How to Calculate a Single-Phase Transformer


In a single-phase power distribution system, all the voltages of the supply vary in unison; in contrast, a three-phase system is where the voltages in each phase lag the other phase. Most high-power utility distribution systems use three-phase 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 three-phase 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 line-to-line voltage, or LLV. Use this formula: A = (KVA --- 1000) / LLV. For example, if you have a 25 KVA, 120/240 volt, single-phrase 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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