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Re: Single phase load on 208/120Y calculation req'd by Oreg

Originally posted by rattus: The apparent power in this case is provided by two legs of a wye. From the transformers' point of view, each is providing 120 x I VA,

** and that is of prime importance. **
I would say that it is of no importance. It fits into the category of “true, but irrelevant.” The load taken by a single phase 208 volt load is indeed shared equally between two legs of a WYE secondary. But so long as we make a reasonable effort to balance the loads among the three phases, we do not ever need to know the exact amount of load that any given secondary winding is supplying to any given load.

I use a Panel Schedule with built-in demand factors (and the occasional 25% add on factor) to calculate the total load on a transformer. The panel schedule computes the VA that each load takes from each phase. This is a “background” part of the process of sizing a transformer; it does not get any of my attention. I only need to check to see that the load is reasonably balanced, and then to look at the total load.

**However, the load's point of view, it is only receiving 208 x I VA. It feels slighted. How come? It is because the phase angles of the phase voltages cause the vector sum to be 208 instead of 240. **
It does not feel slighted. If the load is rated for 208 volts, and if it gets 208 volts, then it is getting what it needs.

I again suggest that the confusion here comes from the following three concepts:

</font>- <font size="2" face="Verdana, Helvetica, sans-serif">A load can be rated for 208 volts. A load can be rated for 240 volts. A load can be rated to handle any voltage in a given range (e.g., from 208 to 240 volts).</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>- <font size="2" face="Verdana, Helvetica, sans-serif">A panelboard can be rated for 120/208 volts (these are 3 phase boards). A panel can be rated for 120/240 volts (these are single phase boards).</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>- <font size="2" face="Verdana, Helvetica, sans-serif">A power source (e.g., transformer secondary) can supply 208 volts. A power source (e.g., transformer secondary) can supply 240 volts.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">We will never understand the purpose (if any exists) of the 1.154 factor until we know for certain what the question uses as its given configuration, in terms of the rating of the load, the rating of the panelboard, and the rating of the power source.

Charles E. Beck, P.E., Seattle

Comments based on 2014 NEC unless otherwise noted.

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