125% Output current is 0.14 over a standard breaker size

JimmysLimeade

JimmysLimeade

EE Student & PV Design
Location
Utah
Occupation
PV Design
I am working on a job that has an AC output current of 180.11A before the 125% adder. After 125% it comes out to 225.1375A. Does that mean I need to upsize to a 250A OCPD or is there a way I can use a 225A OCPD?

I have read a few different things regarding this type of scenario but I can't find them anymore, so any help would be appreciated.
Thanks!
 
Eddie702 said:
I misread it. When he stated AC I was thinking air conditioner. My mistake.
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PV Inverters sometimes specify a max OCPD on their output AC conductors, so I thought that was what you were referring to.
 
I've always found the rounding rule in the NEC to be absurd. I can round 10.4A to 10A but I can't round 1004A to 1000A. Rounding the 10.4A to 10A introduces a 4% error and rounding 1004A to 1000A is a 0.4% error. Ridiculous, but I guess the NEC does not think users can do math.
 
Eddie702 said:
Doesn't the nameplate specify the maximum OCPD?
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The UL Listing for inverters requires that the inverter have a maximum AC OCPD value and if that OCPD is added external to the inverter the rating has to be given in the installation manual. Sometimes they will also be in the datasheet. I can't say I have seen one on the nameplate. With string inverters they are usually external.
 
jim dungar said:
But if you have 1.004kA aren't you allowed to round down to 1kA?
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220.5(B) only talks about fractions of an amp and I only do that with a final result of a calculation. I guess you could make a case for rounding terms internal to a calculation, but one needs to be careful about doing that.
 
pv_n00b said:
I have never seen anyone change a number using notation to allow rounding. Can I call it 0.001004MA and then round it to 0?
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However, rounding based on significant figures is perfectly reasonable and done all the time. So if, for example, the inverters in the OP are microinverters in which the maximum output current is only specified to 3 significant figures, it is reasonable to say that the result of the calculation is 225A.

I.e. where did 180.11A come from?

Cheers, Wayne
 
Sorry for the duplication, but this from the other thread is more pertinent to this one:

I saw a system recently that used (5) PV inverters that each had 48.25A max output current. (1.25)(48.25A) = 60.3A, and 60A inverter OCPDs were used per 220.5(B). For the combined inverter output, however, (5)(1.25)(48.25A) = 301.6A, so no rounding; a 350A OCPD was used.
 
wwhitney said:
However, rounding based on significant figures is perfectly reasonable and done all the time. So if, for example, the inverters in the OP are microinverters in which the maximum output current is only specified to 3 significant figures, it is reasonable to say that the result of the calculation is 225A.

I.e. where did 180.11A come from?

Cheers, Wayne
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Are we now saying that the NEC is reasonable? :) NEC 220.5(B) is pretty simple, write out the amperage and round based on what is to the right of the decimal point. No scientific notation, no kA or mA notation to create a new decimal location, no rounding based on significant figures. Now I'm not saying rounding to significant figures is not a reasonable thing to do in math, but the NEC does not say it's allowed so in the universe of the NEC it's not allowed. Maybe make a public input to allow that in the next version. But I really think the people behind the NEC don't want users to do more than simple math.
 
ggunn said:
225A is OK. See 220.5(B)
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There have been a number of proposals to make that rule apply through out the code, but they have been rejected and that rounding rule only applies to for the purposes of load calculations and not to OCPDs and conductor sizing.
The rejection statements say the rounding rule only applies within Article 220.
 
don_resqcapt19 said:
There have been a number of proposals to make that rule apply through out the code, but they have been rejected and that rounding rule only applies to for the purposes of load calculations and not to OCPDs and conductor sizing.
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The basic Article 210/215 OCPD and conductor sizing rules refer to the "load", so 220.5 is in play. Whereas Article 690, for example, does not refer to "load" for conductor or OCPD sizing, and hence 220.5 is not applicable.

220.5(B) says "Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped." Now the question is did we need that permission to do any rounding? Is there some other section or principle that prohibits rounding? Or in the absence of any verbiage on the matter, is it acceptable to round on the basis of significant figures or otherwise employ some judgement about rounding?

Cheers, Wayne
 
wwhitney said:
The basic Article 210/215 OCPD and conductor sizing rules refer to the "load", so 220.5 is in play. Whereas Article 690, for example, does not refer to "load" for conductor or OCPD sizing, and hence 220.5 is not applicable.

220.5(B) says "Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped." Now the question is did we need that permission to do any rounding? Is there some other section or principle that prohibits rounding? Or in the absence of any verbiage on the matter, is it acceptable to round on the basis of significant figures or otherwise employ some judgement about rounding?

Cheers, Wayne
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The scope of Article 220 limits any rule in that article to the calculation of loads.
220.1 Scope.
This article provides requirements for calculating branch-circuit, feeder, and service loads. ...
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As far as your comment, there is nothing in 220 or any other article that precludes using significant figure rounding, and 220.5 probably should be deleted.
 
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