In a singlephase power distribution system, all the voltages of the supply vary in unison; in contrast, a threephase system is where the voltages in each phase lag the other phase. Most highpower utility distribution systems use threephase transformer systems to meet the power requirements. Single phase systems are mostly used in applications such a residential lighting, receptacles, air conditioning and heating. They are also used in rural areas that can't afford a threephase system.
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Calculate the amps, or "A," a single phase transformer can handle when you know the transformer size, or "KVA," and the linetoline voltage, or LLV. Use this formula: A = (KVA  1000) / LLV. For example, if you have a 25 KVA, 120/240 volt, singlephrase transformer with an LLV of 240 volts, you can calculate the amp capacity: A = (25 KVA  1000) / 240 = 25000 / 240 = 104 amps.

Calculate the size of the transformer, or "KVA," needed when you know current, A, and LLV. Use this formula: KVA = (LLV  A) / 1000. For example, if your LLV is 240 volts and your current is 200 amperes: KVA = (240  200) / 1000 = 48. This means a 48 KVA transformer is required to handled a line voltage of 240 volts and a current of 200 amperes.

Calculate the LLV needed when you know KVA and A. Use this formula: LLV = (KVA  1000) / A. For example, if you have a 15 KVA transformer and the required current flow of 150 amps, LLV = (15  1000) / 150 = 15,000 / 150 = 100 volts.
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