Voltage drop clarification
Voltage drop clarification
The code requires oversizing for voltage drop so that the total voltage drop, that is feeder + branch circuit does not exceed 5%.
Article 215.2(A)(3) FPN No.2 is for branch circuits and article 210.19 FPN No.4 is for branch circuits.
It should be emphasized:
1) Feeder is from POE to the last overcurrent protection. Most of the times, the distance between the POE and the main switchboard or distribution board is overlooked.
2) The distance for branch voltage drop calculation should be to the farthest outlet (not to be confused with receptacle).
3) The calculation shown in the code for voltage drop is a modified DC formula . The AC formula should be used or alternatively as shown on example 1 (page 1184 of the 2005 NEC handbook)
Per 250.122 (b), the equipment ground must be adjusted accordingly.
Service ground is oversized automatically when due to voltage drop either service feeders are increased in quantity (same size) or decreased in quantity, but increased the size (service ground per table 250.66). For cases in which the leg size does not increase to the next step in table 250.66, I utilize the same method as for the equipment ground.
For the question in this post, #10 is good for 30A (table 310.16). If voltage drop calculation dictates that #6 should be utilized, then refer to chapter 9, table 8 for conductor properties; #10 - 10380 cmils, #6 26249 cmils. Adjust the equipment ground conductor (table 250.122) by the same ratio, that is, 26249/10380 = 2.53. Thus, #6 ground in required.