The "125% rules" and the NEC

Its been cold here in the northeast and I have not been motivated to do much else other than sit inside, so I though I would write up a brief piece on the "125% rules" in the NEC as they are frequently misunderstood and not always applied correctly. (Mods feel free to move this do a different section if deemed more appropriate).

First, lets just clarify that sometimes these are referred to as the "80% rule" and sometimes as the "125%" rule. 80% is the reciprocal of 125% so they are both the same thing, it just depends which "direction" we are going from - sizing say an OCPD (overcurrent protective device) from the current, or selecting maximum ampacity from a given OCPD. I'll just stick with "125%" here as that is the wording in the NEC.

There are several 125% rules, one of which is commonly stated, but is not even a requirement, and also a "150%" rule for diversion controllers. Lets take a look at each:

1) Continuous loads. This requirement is spread out in several places in the NEC, and applies to service entrance conductors, feeders, branch circuits, PV circuits, and OCPD's (unless listed for operation at 100%). 210.19(A)(1), 210.20(A), 215.2(A)(1), 215.3, 230.42, and 690.8(B) are some code references. Two important points here: First, is that this applies to ONLY the equipment I just mentioned, IT DOES NOT GENERALLY APPLY to other equipment. For example, you can load say a panelboard or charge controller continuously forever at it rating (although an equipment manufacture could require a derating in their instructions and NEC 110.3(b) would require you to follow it). The second important point is the definition of "continuous load" and this is defined as "A load where the maximum current is expected to continue for three hours or more." 690.8(B) clarifies (in case one was in doubt) that PV system circuits shall be considered continuous, but note that this doesn't necessarily apply to every circuit in/connected to a PV system, i.e. a standalone inverter input and output circuit is not necessarily continuous.

2) The “solar derating”. This 125% factor is from 690.8 and although its purpose is not stated in the NEC, the NEC handbook commentary states that it is to account for conditions where STC ISC can be exceeded. Note that is just telling you HOW to calculate the current, its not telling you WHAT things you size to this number. 690.8(B) tells you to use these currents, add the 25% because they are continuous, then what must be sized to them (conductors and OCPD's). Note that the 125% "solar derating" along with the 125% "continuous derating" result in the “156% rule” for PV source and output circuits.

3. The 125% rule for charge controllers. Although frequently stated in general discussion and many product manuals, there is no NEC requirement for this. Presumably this “requirement” is drawn from the current calculated in 690.8 (#2 above) in conjunction with 110.3(B) (manufacturers instructions and ratings to be followed). Although on the surface this may seem like a valid conclusion, it falls apart when one considers: A) 690.8 does not mention equipment, unlike 690.7(A) “maximum voltage” which specifically states it applies to equipment. 690.72 “charge control” mentions nothing about device ratings relative to circuit current for other than diversion control. C) I am not aware of any charge control that will destroy itself trying to utilize excess current. D) grid tie inverters can be rated less than the PV output ISC. These arguments could logically be extended to claim that not only is there not a “125%” rule for charge controllers, but there is no requirement whatsoever that dictates charge controller size relative to PV output. Finally, I will just reiterate that the “continuous loading 125% rule” does not apply generally so it is up to the manufacturer to state if there must be a derating for continuous loading.

4. Diversion control 150% rule. Thought I should throw this in for completeness. 690.72(B) gives requirements for diversion control which include a second independent means to prevent overcharging, and states that the power rating of the diversion load shall be at least 150% of the power rating of the PV array and that the conductor ampacity and rating of the OCPD for this circuit shall be at least 150% of the maximum current rating of the diversion charge controller.

5. Others appearances of 125%. Not so relevant for this audience, but the 125% factor also appears in article 220, load calculations, and article 430, motors, for calculations and sizing branch circuits and feeders serving motors.

I hope this clarifies this issue for those who need or want to design to the NEC. Please feel free to make any comments or corrections.