HP RATING 2

[tulum]

How do fused disconnects get a Horsepower rating?

I have a squared fused disconnect (600V, 60A, c/w 60A fuses)...it has a rating of 30HP on the box from the manufacturer...

Any Idea...

Regards,
TULUM

 

[waross]

You have to use dual element fuses, or time delay fuses.
respectfully

 

[tulum]

Not sure I get you...

So because the fuses (in this case AJT60) is time delay it better protects motor loads...not sure though how 30HP is the magic number?

Regards,
TULUM

 

[waross]

I may have misunderstood your question.
Unless the switch is rated at 100%, it will be suitable for 48 amps, continuous current, under CEC. That is probably insuficient for a 50 hp. motor, particularly when you add the extra 25% ampacity required for motor circuits..
I thought you were wondering how you could start a 30 hp. motor on a 60 amp switch. You can't with the normal fuses.
respectfully

 

[CJCPE]

In general, I believe that fused switches get a horsepower rating by design and test to CSA or UL standards. That would include sizing to CEC and NEC code requirements for continuous current plus code and/or agency requirements for starting currents and interrupting load (locked rotor?) current.

 

[dpc]

Refer to NEMA KS-1 - this is available for download at

http://www.nema.org.

The horsepower rating is defined differently for fused and non-fused disconnects.

For fused disconnects, it's the largest horsepower that can be started using standard fuses sized at the disconnect switch max current rating. There's a "standard" horsepower rating that applies to any type of fuse and a "maximum" horsepower that applies to time-delay fuses.

In the US, any disconnect sw used as a motor disconnect must be "horsepower rated", but I've never seen one that wasn't, so it doesn't seem to mean much in reality.

 

[tulum]

Thanks for the tips guys...

I will look into th download

Regards,
TULUM

 

[jraef]

Notwithstanding the fuse issue mentioned by dpc which can serve to lower a HP rating of a fused vs non-fused disconnect, disconnect switch maximum HP ratings have to do with inductive load breaking capacity, the ability of the switch to interrupt locked rotor current safely as mentioned by CJCPE. Maximum current load capacity is based on resistive loads.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[rbulsara]

What jraef said..