Voltage Drop 3% or 5%

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zcanyonboltz

Senior Member
Location
denver
How do you know when to use the 3% or the 5% recommendation?
[90.5 (C) 210.19 (A) Note 4, and 215.2 (A) (1) Note 2]

I'm guessing 3% for 120 branch circuits and 5% for 240 circuits and feeders? Not sure about other voltages. Thanks

Test Question-

What is the allowable voltage drop for a 240V branch circuit?

For a 208V branch circuit what is the maximum allowable voltage drop?

Not what rules are for 3% or 5% application.
 

kwired

Electron manager
Location
NE Nebraska
How do you know when to use the 3% or the 5% recommendation?
[90.5 (C) 210.19 (A) Note 4, and 215.2 (A) (1) Note 2]

I'm guessing 3% for 120 branch circuits and 5% for 240 circuits and feeders? Not sure about other voltages. Thanks

Test Question-

What is the allowable voltage drop for a 240V branch circuit?

For a 208V branch circuit what is the maximum allowable voltage drop?

Not what rules are for 3% or 5% application.
The NEC mentioned 3 and 5 percent figures are nothing but suggestions as they are in an informational note. They are not a requirement. You may find local jurisdictions do have a requirement though -but would have to check with the local jurisdiction in question to find out. You may also run into requirements from equipment stating allowable voltage variances or some other design requirement for a certain level of voltage drop, but those are beyond NEC general requirements.
 

david

Senior Member
Location
Pennsylvania
How do you know when to use the 3% or the 5% recommendation?
[90.5 (C) 210.19 (A) Note 4, and 215.2 (A) (1) Note 2]

I'm guessing 3% for 120 branch circuits and 5% for 240 circuits and feeders? Not sure about other voltages. Thanks

Test Question-

What is the allowable voltage drop for a 240V branch circuit?

For a 208V branch circuit what is the maximum allowable voltage drop?

Not what rules are for 3% or 5% application.

I was always taught 3% on the branch circuit
I was taught 3% on the feeder.
Way back when>>>>>>>>> the teacher said this is probably the only time in math that 3 + 3 will equal 5 and be correct.

The overall voltage drop for the circuit is 5% feeder plus branch circuit
 

Pharon

Senior Member
Location
MA
The way I read it is no more than 3% for individual branch circuits and no more than 5% for entire run from service to farthest outlet.
 

david

Senior Member
Location
Pennsylvania
If I was taking a test question as presented I would assume the branch circuit originated at the service panel and would apply a 3% max. voltage drop. The question did not imply a feeder was involved

That?s just me
 

kwired

Electron manager
Location
NE Nebraska
If I was taking a test question as presented I would assume the branch circuit originated at the service panel and would apply a 3% max. voltage drop. The question did not imply a feeder was involved

That?s just me
If test answers are based on NEC alone - there is no incorrect answer for general purposes, there is some specific instances in chapters 5-7 applications however where a specific voltage drop may be required.
 

david

Senior Member
Location
Pennsylvania
How do you know when to use the 3% or the 5% recommendation?
[90.5 (C) 210.19 (A) Note 4, and 215.2 (A) (1) Note 2]

I'm guessing 3% for 120 branch circuits and 5% for 240 circuits and feeders? Not sure about other voltages. Thanks

Test Question-

What is the allowable voltage drop for a 240V branch circuit?

For a 208V branch circuit what is the maximum allowable voltage drop?

Not what rules are for 3% or 5% application.


What is the allowable voltage drop for a 240V branch circuit?
between 0 and 7.2 volts

For a 208V branch circuit what is the maximum allowable voltage drop?
6.24
 

david

Senior Member
Location
Pennsylvania
If test answers are based on NEC alone - there is no incorrect answer for general purposes, there is some specific instances in chapters 5-7 applications however where a specific voltage drop may be required.

I know but I never taken an exam that didn't want the question answered with based on a percentage of the voltage.
 

kwired

Electron manager
Location
NE Nebraska
I know but I never taken an exam that didn't want the question answered with based on a percentage of the voltage.
Test writers are human and can make mistakes. You are justified in answering with what you feel is correct answer, even if multiple choice and they don't give your answer as one of the choices. If I were to take such an exam now and had no multiple choice answer that I felt was correct (especially on this topic area) I would write an explanation of my answer on the test and state none of the choices are correct. Can't say I was wise enough to do that when I was still a student though.

Should I fail the test I do have that written on there and can talk with examiner, or whoever is an authority, and see what happens You have nothing to lose if you failed already. If you can prove the test content is inaccurate - it certainly seems fair to at least throw out the portion that is in error.
 

zcanyonboltz

Senior Member
Location
denver
What is the allowable voltage drop for a 240V branch circuit?
between 0 and 7.2 volts

For a 208V branch circuit what is the maximum allowable voltage drop?
6.24

How do you know to use 3% and not 5%? Since the word maximum is there how do you know not to go with 5%?

