3 phase power

Spec says 480V, 3-phase, 4-wire will be provided. My equipment needs a neutral. Does 4-wire mean 3 phase plus neutral or does it mean 3 phase plus ground and no neutral?

In the USA, 3 Phase 4 wire = the 4th wire is earth ground.


Typically, there is no 3 phase neutral as there is no set standard what the voltage to it would be.

It's very odd that a phase to neutral connection is needed. What is the equipment ?


What voltage , phase to neutral, do you need?

To clarify, in the US single phase neutral and ground are tied together at the service entrance from the electric company to the customer. They are both at the same voltage - zero relative to ground. In a single phase system neutral is a current carrying conductor, and ground is provided for safety only and is not intended to carry current in normal use.

There isn't a neutral in a three phase system, altough one leg (delta) or all legs (wye) are usually tied to ground for safety.

I'm guessing that either your equipment is based on some uncommon overseas electrical standard or that you have a three phase supply and need to get to single phase for control. If the second case is true then you need a transformer across two legs of the three phase supply. You ground one leg of the secondary side of the transformer and that becomes your neutral.

If you are at all uncertain about the application I suggest you consult a colleague or other indiidual with more experience. This isn't something you want to learn by trial and error!

In the USA, 3 Phase 4 wire = the 4th wire is earth ground.

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Not always, because MANY manufacuters define a "4-wire" system to be one with a neutral bus. Where there is a chance of misunderstanding, the power system configuration must be clearly defined.

Typically, there is no 3 phase neutral as there is no set standard what the voltage to it would be.

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NO! This is simply incorrect. There are many power systems that do use a 3-phase neutral, and have a dedicated neutral bus. There are plenty of standards and codes that define what a neutral is.

There isn't a neutral in a three phase system, altough one leg (delta) or all legs (wye) are usually tied to ground for safety.

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Careful...I estimated a system yesterday, 480 volts 3-phase, but with a 277 volt phase-to-neutral voltage, so that 1000 watt 277 volt metal halide lighting fixtures could be used. Sometimes a plant has a 3-phase system with a neutral bus but does not use it. Other times, the 3-phase panels have neutral buses that are used, but the electricans are unaware of it. If a 3-phase panel has a used 1-pole breaker, you can be confident that you do indeed have a 3-phase neutral.

Does 4-wire mean 3 phase plus neutral or does it mean 3 phase plus ground and no neutral?

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Without more information, I would say "4-wire" as it is most currently used, means "3 phase conductors and a neutral conductor". Due to NEC grounding requirements, normally the neutral bus for a Wye secondary system would also be grounded (connected to ground).

EDIT: I just pulled out a Square D catalog and looked at Section 6, Panelboards. Square D defines a 3-phase 4 wire panel to be one with a neutral bus.

+1 on what Lancie said.

There is a 480 three phase four wire standard. 277V is widely used in industrial lighting.

Godfrey, you need to clarify the meaning with the manuf. if he means a neutral or a ground. Most of the time the ground conductor is assumed so three phase four wire means a neutral but I've found that all too often its used to mean grounded three phase.

However, if this is a machine tool and the manuf is trying derive a voltage of Line/1.73 for something on the system I think its better to use a transformer.

Our system is UV disinfection for municipal watewater. Our cabinets require 3 phase plus neutral. The ballasts run on 277 so we take each leg to a bank of ballasts and use neutral for return. Sometimes a neutral from the MCC is not available so we provide a delta-wye transformer to get the neutral.
At this point in the bid there are no plan drawings to confirm so I will get confirmation from the consulting engineer.
Thanks for all the comments.

Sometimes a neutral from the MCC is not available...

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That is a common situation because it costs extra to put a neutral bus in a motor control center. It costs also to run neutral current-carrying conductors (insulated and sized correctly) from the main transformer to the MCC. Normally you don't get the MCC neutral bus unless you ask and pay for it.

Hey Lancie, good post. I am a licensed electrician in Tx, and La. I have been seeing your posts on sevseral plc programming questions. I did not know you had electrical skills also. Much respect!

Actually, in addition to single phase 277V lighting, I've seen whole hotels powered by 480V 4 wire where all the AC loads in the rooms are 277V, including 277V single phase fan motors. They divide up the rooms feeding an equal number from each phase to neutral.

I would also say that 480V three phase 4 wire means with a neutral that is grounded at the source transformer.

in 3 phase system ...esp asia ..nuetral will be provideed after wye trasformer connection ..even ground and nuetral are same at this point.
all lighting system will be using nuetral in this case

Hey Lancie, good post.

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Travis, thanks for the kind words.