbers imply three 2KW heaters. Which is it?

rattus,
I will be installing 6 to 10 immersible heater elements of which mount on the rim of their corresponding tank. The tanks are 13 feet side to side and 4 feet deep, they will be sea water filled. The heaters are each 6kw operating on 208Y,3 phase,4 wire. According to the calculations provided in the post by Ed MacLaren, I end up with 20.84 Amps over current protection required in a circuit breaker. My question is that with that result (20.84 A) should I size the CB at 20 amps or go higher ? I'm not sure how you figure the power factor considering that once the desired water temperature is reached the power factor demanded would naturally be decreased to maintain a constant temperature. Am I safe at basing the pf at 1 (100%) or do I need to base it on 1.25 (125%) ?
Carl

 

[Krim]

No disrespect for anyone but why do you think no one on this forum can answer that question? It appears pretty staright forward.

6000/360= 16.6
16.6*125%= 20.8

Your wire has to be sized for 30 amps since #12 is only good for 20. I am sure a 20 amp breaker will probably hold if it is rated for continuous load.
25 or a 30 would be my choice.

Think of a standard water heater. 4500/ 240 = 18.75 amps.

We always use 10 wire with either a 25 or 30 amp breaker.

18.75 * 125%= 23.4

Am I missing something?

Dennis , Thanks for your input ... I'd come up with the 20 Amp as well but was not sure if I should go with that or up size to either a 25 or 30 A, hence the reason for the posted question. I also wasn't really clear on whether to base the power factor on 1 or the 1.25 (100 or 125 %).

Carl

 

[rattus]

rattus,
I will be installing 6 to 10 immersible heater elements of which mount on the rim of their corresponding tank. The tanks are 13 feet side to side and 4 feet deep, they will be sea water filled. The heaters are each 6kw operating on 208Y,3 phase,4 wire. According to the calculations provided in the post by Ed MacLaren, I end up with 20.84 Amps over current protection required in a circuit breaker. My question is that with that result (20.84 A) should I size the CB at 20 amps or go higher ? I'm not sure how you figure the power factor considering that once the desired water temperature is reached the power factor demanded would naturally be decreased to maintain a constant temperature. Am I safe at basing the pf at 1 (100%) or do I need to base it on 1.25 (125%) ?
Carl

Then each heater comprises three 2kW elements and is supplied by a 208V 3phase branch circuit. Got that.

But, you are using "power factor" to mean "duty cycle". And, to compute the duty cycle, one must know the rate of heat loss. That requires special knowledge. I would assume 100% duty cycle, compute the current as you have done, and upsize the breakers and conductors as suggested by Dennis.

"Power factor" is 100% in resistive loads.

 

[Krim]

Then each heater comprises three 2kW elements and is supplied by a 208V 3phase branch circuit. Got that.

But, you are using "power factor" to mean "duty cycle". And, to compute the duty cycle, one must know the rate of heat loss. That requires special knowledge. I would assume 100% duty cycle, compute the current as you have done, and upsize the breakers and conductors as suggested by Dennis.

"Power factor" is 100% in resistive loads.

Thanks rattus, the higher ups are discussing what type of covering they want to use on these tanks to prevent the bulk of heat loss,that would also not restrict the amount of air available.
Carl