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Ok dumb question... Just want to make sure

3K views 25 replies 13 participants last post by  Southeast Power 
#1 ·
120/208 3 phase Y main panel. I want to feed a main lug 100A 120/208 single phase subpanel..... I once heard that the neutral should be one size bigger then the hot because of harmonics?
 
#3 ·
Since the current in the neutral of a feeder that uses two phases of a wye connected 3 phase system is about the same as the highest current in either of the phases, it needs to be the same size as the phase conductors.

In my experience, oversizing a neutral is almost always a waste of wire.

I'd go with the same size for all 3 wires.
 
#13 ·
or they will know someone who knows..... a quote from my electrical engineer:

"if over 50% of the load on a panel is printers, electronic ballasts, computers,
VFD's, or electronic devices that generate harmonics, or the panel is likely
to exceed 50% with those loads with expansion, size the neutral to 200%"

that's how he designs them.....
 
#8 ·
The only time you might upsize the neutral would be for say a bunch of circuits feeding computers or fluorescent lighting. One contract my company just finished required panels feeding the computer circuits to have the neutral upsized 200%.
 
#9 ·
Ok I'm feeding 6 locations that will have pc's, also I have 8 4' wrap around fluorescent fixtures... Maybe I should up the neutral.... I should of been more specific...

Another stupid question..

My panel is 120/240 MLO fed from a 208 3 phase Y panel, do I have to request different stickers from supply house so panel is identified as 120/208... I may be over thinking everything...
 
#12 ·
There is no code requirement to up size the neutral for this work.

There is no reason to do so by choice unless you really load the circuits up to the max with non-linear loads which in my opinion is a poor design.

As long as you keep the loads on each ungrounded conductor reasonable the grounded conductor will be fine.
 
#14 ·
There is no code requirement to up size the neutral for this work.

There is no reason to do so by choice unless you really load the circuits up to the max with non-linear loads which in my opinion is a poor design.

As long as you keep the loads on each ungrounded conductor reasonable the grounded conductor will be fine.
or they will know someone who knows..... a quote from my electrical engineer:

"if over 50% of the load on a panel is printers, electronic ballasts, computers,
VFD's, or electronic devices that generate harmonics, or the panel is likely
to exceed 50% with those loads with expansion, size the neutral to 200%"

that's how he designs them.....

I just attended a Fluke Motors, and Drives Training, and there was a lot of talk about this very subject, because of so many non linear loads being used. They talked about engineering panels with larger neutrals.
 
#17 ·
Code requirements are the minimum safe installation requirements. I understand what you are saying but if an engineer designing a system decides they want to exceed the code and the customer is willing to accept it, then I, the installer, has got to follow the plan where the plan is explicit. And, really, with the rapid advances in computer and other things that draw non linear loads it probably isn't a bad idea to double it up. Just my opinion...... :)
 
#19 ·
if an engineer designing a system decides they want to exceed the code and the customer is willing to accept it, then I, the installer, has got to follow the plan where the plan is explicit.
I agree 100% and have installed many 'super neutrals' as per print.

And, really, with the rapid advances in computer and other things that draw non linear loads it probably isn't a bad idea to double it up.
I agree the number of non-linear loads are increasing but the amount of harmonics each contributes is dropping due to tighter regulations in Europe for the same types of equipment.


In the end I will do whatever they pay me to. :)
 
#20 ·
Briancraig81 said:
Here's another dumb question:
Company I worked for would make us use A and C phase when feeding a single phase panel from 120/208 3p. Was there a reason behind this or just there prerogative?
I see more problems than benefits with that policy. Obviously load balance is an issue.

Its not like its a high leg delta system so they probably did it for consistency
 
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