Based upon the National Electrical Code:

Question:

How many 125 volt, 15 ampere duplex receptacles are allowed to be installed on a 20 ampere, 115 volt branch circuit in a commercial building?

A. 11
B. 10
C. 12
D. 13

B. 10

C. 12

10 Receptacles.

115 VAC × 20 AMPS = 2300 VA

80% of 20 AMP Ckt (Gen. Purpose Recept. Ckt. NEC art. 210-21 and 210-23):

2300 VA × 0.8 = 1840 VA.

180 VA per Duplex Receptacle:

1840 VA ÷ 180 VA = 10.2222

10.222 Receptacles per circuit

That would be 10 complete Duplex Receptacles, and parts of a Duplex Receptacle - to make it 0.222 of a Duplex Receptacle


Easier to just round off to 10 Receptacles! 0.222 of a Receptacle is not useful!!!
<joke>

Scott35

Wow, this question has made it rounds to 2 or 3 forums already.
Section 220.2(a) 2002 NEC®, or 220-2(a) 1999 NEC® and the first words, "Unless other voltages are specified ...".
Read the original question again. I missed it the first time.

Who is permitted by the code to specify a voltage. Why couldn't someone specify 126 volts and install 14 duplex receptacles?

Scott,
The 80% does not apply so the answer based on the 115 volts is 12, but I don't think that anyone has the right under the code to specify a voltage for this circuit. It is my opinon that the calculation should be based on 120 volts.
Don

I will say 10, 11, 12, or 13.

The max is 13. 120*20/180

You can only specify the voltage if you control the voltage.

George, controling the voltage wouldn't be a problem, and if an aspiring young Jman exam taker answered anything but 12, he or she missed the question.

Roger

Roger,
So if I specified a voltage of 126 and installed 14 duplex receptacles, the inspector would have to pass my installation?
I'll say what I have posted on other forums. The question is bogus. The installer does not have the right to specify the voltage.
Don

[This message has been edited by resqcapt19 (edited 08-03-2003).]

Don,
allbeit 'code' spec's are 'nominal', would one say they rely on the manufacturer's listings?

Don, I absolutely agree that if the question used a higher or lower voltage the calculation would have to use that voltage.

I don't see the issue with the question.

The NEC® has decided the wording "Unless other voltages are specified " has a reason to be in the article. With that said, who is going to be available to champion the exam taker when he / she uses the argument that the question is bogus per the experts on these forums?

If we changed the wording of the question to say "using all the applicable articles but substitute 115v for 120v ; would it change the math? Would it change the fact that there is a limit to receptacles per circuit in a commercial facillity and not in a dwelling? These are the real reasons the question would be given.

Roger

[This message has been edited by Roger (edited 08-03-2003).]

So guys, where do we go from here? In my area we have voltages that range from 115 to 125v, every place is different. So what answer are we going to settle on, 10 ,12 or 13? My answer is still 10, due to the fact that we cannot load the circuit above 80%.

Zapped208, please explain or tell where the 80% or 125% numbers are applicable or addressed in a receptacle circuit? How could we know an unknown load would be continuous? The only reason we would apply the 180va to a receptacle is it would be an unknown load.

Roger

Why can't we do the calculation like this:
180va/115v = 1.565 amps. 1.565 amps times 13 receptcles = 20.348 amps. 220.2(B) permits fractional amps less than 0.5 to be dropped.

I always thought the answer was 13

Bill

Don, if the question asked for the total amps of 13 receptacles on a 115v circuit you would be correct.

As bad as we may dislike the question, the answer as it is worded is 12. (sorry Bill)

Roger

Roger,
There is nothing question that tells me how I have to make the calculation. It is simply a load calculation question. Both methods of working the problem are equally correct. One method gives an answer of 12 the other method gives an answer of 13.
Don

Roger,

I was basing my answer of 13 using a nominal voltage of 120v and not applying any 80% or 125% adjustment. Sorry, I was not addressing the original question.


Bill

OK Joe your turn.

If the worms crawl into the ground now that the can was opened, just grab a Megger

My Thoughts:

The NEC does not tell us "who will specify the voltage."

I will research the original proposal when I get home. I should be able to find that proposal in my library of NEC reports, and the substantiation may include answers to this question.

For now, the test writer is responsible for developing a question like this one, and it has caused many to argue about this issue.

The bottom line:

If you don't use the 115 volts and answer 12, you will be with one wrong answer on an examination.

Joe,
Why is the calculation in my post of 08-03-2003 05:27 PM wrong?
Don

I like Don's computation method. I think it makes more sense than the method everyone including myself used.

I think the question about voltage used is intended more for 60 volt or 90 volt systems not 115 volt systems.

