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I have connected a three-phase incoming power to a MCCB, MCCB will distribute to a 3-phase transformer as well as few single phase equipment, which is means outgoing of MCCB L1, L2 & L3 to transformer, L1 and N (neutral is from incoming distribution) to a 1–5A single phase AC-DC power supply, L3 and N to a 10–15A single phase heater. The problem is, is the wiring correct and safe to use?

Purpose of using transformer is to supply 3 phase 230VAC to servo motor as servo motor support only 3 phase 230VAC instead of 3 phase 415VAC. Besides, that's means I am safe to use two MCCBs (one for transformer another one for single phase equipment) from incoming power supply (from factory distribution board)? Will it damage the 3 pole MCB in the factory distribution board which supply to my two MCCBs?

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  • \$\begingroup\$ It's perfectly normal to mix single-phase and three-phase loads on the same supply. But beyond that, it's impossible to say without seeing the installation. \$\endgroup\$ – Simon B Apr 17 '18 at 12:08
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MCCB will distribute to a 3-phase transformer as well as few single phase equipment

That is not correct. The MCCB should connect to a distribution board that contains branch circuit breakers for various three-phase and single-phase loads loads. Alternatively, there could be two MCCBs, one for the transformer and one for a distribution panel for single-phase loads. There is also a question of why there is a transformer if the voltage at the MCCB is suitable for the small loads.

Two circuit breakers can be used as shown below provided that they are properly rated to protect the connected wiring and loads. The wiring from the circuit breaker in the factory distribution board must be sized to be protected by that circuit breaker and that circuit breaker must be rated to carry the current in the phase (marked C) that carries both connected loads.

enter image description here

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  • \$\begingroup\$ Purpose of using transformer is to supply 3 phase 230VAC to servo motor as servo motor support only 3 phase 230VAC instead of 3 phase 415VAC. Besides, that's means I am safe to use two MCCBs (one for transformer another one for single phase equipment) from incoming power supply (from factory distribution board)? Will it damage the 3 pole MCB in the factory distribution board which supply to my two MCCBs? \$\endgroup\$ – Ran Apr 18 '18 at 4:13
  • \$\begingroup\$ Dear Sir, appreciate for the explanation. Unfortunately, I was using this type connection as shown in the picture, however, it had damage C pole of the first MCCB (left in the picture). Equipment using single phase power are heater and a AC-DC power supply, I am not sure whether the heater using too high ampere to cause damage of the C pole of MCCB which cause by not balance of 3 phase. Thank you. \$\endgroup\$ – Ran Apr 19 '18 at 1:50

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