25kW EDH unit

Status
Not open for further replies.
K

kropjl

Member
Location
Cleveland, OH
Hello,
I got an as-built that says that a specified 25kW, 480-3 phase EDH unit has a 60A/3P disconnect switch with 40A fuses. Forgive me, I am a new electrical engineer so if any of my thinking is incorrect here, please let me know where I am going wrong, but the 60A/3P disc. w/ 40AF does not seem plausible to me. The load on each phase for the EDH unit should be 25kW/3 = 8.333kW, which the as-built's panel schedule agrees with. Here is where I may be thinking of things incorrectly, but doesn't a 40A breaker have a load of 40Ax120Vx.8=3840W? A 60A disconnect have a load of 5760W per phase?

Thanks!
 
kropjl said:
Hello,
I got an as-built that says that a specified 25kW, 480-3 phase EDH unit has a 60A/3P disconnect switch with 40A fuses. Forgive me, I am a new electrical engineer so if any of my thinking is incorrect here, please let me know where I am going wrong, but the 60A/3P disc. w/ 40AF does not seem plausible to me. The load on each phase for the EDH unit should be 25kW/3 = 8.333kW, which the as-built's panel schedule agrees with. Here is where I may be thinking of things incorrectly, but doesn't a 40A breaker have a load of 40Ax120Vx.8=3840W? A 60A disconnect have a load of 5760W per phase?

Thanks!
Click to expand...

IMO, 25 kW, 480V, 3-phase heater has a full-load of 30A. A 40A fuse seems right. FYI standard speed fuses have minimum melting amps of about twice the fuse rating. Try surfing the web for "fuse minimum melting" and see it for yourself.
 
kropjl said:
Hello,
I got an as-built that says that a specified 25kW, 480-3 phase EDH unit has a 60A/3P disconnect switch with 40A fuses. Forgive me, I am a new electrical engineer so if any of my thinking is incorrect here, please let me know where I am going wrong, but the 60A/3P disc. w/ 40AF does not seem plausible to me. The load on each phase for the EDH unit should be 25kW/3 = 8.333kW, which the as-built's panel schedule agrees with. Here is where I may be thinking of things incorrectly, but doesn't a 40A breaker have a load of 40Ax120Vx.8=3840W? A 60A disconnect have a load of 5760W per phase?

Thanks!
Click to expand...

Electricians typically calculate this using actual amps, engineers typically divide the VA over the three phases and calculate that way.

As you can see from the calculation Bob posted the actual amps is about 30, multiply by 1.25 for continuous load and a 40 amp overcurent device is necessary.

I think you got the wrong numbers in this formula "40Ax120Vx.8=3840W?"

You said this is a 480 volt unit so you should be using 277 line to neutal volts and not 120. Not sure why you have a .8 factor in there either, should be 1.25 IMO. But you also need to start with actual load and not the overcurrent protection

25kW /3 = 8.333kW per phase

8333 W / 277V = 30 amps (give or take a fraction) This is actual amps and is same as what Bob calculated with a little different method. Again the 40 amp figure is the overcurrent device @ 125% rounded to next standard device.

That help?


Add, when I questioned the .8 factor, I think I see what you were doing, but you needed 277 and not 120 in the other portion of your formula. The .8 was an inverse way of calculating to see if the 40 amp overcurrent device was enough for the load
 
Last edited:
Status
Not open for further replies.
Top