My Holt's book says the answer 6.24 but does not explain why to use the 3%. I guess the consensus is 3% for the branch circuit and 5% for the entire run. In the first question in op the key word is allowable so you can answer 7.2V I don't have the answer to this question, but the second 208V question in op is tricky.

Thanks for the replies, this obviously is not the best question for an exam.
 
Last edited:

jap

Senior Member
Occupation
Electrician
Test writers are human and can make mistakes. You are justified in answering with what you feel is correct answer, even if multiple choice and they don't give your answer as one of the choices. If I were to take such an exam now and had no multiple choice answer that I felt was correct (especially on this topic area) I would write an explanation of my answer on the test and state none of the choices are correct. Can't say I was wise enough to do that when I was still a student though.

Should I fail the test I do have that written on there and can talk with examiner, or whoever is an authority, and see what happens You have nothing to lose if you failed already. If you can prove the test content is inaccurate - it certainly seems fair to at least throw out the portion that is in error.

That's a very good point of view.

JAP>
 

david

Senior Member
Location
Pennsylvania
That's a very good point of view.

JAP>
I agree,

but most test questions are multiple choices, and the criteria usually says pick the best answer.

Now all of the above or none of the above could through the monkey into the exam. My experience in taking exams over the years is they re-visit the subject several times in an exam and most likely you could get the feel from the exam your taking if they want that answer based on a mathematical equation.
 

jap

Senior Member
Occupation
Electrician
I've written reasons for my answers on my CEC courses when this happens.
I do believe for a fact that sometimes the test writers answers are incorrect and sometimes they need to be let known that there answers aren't correct.

JAP>
 

jap

Senior Member
Occupation
Electrician
I also feel its alright to throw an odd test question in every now and then where there may be no definite correct answer but if one runs up against this on more than one question on the exam it would make one question the validity of the exam giver.

There's a certain pride and confidence felt when the answer you came up with is exactly what is given as an answer.

JAP>
 

david

Senior Member
Location
Pennsylvania
How do you know to use 3% and not 5%? Since the word maximum is there how do you know not to go with 5%?

My Holt's book says the answer 6.24 but does not explain why to use the 3%. I guess the consensus is 3% for the branch circuit and 5% for the entire run. In the first question in op the key word is allowable so you can answer 7.2V I don't have the answer to this question, but the second 208V question in op is tricky.

Thanks for the replies, this obviously is not the best question for an exam.

For what it is worth I was always taught that when you are only dealing with a portion of the overall circuit it is 3%.

If you are dealing with the circuit that has both a feeder and branch circuit it is 5% overall.

That could be 2% on the feeder and 3% on the branch circuit.
It could also mean 3% on the feeder and 2% on the branch circuit. They only thing it means for sure is 5% on the circuit overall
 

Carultch

Senior Member
Location
Massachusetts
I was always taught 3% on the branch circuit
I was taught 3% on the feeder.
Way back when>>>>>>>>> the teacher said this is probably the only time in math that 3 + 3 will equal 5 and be correct.


The teacher isn't saying that.

The teacher is saying that either portion of the circuit can be 3%, but the sum total of both portions have to add up to not more than 5%.

So if one portion is 3%, the other portion has to not exceed 2%.
 

david

Senior Member
Location
Pennsylvania
The teacher isn't saying that.

The teacher is saying that either portion of the circuit can be 3%, but the sum total of both portions have to add up to not more than 5%.

So if one portion is 3%, the other portion has to not exceed 2%.

Actually in principle you are correct,
But the teacher said exactly that more or less to make the class laugh as a lead in to teach the principle.
And old friend of mine took the course just graduating one class ahead of mine. We worked for the same electrical contractor for eight years together , soon as we mentioned the instructors name we laughed and said 3 + 3 = 5
 

david

Senior Member
Location
Pennsylvania
For what it is worth I was always taught that when you are only dealing with a portion of the overall circuit it is 3%.

If you are dealing with the circuit that has both a feeder and branch circuit it is 5% overall.

That could be 2% on the feeder and 3% on the branch circuit.
It could also mean 3% on the feeder and 2% on the branch circuit. They only thing it means for sure is 5% on the circuit overall


The teacher isn't saying that.

The teacher is saying that either portion of the circuit can be 3%, but the sum total of both portions have to add up to not more than 5%.

So if one portion is 3%, the other portion has to not exceed 2%.

They only thing it means for sure is 5% on the circuit overall

OK I see you are taking issue with that
You should take in context to it being a 2 % / 3% split I was not implying by that it could be a 1% /4% split I did say 3 + 3 = 5
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
There is only one article in the NEC where you must consider voltage drop.
Hint: The article involves red equipment that rotates.
There is another one, also in Chapter 6.

While the NEC does not have a general mandatory voltage drop rule, some of the energy code that are being adopted do have mandatory voltage drop rules for non-dwelling occupancies.
 
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