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Answer for USA, Canada and countries running a 60 Hertz supply service

If you use 500 MCM wire rated at 90 degrees C which is rated at 515 amps and allow for 80% ampacity of the wire which equals 412 amps, the conduit size will be three inches. If you use 700 MCM wire rated at 60 degrees C which is rated at 500 amps and allow for 80% ampacity of the wire which equals 400 amps, the conduit size will be three and one half inches.

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Q: What size conduit for 400 amps?

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4 inch rigid conduit

The question can't be answered without knowing what the voltage is. That is because the allowed voltage drop is 5% of the supply voltage, so you need to know the supply voltage. Then the wire calculation aims to find out what the minimum size of wire is that produces that voltage drop or less.

400 Volts X 45 Amps = 18,000 Watts

The sizing of conduit systems that wire has to be pulled into is dependant on the physical size of the conductor being drawn into the conduit. The second governing factor is the conductor count of the specific conductor that is needed. A 500 MCM copper conductor with an insulation factor of 90 degrees C is rated at 430 amps. This is where the conductor count is need by the service, as to whether the service is single or three phase. In this specific case, the electrical code will allow both single and three phase installations to use the same size conduit. A maximum total of four 500 MCM conductors can be drawn into a 78 mm diameter conduit.

The cable that is used to connect from the generator is based on the size in kW's of the generator and the generator's breaker size rated in amps. It usually is a flexible cab-tire 4 wire cable if the generator is a portable type. If it is a stationary generator is has to be wired with a flexible conduit so as not to transmit vibration from the generator to the conduit system.

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120mm

4 inch rigid conduit

4 inch rigid conduit

Couple of things wrong here. 30 isn't a wire size. If you mean a wire that can carry 30 amps that would be # 10. Now it depends on how many # 10 wires you want to put into a conduit that governs the size of the conduit.

No, the conduit has to be supported individually from a supportive structure. The spacing of the strapping is related to the size of the conduit.

The question can't be answered without knowing what the voltage is. That is because the allowed voltage drop is 5% of the supply voltage, so you need to know the supply voltage. Then the wire calculation aims to find out what the minimum size of wire is that produces that voltage drop or less.

A #10 copper wire with an insulation factor of 90 degrees C is rated at 30 amps. If you need the full 30 amps for the load use a #8 copper wire. Loaded to 80% of rating will give you 32 usable amps.

400 Volts X 45 Amps = 18,000 Watts

For a distribution panel rated at 400 amps parallel runs of 3/0 conductors will do the job.

A #4 copper conductor will limit the voltage drop to 3% or less when supplying 25 amps for 400 feet on a 120 volt system.

The sizing of conduit systems that wire has to be pulled into is dependant on the physical size of the conductor being drawn into the conduit. The second governing factor is the conductor count of the specific conductor that is needed. A 500 MCM copper conductor with an insulation factor of 90 degrees C is rated at 430 amps. This is where the conductor count is need by the service, as to whether the service is single or three phase. In this specific case, the electrical code will allow both single and three phase installations to use the same size conduit. A maximum total of four 500 MCM conductors can be drawn into a 78 mm diameter conduit.

50 mm or 2 inch will handle the 3/0 wires for a 200 amp service.