Neutral Calculation

[eddiet]

There is some confusion about the sizing of neutral service conductors.
When a large % of the loads are 240 or 208 volt and neutral calc is very low.
NEC tell us that regaurdless of load the neutral or (grounded conductor) can not be smaller then the EGC 215.2(A)(2) per Tbl.250.122
220.61 (A) tells us that the max unbalanced load shall be the max net calculated load between the neutral and any one ungrounded conductor.
So if I have a 200 amp single phase service with 121 amps on line one and 96 amps on line two.
Do I size the neutral to carry
A. The 121 amps on line one (even though much of the load is 240 volt) # 1Cu. Tbl. 310.15(B)(16)
B. My calculated load of 86 amps? # 4 Cu. Tbl 310.15(B) (16)
C. Or by Tbl 250.122 200A = #6 Cu.

 

[augie47]

IMO, Your calculated load but not smaller than required by 250.66 (not 250.122) per 250.24(C)

 

[texie]

There is some confusion about the sizing of neutral service conductors.
When a large % of the loads are 240 or 208 volt and neutral calc is very low.
NEC tell us that regaurdless of load the neutral or (grounded conductor) can not be smaller then the EGC 215.2(A)(2) per Tbl.250.122
220.61 (A) tells us that the max unbalanced load shall be the max net calculated load between the neutral and any one ungrounded conductor.
So if I have a 200 amp single phase service with 121 amps on line one and 96 amps on line two.
Do I size the neutral to carry
A. The 121 amps on line one (even though much of the load is 240 volt) # 1Cu. Tbl. 310.15(B)(16)
B. My calculated load of 86 amps? # 4 Cu. Tbl 310.15(B) (16)
C. Or by Tbl 250.122 200A = #6 Cu.

Since you said these are service conductors, as Augie said, you would use 250.66 as the minimum even if your Art 220 calculated neutral load is less. If this was a feeder instead of service conductors the rules are similar except you would use 250.122. The key here is 250.66 is based on conductor size and 250.122 is based on the OCPD size. Keep in mind that even for a service that has ZERO neutral load you still must have a neutral sized per 250.66. However, a feeder would not be required have a neutral if there is zero neutral load-just a EGC per 250.122.

 

[don_resqcapt19]

To find the maximum grounded conductor calculated load, you do not look at the line to line loads. You only look at the line to neutral loads. The maximum neutral load is the line to neutral load on the line with the highest line to neutral load, that is with all of the line to neutral loads on that line turned on and all of the line to neutral loads on the other line turned of so there is no neutral current cancellation. Then as others have said, you size it to the larger of the calculated neutral load or the size from Table 250.66.