I think the questions on the exam should reflect usual practice not practices that "game" the code.

George, why is 115v a standard voltage in article 430? This is part of the same NFPA (70) document.

Roger

Gentlemen: In addition to Article 430 in Article 440 it says:

What is the determining factor on how many receptacles can be placed on the circuit in question?
Don

Ohms law.

Don, how long can a 20 amp breaker hold 20.35 amps in true design. We know in real life 20 amps is a close number in time curve and duration but in reality a 20 amp OCD is just that, 20 amps.

See table 210.24 "maximum load".

You and I both know what 220.2(A) & (B) says, and we both know the way you are using it is to prove the article wrong and poorly worded. You and I also know the "spirit of the question is to test an exam takers skill and knowledge.

You, yourself have been "troubled" on another forum today with someones claim of being a "master electrician", and with this persons question, it would be in doubt he would have a clue to the answer to this question even if it used 120 v.

My point here is, that if the exam taker had the insight to answer this question with 12, I would feel good about his / her understanding of the determining factors.

Roger

Roger,
The load is the only factor that can be used to answer this question. Per the code 13 duples receptacles on a 115 volt cirucit is a load of 20 amps and is permitted to be protected by a 20 amp overcurrent protective device. How can we say that the calculation has to be made the other way? What code section tells us that? As far as the trip time, the time trip curves for a major brand of breakers show that this load may trip the breaker somewhere between 10 minutes and never.
Don

Don, I can't stand to see a grown man cry. You are right! You are right! You are right!

I know I feel better now.

BTW, let's remember this.


Roger

Roger,
I'm not crying. No one has cited any code section that says the alternate method of calculation that I used is wrong. Until they do, I will insist that 13 duplex receptacles are permitted on a 20 amp 115 volt circuit.

The wording specifing the voltages to be used in the calculations was a result of proposal 2-118 in the 1983 TCR. Even with the substantiation, it is not clear what is meant by the words "unless other voltages are specified". The substantiation said: "There is a need to specify what voltage level the panel instends should be used for branch circuit and feeder calculations. The voltage levels selected are in keeping with the nominal system voltages listed in ANSI Standard C84.1".
The panels comment on proposal 2-208 in the 86 TCR seeking to add the voltages 380Y/220 to the section seems to indicate that I am wrong about who can specify a voltage. The panel rejected the proposal with the comment that: "present wording allows specifing other voltages".
Don

i see the problem, yet am fuzzy on a fix for it.

would a definition help?

meanwhile, it would be ashame to have brighter students balk at a loaded Q here.

Decrease the voltage...

Power systems are rated for nominal voltages such as: 120, 208, 240, 480, and 600V. These voltages are specified by ANSI and have been the "correct" voltages to use since the early 1950's.

110.4 says in part "...nominal voltage of a circuit..."

220.2 says "Unless other voltages are specified, for purposes of computing... nominal voltages ... shall be used."

Motors are rated for utilization voltages (they allow for voltage drop from nominal) of: 115, 200, 230, 460, and 575V. These are the nameplate voltages referred to in Articles 430 and 440.

In my opinion, the question was intended to test the knowledge of the formula (va = volts x amps). Art 220.2 gives the writer the authority to specify the voltage (yes it appears to be a trick question).

Isn’t the whole point of 220.2(A) to make us use nominal voltages for computations?

Wouldn’t the “Unless other voltages are specified” refer to other voltage systems not listed at the end of this section? (not any arbitrary voltage by any entity)


[This message has been edited by engy (edited 08-07-2003).]

When we figure what the VA is per outlet (strap), the result is less than 180 VA. Section 220.3(B)(9) indicates that the minimum per strap is 180VA.

20 x 115 = 2300 VA 2300VA/13 = 176.9 or 177 VA per outlet, less than the required 180VA

Joe,
I did my calculation at 180 va per strap. I just chose to do the calculation in a manner where the code clearly permits rounding. Even if I do it like this, I still get a 20 amp "code" load.
13 x 180va = 2340 va 2340va/115v = 20.348 amps. The code permits me to drop the .348 amps giving me a "code" load of 20 amps which is permitted on a 20 amp circuit.
As I told Roger, we'll have to agree to disagree on this one. I will insist that 13 are permitted until someone can show me a code rule that says 2340 va is not permitted by the code on a 115 volt, 20 amp circuit.
Don

It may be worth noting that for receptacle branch circuits, utilization equipment is of two types. Loads can be constant-resistance or constant kVA—corresponding to heating/incandescent-lighting and inductive/motor loads. The point is—within limits—resistive-load current increases with increasing line voltage, and inductive-load current decreases with increasing voltage. Equipment nameplates usually list 115V, nominal system voltage is 120V, [and the wiring-device rating is 125V.]