3 phase

[keniff]

Hi guys don't do alot of three phase so bare with me, am I right in thinking for example if your feeding three phase equipment say 60 amp, you size the cable on 20 amps per phase rather than 60 amp per phase, also where can I find info on neutral sizing in 3 phase systems, thanks

 

[malcolmsanford]

Hi Keith yes if it is 3 phase 20amps per phase then you size the cable as you would per phase.

In all honesty mate unless you are going to be working with some pretty hefty cables then basic rule of thumb is neutral the same size as phase.

It is quite an art to design a system where you have to be completely sure that if selecting a smaller neutral there is no chance of that installation every getting so out of balance that a smaller neutral is deemed acceptable. If you do go down the road of selecting a smaller size and the system starts to become out of balance then you can have real problems. I did once use smaller neutrals when designing motor circuits, where we were talking 95mm^ supplies and we used a 35mm^ neutral as all the loads were balanced and the neutrals were mainly for controls.

I do have some old college papers on this and If I can find them I'll post, but as I said for most designs unless your talking about very large cables I would design the neutral the same size as the phase.

 

[keniff]

Thanks malcolm that would be usefull I tend to forget three phase doing so much single phase domestic

 

[RWJ]

Hi Keith yes if it is 3 phase 20amps per phase then you size the cable as you would per phase.

In all honesty mate unless you are going to be working with some pretty hefty cables then basic rule of thumb is neutral the same size as phase.

It is quite an art to design a system where you have to be completely sure that if selecting a smaller neutral there is no chance of that installation every getting so out of balance that a smaller neutral is deemed acceptable. If you do go down the round of selecting a smaller size and the system starts to become out of balance then you can have real problems.

I do have some old college papers on this and If I can find them I'll post, but as I said for most designs unless your talking about very large cables I would design the neutral the same size as the phase.

As above ^^^^ well explained without going in to it too far....

Also my gaffers name is Keith Sims... it's not you Keith??? His location isn't Sussex and is flaming useless on a computer so I assume I'm still safe!!!

 

[keniff]

Lol no it's not me, well it is me, but not the me your looking for!!

 

[RWJ]

Lol no it's not me, well it is me, but not the me your looking for!!

Your sounding like Lionel Richie now!!...... HELLLO

Just a confidence, allot of people put two m's in Sims..... We do it on purpose just to wind him up lolol

Anyway good to meet you other Keith

 

[keniff]

hmmm ill remember to tell the next person where to shove the extra m then!! lol.
have a good day, the weather is roasting down here!