Math 101

[Roger]

Check out the math in the answer and what the heck is a branch branch circuit?

3. What is the minimum number of 20 amp branch circuits required for a 1500 sq.ft. house?

(A) 6 (B) 4 (C) 8 (D) 2


Item A was the correct answer.
Step 1: Compute load - Gnerals lighting load 1,500 sq. ft. [NEC 210-10] at 3 VA per sq. ft. {NEC Table 210-3(a)] = 4,500 VA.
Step 2: Find Minimum Number of Branch Circuits Required. Find Example in Appendix D. General Lighting load: 4,500 VA/120 volts = 37.5 amps. This requires (2) 20 amp, 2-wire circuits [NEC 210-11(b)].
Small Appliance Load: This requires (2) 20 amp, 2-wire circuits [NEC 210-11(c)(1)]. Laundry Load: This requires (1) 20 amp, 2-wire circuits [NEC 210-11(c)(2)]. Branch Branch Circuit: This requires (1) 20 amp, 2-wire circuits [NEC 210-11(c)(3)].
Answer: 2 + 1 + 1 + 1 = 6 circuits.

How about this one

11. What size dual-element fuse does the Code require for a 2 hp, 208 volt, single-phase motor?

(A) 30 amp (B) 45 amp (C) 20 amp (D) 35 amp

Item C was the correct answer.
Step 1: Look up motor, short circuit protection and full load current in index.
Step 2: Find 13.2 amps [NEC Table 430-148] and 175% in [NEC Table 430-152].
Answer: 13.2 amps X 175% = 23.1 amps. Note [NEC 430-52(c)(1) Exception No. 1] permits going to the next higher size, which is 25 amps (not a choice) and not 30 amps.

[Seal]

so is it right or wrong?

[Wgoodrich]

YOU SAID;
Check out the math in the answer and what the heck is a branch branch circuit?

3. What is the minimum number of 20 amp branch circuits required for a 1500 sq.ft. house?

(A) 6 (B) 4 (C) 8 (D) 2


Item A was the correct answer.
Step 1: Compute load - Gnerals lighting load 1,500 sq. ft. [NEC 210-10] at 3 VA per sq. ft. {NEC Table 210-3(a)] = 4,500 VA.
Step 2: Find Minimum Number of Branch Circuits Required. Find Example in Appendix D. General Lighting load: 4,500 VA/120 volts = 37.5 amps. This requires (2) 20 amp, 2-wire circuits [NEC 210-11(b)].
Small Appliance Load: This requires (2) 20 amp, 2-wire circuits [NEC 210-11(c)(1)]. Laundry Load: This requires (1) 20 amp, 2-wire circuits [NEC 210-11(c)(2)]. Branch Branch Circuit: This requires (1) 20 amp, 2-wire circuits [NEC 210-11(c)(3)].
Answer: 2 + 1 + 1 + 1 = 6 circuits.

CORRECTED REPLY;

1500 X 3 = 4500 Va + 1500 Va Laundry + 3000 Va Small appliance branch circuit = 4500 + 1500 + 3000 = 9000 Va. Adjust first 3000 @ 100% then add remainder at 35% = 6000 X 35% = 2100 Va + 3000 @ 100% = Total General Lighting adjusted load 5100 Va.

Then 20 amp X 120 Volts = 2400 Va Capacity of a 20 amp circuit @ 100 % for a dwelling general lighting load.

Now divide 5100 Va general lighting calculated load by 2400 Va maximum capacity of 20 amp circuit = 2.25 circuits

Answer would be considering the .25 over 2 general lighting circuits makes the answer 3 general lighting branch circuits.


Now the person that wrote that question did not give you the correct answer in your choices. Your answer would then have to be none of the above in their choices. I would have written in NONE OF THE ABOVE ! Then I would have challenged the skill of the test writer. Seen this problem with test writers not knowing their own answers to their own questions many times before.



YOU SAID;
How about this one

11. What size dual-element fuse does the Code require for a 2 hp, 208 volt, single-phase motor?

(A) 30 amp (B) 45 amp (C) 20 amp (D) 35 amp

Item C was the correct answer.
Step 1: Look up motor, short circuit protection and full load current in index.
Step 2: Find 13.2 amps [NEC Table 430-148] and 175% in [NEC Table 430-152].
Answer: 13.2 amps X 175% = 23.1 amps. Note [NEC 430-52(c)(1) Exception No. 1] permits going to the next higher size, which is 25 amps (not a choice) and not 30 amps.


