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My apartment house feed is 120/208v 4 wire (plus ground) wye connected.

Apartments have 120/240v service.

I can't see how the 240v circuit is derived from 120/208.

I'd welcome an answer or a link!

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    Are you sure the apartment are 120/240v and not 120/208? The configuration in an apartment unit would be very similar, 2-120v legs, a neutral and a ground. 120v phase to neutral and 208 phase to phase.
    – JACK
    Jul 14 '20 at 2:15
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    Can you measure the phase-to-phase voltage in an apartment? (Say, at a NEMA 10, 14, or 6 receptacle) Jul 14 '20 at 2:16
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    the term "wye connected" and 208 tells me 3 phase, which I won't try to solve bc I don't understand it well enough. OP must be in Europe someplace. Jul 14 '20 at 14:15
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    The short answer is that in general, it's not -- most 240V appliances also work fine at 208V by design, to handle this exact situation. If you actually need to get a true 240V from a 208V supply, you can use a step-up transformer, but most likely you'll have no need to.
    – Nate S.
    Jul 14 '20 at 16:06
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    @george Anderson it is fairly common in the US for larger complex’s to be fed with 3 phase and at the main broken down to feeds like jack said+ I have worked on the heat pumps in a complex and there was no problem there most inverter based units today are universal voltage and even a old school compressor will work in a worst case the start /run cap may need to be changed but they do function.
    – Ed Beal
    Jul 14 '20 at 16:55
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It isn't.

120/240V is a 240V single phase supply, center tapped to produce a "split phase" with 120V between either side to neutral, and 240V between the two hot lines.

120/208V is two phases from a three-phase supply. There is 120V from either phase to neutral, and 208V between the phases.

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Apartments have 120/240v service.

Says who? Apartments have the service they have. 120/240V implies a “split-phase” service which is universal for single-family dwellings outside NYC, but apartment buildings are actually commercial and are often wired to commercial standards. It really depends what the power company has in the area, and what they are willing to provision to the development.

My apartment house feed is 120/208v 4 wire (plus ground) wye connected.

Well, then, that is that. Your apartment does not have 240V service.

Water heater, range and dryer are resistive loads, and they will work alright on 208V instead of 240V, just will heat at a 25% reduced capacity. Consider that when sizing those appliances, although it’s usually not a big deal.

Motor loads (A/C; heat pump water heater or dryer) will care about 208V vs 240V, but many of them can be internally jumpered for either voltage. Shop carefully!

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the advantage of feeding a complex with 3 phase is the phases can be split up and balanced allowing smaller feeders at the main. The only negative the water heater will be slightly slower to heat up same with the oven or range and dryer. The dryer motor is usually 120 v so that is not a problem. In residential complexes there are normally very few 240 motor loads, a 240 motor will usually work ok , remember the voltage described by code is 208 / 230 so we are right at +-10% so most motor loads won’t have a problem. The big advantage comes when the complex has elevators or provides hot water or cooling 3 phase power is more efficient than single phase .

Today 208 is the common “low voltage” 3 phase years back it was 230v

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