3ph-3w vs 3ph-4w

[Designer69]

why do you sometimes need to run a neutral to a 3 phase load?

IE, a 3 phase air compressor with a control panel, design shown is 4 pwr and 1 gnd.

Thanks

 

[Jraef]

why do you sometimes need to run a neutral to a 3 phase load?

IE, a 3 phase air compressor with a control panel, design shown is 4 pwr and 1 gnd.

Thanks

Only if they are using the neutral for something. For example in some 208V panels, they use 120V controls by connecting L-N rather than use a Control Power Transformer. A 3 phase motor doesn’t care though.

 

[adamscb]

If a load is three-phase, then all line currents should be equal, and therefore no current travels on the neutral. If a panel is 3ph-3w then it can't serve single-phase loads. If it's 3ph-4w then it can serve single-phase loads.

 

[smoothops10]

Both good responses but keep in mind I believe NFPA 79 requires the CPT be less than 1000 VA to limit short circuit current available to the 120V control pilot devices and relays. Probably only a real factor if the control panel is real close to the source but in general I've defaulted to specifying all 208V 3 phae controls panels as 3W+G w/ CPT in lieu of 4W+G w/o CPT so I don't have to worry about it. Unbalance due to a 50 or 150 VA CPT is generally negligible and can be ignored.

 

[kwired]

If a load is three-phase, then all line currents should be equal, and therefore no current travels on the neutral. If a panel is 3ph-3w then it can't serve single-phase loads. If it's 3ph-4w then it can serve single-phase loads.

If load is three phase only there isn't even a neutral conductor on the load to connect to.

A 3 ph-3w source can supply single phase loads - they will have to be line to line and not line to neutral (ie. 208 volts and not 120).

 

[smoothops10]

Again NFPA 79 I believe limits control voltage to max 120V or 200V nominal. I don't have it it in front of me but from what i recall utilizing 208V LL single phase for pilot control ckts would not be allowed if building to this standard.

 

[adamscb]

If load is three phase only there isn't even a neutral conductor on the load to connect to.

A 3 ph-3w source can supply single phase loads - they will have to be line to line and not line to neutral (ie. 208 volts and not 120).

Yes k-wired, you are right, thanks for clarifying. Should have said it can't supply L-N loads.