CORRECTED REPLY;

20 amp 208 volt motor is a three phase motor not a single phase motor. Right then again I would answer none of the above and challenge the skill of the test author not knowing enough skills to write a test and know the answers to his or her own question.

Now lets say you have a 240 volt motor allowed per the manufacturers installation instructions to be ran on a 120/208 wye power system you would calculate approximately a 10% increase of amps due to the lower voltage feeding that motor. Go to Table 430.248 single phase is listed in the single phase motors. at 12 amps . If you increased the 10% to 12 amp for 240 volt motor on 208 volts you would get 13.2 amps. The chart in the 208 column getting 13.2 amps same as the 10% increase used to be done.

Now take the 13.2 and increase it for maximum overcurrent device allowed time delay and you get 23.1 amps. Then checking for a branch circuit you are allowed to install the next larger fuse amp rating when found not meeting those amps listed in 240.6 allowing a 25 amp fuse to be used per 240.6.

However the question has a trick in it whether the author of the test is aware of that trick or not is in question. It stated what is the fuse size required. It did not ask what the maximum fuse size allowed. Therefor the Code requires the fuse to be adequate in size to carry the load. This would require at least 25% higher than the amp rating of the 13.2 amps being 16.5 amps REQUIRING a 20 amp fuse. Answer would invite maximum time delay fuse of 25 amps but code only requires adequate fuse size and the maximum fuse size would only be used if needed due to high start load such as a direct drive motor with a cast iron heavy metal attached taking a longer higher amp start time. allowing the maximum amp fuse to be used. You should only size your fuses to the minimum safe starting size not always the maximum size allowed. Therefor the true answer to the question in that test would have been 20 amp fuse.


I know this is long winded but it brings out the point that those writing tests are often not skilled test writers and the answers may never be found to meet their question demand because the test author did not have the skill to write a question that didn't have way too many answers. I am sure the test writer knew what they meant in writing the question but they didn't have the skills to write a question that is without challenge due to lack of test writing skills.

Hope this gets you to thinking the life we work in the industry.

Lack of skills and inaccurate instructions when taught is our industry's worst enemy. They don't know the don't know.

Good Luck

Wg

[Roger]

Wg, the first example is just poor proof reading and a typo but it is the test as given and the authors master copy with answers for grading purposes and supplied to the students. This test is also offered online.

You hit the nail on the head in the second example. My problem is in the answer as given. The answer is to prove the 20 amps not the 25 amp next size up rule for maximum time delay dual element fuse.

To further add....

And is that really the maximum... I believe you can go to 225% if that 175% still isnt sufficient to start the load. So I could see where 30 amps is also an answer.

The 25% minimum is for the ampacity of the conductors. I believe the time delay dual element fuse is supposed to be the minimum for the starting current of the motor.

Now I'm not trying to make fun of this test just trying to bring to the attention of readers to be careful of tricks.

Is see no advantage to tricks in skilled trades test formats. the question should be worded correctly and the reasoning should support the anwser the test writer wants.

I'm glad I dont write electrical exam tests......:D

[Wgoodrich]

Look in table 430.52 under single phase or polyphase and you will find dual element fuses to be 175% maximum short circuit. Nowhere is 225% used in sizing short circuit. There are many special circumstances that would allow much higher than that 175% increase but those specials were not included in the question specific wording.

Now to get more particular the test author did not mention if this dual element fuse was also being used as an overload device as allowed per Code rules. If he had mentioned overload in his question then the whole answer would change to overload rules of 430.32 being size a maximum of 115% of the full load current of the motor unless a factor was stated as a minimum then a maximum would allow up to 140% in that same rule. Then considering the required by code stated in the quetion you would not have enough info to answer that part accurately . I didn't mention that twist involved because the author of the question did not mention overlaod in the question. However the Code rules allow a dual element fuse to be used both as short circuit and overload that could also chage the whole crux of the question. Just reading a little more into the question that was not clarified in teh words of the question.

Having fun

Wg

[Roger]

I'm having fun......:D

And you are correct the test question as written doesnt reflect what you would do if the 175% maximum required fuse wouldnt start the load. I was refering to the 2nd exception in 430.52.

