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. B) 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.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,596 admin
    Thank you Ethan.

    One point of confusion for me... The 80% derating of the branch circuit loads... For a 15 amp breaker/wire branch circuit, 80% would be 12 amps maximum... Is this only applying to 3+ hour loads?

    Or does this apply to all loads except "starting current"/surge type loads (starting induction motor, etc.).

    Most (common/typical) circuit breakers (as I recall) would typically trip before 3 hours if operated at 100% of rated load (could be minutes to hour or more -- per spec. sheet).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarpatsolarpat Registered Users Posts: 5
    The derating of the branch circuits applies to all the circuits except for "lighting only circuits" You can run 100% on the lighting circuits. And yes you can load the 15 amp circuits to 12 amps max and your 20 amp circuits to 16 amps max.
  • Ethan BrushEthan Brush Solar Expert Posts: 235 ✭✭
    Bill,

    Yes the "continuous 125% rule" only applies to loads that are of 3 hour duration or longer. If the time period is less than that, then it is not a continuous load. Technically, I can run 20 amps of lighting on a 20 amp circuit for 2 hours 59 minutes. If I plan on running it for three hours, I can only load it to 16 amps. Starting surges typical of motors and transformers are accounted for by the allowance of a larger OCPD than we are used to for a giving wire size, sometimes as high as 250% larger. Its a common "rookie inspector" mistake to see a 40 amp OCPD serving 12 gauge wire feeding a air conditioning unit and call it a code violation.


    I fully agree with you about being concerned about running an OCPD at or close to its max rating for even durations well under 3 hours. I did some work in a greenhouse where they were having trouble with circuit breakers tripping at 75% loading. It was because of the high ambient temperature in there. There are two things here that I should have mentioned: First, I am just saying what the codes says, not necessarily that it is a good design or should be done that way. Secondly, the current may be calculated from an article 220 load calculation, not by reading your clamp meter. The article 220 figure will generally have quite a bit of fudge and guessing (depending on what we are calculating of course, it could be pretty close to actual in some cases) so that calculated load is not likely to be the actual load.

    Solarpat,

    I do not believe that is correct. I think perhaps you meant the opposite of what you said? For a dwelling unit NEC load calculations, the lighting AND general outlet load is determined by using 3 watts per square foot. It is considered non-continuous, so if you were to work that out and get 60 amps at 120V, that would be 4 15 amp circuits or 3 20 amp circuits - period. An office lighting load would be continuous. I think NEC aticle 220 load calculations are not very relevant for us off grid folks. Most of the time we are dealing with nameplate and more definite values.
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