Sizing grounded conductor?

[jahilliard]

I am attempting to create a little more space to pull aluminum conductors by using a smaller neutral but I can't find the rule allowing me to do that so I'm not entirely sure I can. the scenario is a 10 meter, meter center with 150 Amp breakers per unit. I would like to pull 2/0 Al with a smaller grounded conductor to each unit...just need some help locating the section in the NEC...little help please!

 

[augie47]

I would start with 220.61

 

[Dennis Alwon]

Also look at 250.24(C)(1) once you have calculated the load for the neutral.

 

[jahilliard]

yea I'm aware of those references but having a little trouble making much practical sense out of either one.. any chance I could see an example to make this more clear to me? I know for single family we would use 2-4/0 and 1-2/0 from the meter..and of course SER cable comes sized accordingly as well...just trying to actually understand the application of the rule... I don't mean to be a pain my brain just doesn't seem to be at full speed today!

 

[augie47]

The examples in the NEC Annex D give some pretty good data.
The only true method is to follow 220.61, but the wording makes it pretty obvious that you can almost always count on a minimum of one wire size reduction minimum on a residential load.

 

[jahilliard]

I appreciate the feed back. I Really wanted to be certain before we pulled all the wire and realized the rule didn't apply to this application fro some reason. I'll check out the examples. thanks again. I RARELY use aluminum for anything other than single family but the owner wanted the difference in cost between copper on all 6 of the 10 unit bldgs we are doing. I guess on one hand I haven't really heard too many instances of Aluminum wiring being the cause of for any real issues.

 

[Dennis Alwon]

We are allowed to downsize the neutral only if the calculated load on the neutral is less than the ungrounded conductors. In general for a residence one can usually drop at least one size or two on the neutral because there are often many 240v loads which do not use the neutral.

If you are asking how to calculate the load then the annex examples would be a good way , as Gus stated. Of course the minimum size cannot be smaller than the grounding electrode conductor .

 

[Dennis Alwon]

I appreciate the feed back. I Really wanted to be certain before we pulled all the wire and realized the rule didn't apply to this application fro some reason. I'll check out the examples. thanks again. I RARELY use aluminum for anything other than single family but the owner wanted the difference in cost between copper on all 6 of the 10 unit bldgs we are doing. I guess on one hand I haven't really heard too many instances of Aluminum wiring being the cause of for any real issues.

The new aluminum wire has different properties than the old stuff and is quite safe.