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How do you convert 208V to 240V?

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How can I get 240v single phase power from a 3-phase 208v line?

You can’t get it directly, though there’s a lot of equipment rated for 240V (usually specified at 230V) operation, that will still function at 208V, though it will produce 25% less power at that voltage.

If you actually want 240V, you’ll need either (a) a two-winding isolation transformer rated for the full load kVA, or (b) a buck-boost transformer, connected in boost mode, and rated for the product of the load current and the difference between the input (209V) and output (240V) voltage.

The isolation transformer solution will be connected as follows:

The buck-boost configuration would utilize a two-winding transformer connected in boost mode, as follows:

As an application example, assume the load is rated for 10kVA at 240V. If you go with an isolation transformer, it will need to be rated for the full 10 kVA, with a 240V primary winding and a 208V secondary winding, and thus be considerably heavier and more expensive than the buck-boost transformer solution. An example of such a transformer would be this one.

If you go with the buck-boost transformer solution, its full load current at the specified load voltage will be 10,000/240 = 41.7A. That means the VA rating of your buck-boost transformer will only need to be (240 - 208) x 41.7 = 1.33 kVA or higher. An example of such a transformer would be this one.

Note that with the isolation transformer, you are creating a separately-derived system and must ensure the secondary neutral is bonded to the grounding electrode system. And with either transformer type, you must provide overcurrent protection on the transformer primary and/or secondary per NEC 450.3,

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