Just trying to show the overall knowledge that is required to decifer these electrical exam questions and not fall through the holes and land on the traps they set for you.

[suemarkp]

I didn't like the first question at all. My answer would have been two branch circuits. Who is to say a house must have a kitchen and bathroom? Yes, most all do, but if you don't have them I don't believe the NEC requires circuits for them. Same with the laundry.

Wg - I don't see where in the NEC that the number of general lighting branch circuits only has to match the corrected demand load. You added the kitchen and laundry then applied a demand factor. This would be correct for the overall service calculation, but how could the general lighting circuits be allowed to have a demand load applied when non-general circuit were added?

I would have thought that the number of required general lighting branch circuits is (square feet * 3 / 2400) for twenty ampere branch circuits.

[Roger]

Mark, just for a little background this is test offered up by a local establishment that specializes in teaching skilled trades. My intention in putting this on the forum is not to make footprints on their test, I dont have the credentials to be going around trashing others tests. My experience with testing in the electrical trade has caused my patience to be worn thin by these ambiguous/trick questions. I'm not sure if you can write a question that some people will not veer down a different road to the wrong answer. But jeez at least write a question that doesnt invite 3 or 4 or more correct answers.
I have to admit that I didnt take a real close look at the first example I just saw the 5 = 6 deal and the typo sticking out. I see now that it over simplifies and assumes a great deal.

The #2 example is written very poorly IMO. Leaves you out to dry and is totally a mess. It gives a reasoning that doesnt support the given answer. It says the solution is 13.2 x 175% = 23.1 amps so he/she intended this to be maximum ocpd using a dual element fuse. Then sites next size up rule. So in my opinion their was no correct answer under that analysis.
I agree in general that the 20 amp is correct ....ie....the minimum required if the motor starts. My problem is with teachers solution.

I get little ticked at these seemingly intentional attempts to see if you can decifer the intent of the question. It does seem to me to be rather common in electrical exams. It would seem to me you want to know if the the person can get to the right answer with given information. Not a play on wording. Now it is posssible that the teacher thought it was clear as a bell, though unlikely as the solution doesnt support the correct answer. I see no gain educational to students or apprentices to find a complicated way to ask for the minimum fuse allowed for that motor. If you structure the question to include 25 amps and 20 amps as answers and use the word require and 35% of the class gets it right at 20 amps and the other 65% get it wrong......then use the word minimum in place of required and that same class 80% get the answer right and 20% wrong what have you proved? Pretty much that a pretty good precentage that got it wrong the first time knew how to get the right answer but got hung up on the use of a word. Darn unfair in my opinion especially on licensing tests where your livelyhood may suffer.

No having said all that we all know that is how it is on tests and we generally prepare for that eventuality. I just wonder the merits to keywords being intentionally misleading. Is the persons intelligence really being tested in the electrical field or are we seeing if they can find new definitions to words.

Jeez I dont think you have to challenge students or apprentices by using "tricky keywords" on exams seems challenging enough without playing games with words.

I disagree that the word requires means minimum. But hey Tom Henry offers a book about keywords that arent necessarily "websters correct" and determining the intent of an exam question.

[Seal]

Great to read, but I have no idea what the heack you guys are talking about!
Thanks for the replys!bow_1

[Wgoodrich]

suemarkup the kitchen, nook, laundry, pantry, and bathroom receptacles are considered as general lighting loads. I included the rules that apply in hopes it will clear up your doubts.

220.14
(J) Dwelling Occupancies In one-family, two-family, and multifamily dwellings and in guest rooms or guest suites of hotels and motels, the outlets specified in (J)(1), (J)(2), and (J)(3) are included in the general lighting load calculations of 220.12. No additional load calculations shall be required for such outlets.
(1) All general-use receptacle outlets of 20-ampere rating or less, including receptacles connected to the circuits in 210.11(C)(3)
(2) The receptacle outlets specified in 210.52(E) and (G)
(3) The lighting outlets specified in 210.70(A) and (B)

COMMENT;

210.11.C.3 is the rules governing dwelling unit; small appliance circuits, bathroom circuits and laundry circuits.
210.52.E and G are rules governing dwelling unit; outdoor outlets and basement outlets also considered general lighting loads in loading calculations.

While they are specific in how they are wired they are considered as general lighting loads in calculatons of general lighting.

