Heater Calculations

[jhabby]

Question: 3 phase heater 480v. 45KW. What should the fuse protection be rated at? This is what I believe. 45000/480=93.75 93.75/1.73=54.19 amps.
Next greastest nominal size is 60amps. Is this correct? Also is there deratings or "pro" rating.
Anyone have any input, and exact fuse numbers that will work well. Just making sure these are indeed the right fuses.
Thanks

 

[bennie]

Re: Heater Calculations

The actual current will be less than the calculation. A 50 amp will likely hold.

 

[jhabby]

Re: Heater Calculations

Ok, problem being a 60 amp isnt holding. It is intermittent. Iam trying to solve this systemactically. First issue was making sure the fuses are of the right type and sized properly.
Second step was to check loose connections, megg the wires, ring out the heaters themselves, etc.
This hasnt been done yet. It is actually happening on two different heaters, same applicaton.

 

[bennie]

Re: Heater Calculations

Take a voltage and current reading.

 

[Ed MacLaren]

Re: Heater Calculations

Electric heating loads are normally treated as continuous loads, and would require that the fuse or breaker be rated at not less than 125% of the load current, unless the fuse or breaker is part of an assembly approved and marked for continuous loads at 100% of their rating.

54.19 amps X 1.25 = 67.73 amps - Use 70 amp OCPD minimum.

Ed

 

[bennie]

Re: Heater Calculations

There is something wrong if the heater is taking out a 60 amp fuse.

 

[charlie b]

Re: Heater Calculations

Ed is right, in that the fuse should be "pro-rated" (as you put it) to 70 amps. Bennie is right, in that a 60 amp fuse?s failure to hold is indicative that something must be wrong. But it is your use of the word "intermittently" that concerns me. "Ghost problems" (i.e., that come and go as they please) are tough to troubleshoot. You could have a insulation breakdown with leakage current to ground, but it might pass a megger test today and fail the same test tomorrow. I have one question though: What size of conductor feeds this heater, and how does it look (especially in the area close to the heater)?

 

[bennie]

Re: Heater Calculations

This appears to be a malfunction of a common component.

Are the heating elements connected in a star configuration?

 

[templdl]

Re: Heater Calculations

Another thought, have you determined if the breaker is tripping on thermal? Is the breaker warm and/or do you have to wait for a bit before you can reset the breaker and reclose it? If so this is the may the discussion is going so far. If it can be reset immediately after tripping the breaker is most likely tripping on magnetic or instantaneous as a result of a fault of some type which may be intermittent and darned hard to find.
As Bennie said take a current reading and if you are lucky you may get a reading at the time the breaker trips.
Also, check the load connections on the breaker. If there is heating there it would cause the breaker to derate and nuisance trip.

 

[bennie]

Re: Heater Calculations

I am also bothered by the word "intermittent".

Take a look at the distribution transformer supplying this building.

Is there one or two high voltage bushings on each pot?

[ April 11, 2003, 10:30 PM: Message edited by: bennie ]

 

[jmc]

Re: Heater Calculations

The question did not indicate what the 45KW heater is used for. Article 424 applies to fixed electric heating equipment and Article 427 applies to pipeline and vessel heating. In both cases, the branch circuit must be sized at 125 percent of the load. In addition, Article 424, section 424.22(B) requires that the heating load be subdivided and not exceed 48 amperes per group.