I'm doing a few basic load calculations on a 3 phase circuit at work and I stumbled on a concept I'm not sure how to proceed. Let's say you have a 3 phase load and it is basically a black box. For many reasons, you don't have any access to the load details (no schematic or circuit) but you know nameplate information like the power draw (in VA), the PF (and that it is lagging), FLA, etc. Based on these parameters, I proceeded to calculate the 3phase impedance of each phase so I can combine it with other loads on the system, but I'm not sure how the load should be attached to the system, that is whether if the load should be presented as a Delta or Wye load to the power system. I would think it makes a difference, since if it's Delta, the phase/line currents aren't equal and could throw off my calcs.

I don't do calcs like this very often so maybe I'm missing something very obvious. What is the best way to model/estimate a scenario like this?

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    \$\begingroup\$ How many wires has this "black box" got and what colour are they? \$\endgroup\$
    – Andy aka
    Aug 24, 2018 at 17:10

1 Answer 1


Assuming that you have only three terminals, no neutral connection, you can treat it as either wye or delta. For most purposes, it is probably better to assume it is wye. As long as you know the rated line-to-line voltage, there reason to be concerned about the actual internal connection.


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