220.42 specifically allows the diversification reduction due to intermittent loading for all general lighitng loads including kithchen, nook, dining, pantry, laundry, and bathroom receptacles allowing the first 3000 va to be calculated at 100% and a 35% calculation rating for all remaining loads above the 3000 va. mentioned.

Hope this helps

Wg

[suemarkp]

I guess this is what I was thinking about:

210.11(B) Load Evenly Proportioned Among Branch Circuits. Where the load is computed on a volt-ampere/square meter or square foot basis, the wiring system up to and including the branch-circuit panelboard(s) shall be provided to serve not less than the calculated load. This load shall be evenly proportioned among multioutlet branch circuits within the panelboard(s). Branch-circuit overcurrent devices and circuits shall only be required to be installed to serve the connected load.

As usual, the wording here is kind of vague. I was assuming you can only spread the VA/Sq Ft load among the circuits that were calculated that way and not small appliance, laundry, bathroom or individual branch circuits.

To me, it seems wrong to count small appliance, laundry, and bathroom circuits as part of the general lighting circuits (as far as providing enough circuits) because those circuits are not allowed to have outlets of that type. But I see where you're ocming from and I don't think the code gives a clear direction. No surprise there...

[Wgoodrich]

Suemarkup, let me try to create a picture in you mind that may shed some light in the NEC design. Remember the Code seldom explains why a rule but often we can translate their concerns.

Take the kitchen counter area. Say we have plenty of counter space. Now we place receptacles to meet the 2'/4' rule over the counter making say 8 receptacles over that kitchen counter.

Now lets wire for a rich dude that wants a receptacle every 6" over those counter spaces. The placement of the receptacles and the requirement of those 12 awg wires are designed to carry a concentrated load of a small appliance that pulls about an average of 1500 va per appliance. Two of them buggers would overload even a 20 amp circuit. Say a toaster and a coffee pot being common. Both will hit that 1500 va per appliance as well as most other appliances. Now if a woman is cooking she only has two hands and most commonly piles her appliances in one space for close working conditions all plugged into those two receptacles in that 4' distance. The woman may use three appliances at one time such as a fryer and electric skillet and say a coffee pot. Much more and she would start burning food unwatched. Kind of a habit of a woman cook prediction. So we figure a common small appliance load of say 4500 watts on the kitchen receptacles at any given time.

But we now have a rich dude and that guy want a recepacle every 6" along the counter.

Minimum code is designed to handle the 4500 watt common combined load we discussed. Minimum code put 8 receptacles as we talked over this counter. But that rich dude now has 25 receptacles over that kitchen counter. How much more load will that woman add to the general load of your service as a combined load. My guess would be little or no change.

Now go to a bedroom. Receptacles 6/12' puts about 5 receptacles in a room using a say a TV, radio, Table lamp pulling about 6 amps. Now we put a receptacle on every stud in that bedroom haveing a total of say 50 receptacles in that same bedroom. How much more load is the normal home owner going to use in those two bedrooms? My guess little change in actual combined load on the service feeders.

While we do require larger wire in kitchen, nook, dining, pantry, laundry, bathroom, we seldom increase load by increasing the number of receptacles. Teh evenly balanced also applies to those rooms just mentioned same as general lighting.

When we are sizing a demand load of a service feeder little change is added to the service feeder concerning number of receptacles whether in the kichen, bathroom, laundry any more than we would experience load increase due to number of receptacles on the general receptacles in the normal living areas. This I believe is why when calculating demand load the group bathroom, and small appliance as well as laundry as general lighting loads allowing the intermittent load deration from that chart.

The Code committees are convinced rule of thumb 1500 va for laundry, 3000 va for small appliance branch circuits and 3 va per square foot of general lighting load all adjusted for intermittent load credit of the deration chart. They require the first 3000 va to be at 100% I suspect this is because the one working in that home is concentrated either in the bathroom, laundry or kitchen but not all at the same time. They figure the 3000 at 100% then 35% for the remainder should cover common loading of a family with kids.

If you put 20 - 20 amp circuits for receptacles in that kitchen you would not be inviting more loading more than normal habits of a family when cooking. If abnormal happens that is why we have breakers to trip due to overloading to protect the wires that may occur unexpectedly by an adverse habit say Thanksgiving day.


Just my thoughts hope it helps

Wg