Single phase to 3 phase converter calculation

[katwalatapan]

Hello,

I wanted to inquire, if my understanding of single phase to 3-phase converter application is correct in the following scenario:

For a facility with 1-phase, 120/240V, 3W, electrical service, an elevator is planned to be installed with 20 HP rating, 3-phase, 208V, FLA - 72A, LRA - 166A, via a digital 3-phase converter. If the facility has sufficient capacity to accommodate the load, the two (2) line conductors going into the 3-phase converter and the three (3) line conductors coming out of the to feed the 3-phase load, should be sized at 250% of 72A?

I'd appreciate if you would provide comments to verify my understanding.

Thank you.

 

[jraef]

Huh. Seems to me I have seen (and responded to) this post somewhere before.

You are confusing the minimum and maximum limits of the NEC as being a design tool, they are not. You are permitted to over size conductors in any way you see fit, although the ability to terminate them becomes problematic at some point. You just cannot UNDER size conductors, and you cannot OVER size protective devices.

Also, when feeding a phase converter, the single phase input must still provide ALL of the same kW as the 3 phase output is delivering. So the single phase input current is higher than the 3 phase output current by a factor of the square root of 3, 1.732. So size your output conductors appropriately for the load, then make the input conductors larger by 1.732x. Protection of both sides should be selected by consulting the installation manual of your phase converter first, because they often have specific requirements that may be above and beyond the minimums required by code.


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