Problems calculating neutral conductors please help

[Matt Mckenzie]

Reviewing for test and having problems with calculating neutral size for for single phase sub panels. Can someone please help. I am having problems with balancing the loads. Balance the loads and add the amps for each side of each leg and use the highest amperage of one leg to get your neutral size from the wire tables? Sometimes your neutral can be reduced in size and sometimes your neutral can not be smaller that your ungrounded conductors. Anybody have any pointers to get me on the right page? Please help. Any pointers for sizing neutrals for 3 phase too please. I remember doing a job where the electrician down sized the neutral on a delta 3 phase service and the inspector tried to say that he could not do that and he showed him in the book at 220.61(B)

 

[Dennis Alwon]

For sizing the neutral or grounded conductor look at 250.24(C). This has change in the 2014 and 250.24(C)(1) directs us to T250.102 (C)(1) which is the same as T250.66

(1) Sizing for a Single Raceway. The grounded conductor
shall not be smaller than the required grounding electrode
conductor specified in Table 250.66 but shall not be required
to be larger than the largest ungrounded serviceentrance
conductor(s). In addition, for sets of ungrounded
service-entrance conductors larger than 1100 kcmil copper
or 1750 kcmil aluminum, the grounded conductor shall not
be smaller than 121?2 percent of the circular mil area of the
largest set of service-entrance ungrounded conductor(s).

 

[augie47]

Dennis has shown you 250.24(C) concerning sizing the grounded conductor to service equipment.
You would use this once you had satisfied 220.61 for sizing to match the load.
220.61 is also used for feeder neutrals but keep in mind, for feeders 215.2 provides you with a minimum size.
There are a lot of "tales" of "drop one size, drop two sizes", but the true methods are derived mostly from 220.61.