Voltage Drop Based off of Breaker Size

Grady_G

Member
Location
Tampa FL
In my line of work i like to check the engineers riser diagrams to make sure voltage drop has been accounted for. Today i was informed that i should be checking it based off of 80% of the main breakers rating.
So if the main breaker in a section of switch gear is 2000A is it common practice to use 1600A when running the voltage drop for the feeders or would it be proper to use the full 2000A for the voltage drop equation?
I know a breaker unless specifically rated at 100% should only be loaded to 80% of its rating so it kinda makes sense to me to do voltage drop based off of the 80% but since i can not seem to find anything that specifically discusses this i figure i would ask this question here. Most of what I'm reading about voltage drop deals with the actual load and since switch gear can feed many loads i figure its easier to just base the calculation off of the main breaker instead of trying to determine every load from every sub panel and such. Please include any code reference in your answer.
 

Grady_G

Member
Location
Tampa FL
I know the code only talks about voltage drop as a recommendation and not as a rule but is there an industry standard that might apply?
 

iwire

Moderator
Staff member
Location
Massachusetts
I doubt it, it would greatly depend on the customers wishes. Do they want a design based on todays needs or do they want a design for the future?
 

topgone

Senior Member
In my line of work i like to check the engineers riser diagrams to make sure voltage drop has been accounted for. Today i was informed that i should be checking it based off of 80% of the main breakers rating.
So if the main breaker in a section of switch gear is 2000A is it common practice to use 1600A when running the voltage drop for the feeders or would it be proper to use the full 2000A for the voltage drop equation?
I know a breaker unless specifically rated at 100% should only be loaded to 80% of its rating so it kinda makes sense to me to do voltage drop based off of the 80% but since i can not seem to find anything that specifically discusses this i figure i would ask this question here. Most of what I'm reading about voltage drop deals with the actual load and since switch gear can feed many loads i figure its easier to just base the calculation off of the main breaker instead of trying to determine every load from every sub panel and such. Please include any code reference in your answer.
NEC has recommendations, but only as "Fine Print Notes" (FPN). The whole idea is to prevent occurrence of a large voltage drop that will make your system operate inefficiently.
Please see:
210-19(a) FPN No. 4,
215-2(d) FPN No. 2,
230-31(c) FPN,
310-15 FPN No. 1,
455-6(a) FPN, and
210-19(a) FPN No. 4
551-73(d) FPN.
I guess, one would err on the safe side if you use the breaker rating amps when calculating VD of the particular feeder. The way I see it, even if breakers are sized to be loaded 80% of nameplate, that doesn't prevent an electrician from adding small loads to existing feeders by just verifying the feeder load using clamp meters!
 

kwired

Electron manager
Location
NE Nebraska
Totally a design issue. General usage - keeping it minimal enough to not effect lights too much when starting certain loads may be all that is important.

Specific sensitive equipment may have specification that exceeds the recommendations in NEC's informational notes.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
In my line of work i like to check the engineers riser diagrams to make sure voltage drop has been accounted for. Today i was informed that i should be checking it based off of 80% of the main breakers rating.
So if the main breaker in a section of switch gear is 2000A is it common practice to use 1600A when running the voltage drop for the feeders or would it be proper to use the full 2000A for the voltage drop equation?
I know a breaker unless specifically rated at 100% should only be loaded to 80% of its rating so it kinda makes sense to me to do voltage drop based off of the 80% but since i can not seem to find anything that specifically discusses this i figure i would ask this question here. Most of what I'm reading about voltage drop deals with the actual load and since switch gear can feed many loads i figure its easier to just base the calculation off of the main breaker instead of trying to determine every load from every sub panel and such. Please include any code reference in your answer.
As others have said, it's a design issue and a tradeoff between performance and expense. In solar, where I work, it's largely a tradeoff between energy lost to voltage drop over the life of the system (typically 20 years) vs. the expense of wire up front. In my company we have Vd targets we use in our designs, and sometimes customers or AHJs will tell us the losses they can tolerate, but there are no hard and fast rules.

But to answer your question, in our Vd calcs on the AC side we use the maximum rated output current of the inverter(s), which is 80% or less than the rating of the interconnecting OCPD.
 
Last edited:

ron

Senior Member
The voltage drop calculation is generally done based on the calculated load for the feeder / circuit. That will tell you the intended current on the breaker, whether it is 80% or 100% of the breaker handle rating.

You will find that almost every adopted energy code has a voltage drop requirement.

For example an ASHRAE interpretation for the 90.1 version of the Energy Code (although this is an older version), shows that where and how to do the calc is not always clear and needs interpretation.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It's kind of apples and oranges though in my opinion. The so called "80% rule" in breakers has to do with continuous current, defined as 3 hours or more. Voltage Drop has no time frame associated with it. So for example if you have a high inertia machine that pulls high current for 30-60 seconds, the 80% issue of the breaker size is irrelevant because that higher current is temporary. But that high current can still cause voltage drop problems that should be considered in the conductor sizing.
 

Grady_G

Member
Location
Tampa FL
It's kind of apples and oranges though in my opinion. The so called "80% rule" in breakers has to do with continuous current, defined as 3 hours or more. Voltage Drop has no time frame associated with it. So for example if you have a high inertia machine that pulls high current for 30-60 seconds, the 80% issue of the breaker size is irrelevant because that higher current is temporary. But that high current can still cause voltage drop problems that should be considered in the conductor sizing.
I agree that we have all seen breakers feeding motor loads that would surpass the breaker rating if all the motors or any load for that matter were to start at the same time. So there is a chance that the load could exceed the 80% rating at some point even if for a short while. The breaker might not trip due to the trip curve but it would make sense to me to base the voltage drop off of the 100% rating of the breaker to be on the safe side. This would allow for any additional growth from the equipment served.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I agree that we have all seen breakers feeding motor loads that would surpass the breaker rating if all the motors or any load for that matter were to start at the same time. So there is a chance that the load could exceed the 80% rating at some point even if for a short while. The breaker might not trip due to the trip curve but it would make sense to me to base the voltage drop off of the 100% rating of the breaker to be on the safe side. This would allow for any additional growth from the equipment served.
That is only realistic if you have some reason to believe that a transient voltage drop will be in some way harmful.
 

Sahib

Senior Member
Location
India
Grady_G:
Calculate voltage drop based on the actual load currents of individual devices. But when you calculate the size of the main breaker, take into account the demand factors of branch/feeder circuits and apply the 80% factor on the resultant actual current through the main breaker. You may also add an additional growth factor say 20% to the figure thus arrived.
 

iwire

Moderator
Staff member
Location
Massachusetts
So you think its best to base it off of 80% percent breaker rating?
No, I think it is best to base it off the load unless the person paying the bill wants to pay extra now for a possible increase of load later.

There is no right answer, it really should be based do what the customer wants to pay for.
 

kwired

Electron manager
Location
NE Nebraska
No, I think it is best to base it off the load unless the person paying the bill wants to pay extra now for a possible increase of load later.

There is no right answer, it really should be based do what the customer wants to pay for.
Like making the decision to buy the Cadillac instead of the Buick.
 
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