Sizing Disconnect for Equipment

[reelfish]

Where is best place to start looking to size a disconnect for equipment Im going to build?(480v)I hear to add up all 480v power source amperages at full running capacity. Then hear add 1.2 percent.
Does anyone have good place to start with this. Adding up motors and other items using 480v Im coming up with over 800amps. (Messing with allot of amperage). Just the one motor alone pulls 386amp.
There are allot of calculators out there but there must be some rule of thumb?

 

[jraef]

125% of the FLA of everything that could be running at the same time. You have to cover for the possibility.


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[rbulsara]

Also allow for the motor starting currents. Best place to start is the refer to NEC.(regardless of it is legally applicable or not).

If a large motor constitutes a significant % of the total load, a good point to start is to compare what jraef said to

(100% of all loads that could be running at the same time plus 250% of the largest motor FLA.). Use the larger of the two.

It gets a bit complicated if you want to save on the wire size.

Rafiq Bulsara

http://www.srengineersct.com

 

[Mobius44]

Depending on where you are located and the equipment is being installed, you have to follow a specific code.

I can't stress enough that you should research what code(s) you must meet, and understand them. Using a "rule of thumb" can get you (and others using your equipment) in trouble.

For example in the USA I follow NEC, which provides a couple methods to size motor disconnecting means. I use the largest disconnect size found from these two methods.

Method 1:
1A) Add FLA of motors plus all other loads.
1B) Multiply total amp load found in step 1A by 115%.
1C) Select disconnect with amp rating greater or equal to result of step 1B.

Method 2:
2A) Use tables in NEC to determine LRA of each motor.
2B) Add LRA of motors plus all other loads.
2C) Use the LRA tables to determine equivalent motor HP from the total locked rotor amp load found in step 2B.
2D) Select disconnect with HP rating greater or equal to result of step 2C.