continous and non-continous loads

[lrob]

I have small piece of industrial type equipment with DC drives, heaters, dc power supply, PLC and servo drive. The motor and heaters all run in cyclic mode. When I calculate my protection current the total load is almost 30 amps of 230vac. When I place a clamp on amp meter and monitor the peak load I max out at around 8 amps. My real question is on an application like this what is the correct way to calculate total system load. I need to protect the system and also know the size of the power feed or transformer. Does any one have a reference I can look at that would help me with load calculations for systems such as these?

 

[dpc]

Per NEC, you take 125% of continuous load and 100% of the non-continuous load. "Continuous" is defined as three hour or more.

That's the basic rule.

Better too big than too small.

 

[bigbillnky]

Hello lrob

If you have already used an amp clamp to read actual current, this equipment must have already been assembled. Are you attempting to reverse-engineer the protection based on actual amp draws? What are you referring to when you state the "protection" current, and how did you calculate it? Do you need to know how to calculate the short-circuit and ground-fault protection, as well as the proper overcurrent protection?

It is not as simple as continous and non-continous loads. Calculating a feeder for multiple motor loads requires a different method than non-motor loads. You must also use the full load currents listed in section 430 of the NEC, not the motor nameplate. Another variable is whether you use fuses or breakers, and what types. Mike Holt and Tom Henry both have excellent books on calculations. I would actually recommend consulting an EE or a master electrician to ensure proper protection. This isn't an area for do-it-yourself type guesswork. You either fully understand how to do it or you don't.


Bigbillnky,C.E.F.....(Chief Electrical Flunky)