How Do You Size Panelboard Ampacity?

You must first do a load calculation for the panel. Use section 220 of the NEC to assist you.

Are you referring to the demand factors that must be taken?

Appearantly they are.

Where do you find Ampacity just out side of insanity.

Be sure to keep in mind any future expansion. Also, panelboards are notoriously overloaded. If someone sees a space available, they assume that they can add a load. If you're project will utilize the max load of your components, but you still have spaces available in the panelboard, it's always a good practice to put a notice in the panel. I also find that most panelboard drawings don't have the loads documented. While this is required by the AHJ, I still run into it quite often.

Further to ordinary load current, it is much more important to calculate the three phase, symmetrical short-circuit current.

Remember that the short circuit current will be the same irrespective of the load charge of any of the feeders.

To calculate the three phase, symmetrical short-circuit current you will need data about the source of the panel.

For instance, if the panel is fed tru a 13800-220/127V, 60Hz, 500kVA, you may consider the transformer impedanc as beeing 5% and consider as if it is connected to an infinit bus.

Therefore, you will have a secondary short circuit of 26kA!

This will be the short circuit current for which your panel must be rated.

Regards

Hello Guest,

Can you elaborate or detail the calculation? Can you please do the computation for me based on the example you had given. Im trying to follow it but I guess miss some important data.

Thanks.