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Three Phase Power

December 14th Fact-of-the-Day

The current in leg B lags the current in leg A 120 degrees (1/3 cycle) and the current in leg C lags the current in leg B 120 degrees (1/3 cycle) in three phase power systems. The neutral leg current is zero if leg A, B and C currents are equal and each load is linear. Over-current protection is not needed on the neutral circuit with linear loads, because even if the load currents are unbalanced the neutral leg current can never exceed the highest current flowing in any of the three phase legs, so their over-current protection serves to also protect the neutral leg. This is well-known and easy to understand, but many people do not realize that neutral leg current can exceed individual leg currents if the loads are non-linear. In this case the neutral not only needs over-current protection but also needs larger-gauge wire than is needed by the other three legs. ©2004 Martek International All rights reserved.


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