# Service conductor derating



## george nicholson (Apr 19, 2008)

Until about 2 years ago I used the conductor size straight from the table but since that time I have been forced to use the 80% derating for more than 3 conductors in the raceway in most all local municipalicities.

The electrical inspectors and plan review people in my area all say that the following is why the service conductors must be derated:

1) "On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor."

2) Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled in lengths exceeding 24 in., the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(A). 

3) The Article 310 requirement seems specific. What's missing, however, is a definition of what constitutes "major," as well as any guidance as to how much neutral current might be expected. Likewise, in Articles 210 and 220, how high are "high" harmonic currents?

4) Most buildings can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern, but, when the nonlinear loads exceed 15% some non-apparent negative consequences can be expected. One plan reviewer uses the estimates from EPRI to determone that non-linear loads must be considered.

5) The Electric Power Research Institute (EPRI) estimates that in 1992, 15% to 20% of the total load was nonlinear, and by the year 2,000 it is expected that 50% to 70% of all loads will be nonlinear.

These inspectors say that with almost all lighting being electronic ballast, HVAC and refrigeration being variable speed, many personal PC stations, UPS's, printers, etc that the neutral is definately carrying current and must be counted. 

Therefore the nonlinear loads may exceed 15% which they consider "major". Thus derate the wire to 80% of the table is what they are requiring.


Is this correct?


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## JohnJ0906 (Jan 22, 2007)

I'm not sure that I agree, but, IMO, "major" is open to interpretation. And who does that interpreting? The local AHJ. 
At least this is at plan review, so you know up front, and it applies across the board, and doesn't depend on which inspector you get that day.


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## macmikeman (Jan 23, 2007)

It sounds to me like your local inspectors have been studying and paying more attention to the codes as of late. This isn't such a bad thing. There are tru rms meters available to record the levels of harmonic disturbance present on a system. Allowance for future needs is always a good idea. Put the two things together and you have a good reason for the derating. Bid the work accordingly.


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