Exam Question: Feeder Size

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"A single-family dwelling unit has a net load of 4000VA. A 6kW range will also be included in the design. The dwelling unit is supplied by a 120/240V, three wire, single phase copper feeder of type THW. According to the NEC, what size feeder is required? - AWG3/AWG4/AWG6/AWG8."

The solution references NEC 230.79 - Rating of Service Disconnecting Means and NEC 310.15(B)(7) Single family dwelling services and feeders.

The way this question reads, it sounds like an apartment unit, not a stand alone house. NEC 100 definitions back me up here. (uses term 'unit')

Im thinking that this solution is wrong because the 'feeder' is a feeder and not a service.

the second question is the solution doesnt use the demand factor table of 220.42 for the 4000VA load. Is this because its not specified as a lighting load?
 
If I recall right, they are showing the 4000 load as from a square foot calculation..think it is 3 per sqr foot, and then you add in the range or other heavy loads...
the question is not asking about lighting as that is in your basic calculation using the square foot for 15 and 20 amp circuits.. in other words, the standard residential calculation that is based on outside walls to outside walls... not on inside walls dimensions, straight from home plans.
Note, if doing this in real life, you do not use the square for the realtors use but use the foundation exterior square foot measurements.

then you add washers, dryers, ranges, ovens, AC, pool motors etc...

Pretty basic design stuff for testing... but I dont have the latest tables in front of me so won't try to give you the answer... I would assume around a 75 amp main breaker roughly but probably lower..lol


Oh, and without doing the math, but just a guess, would say 4 gauge service, 6 gauge to range... but just guessing..watching heathcliff with grandsons and all books upstairs
 
TheSmoothestCriminal said:
"A single-family dwelling unit has a net load of 4000VA. A 6kW range will also be included in the design. The dwelling unit is supplied by a 120/240V, three wire, single phase copper feeder of type THW. According to the NEC, what size feeder is required? - AWG3/AWG4/AWG6/AWG8."
Click to expand...
TheSmoothestCriminal said:
The solution references NEC 230.79 - Rating of Service Disconnecting Means and NEC 310.15(B)(7) Single family dwelling services and feeders.

The way this question reads, it sounds like an apartment unit, not a stand alone house. NEC 100 definitions back me up here. (uses term 'unit')

Im thinking that this solution is wrong because the 'feeder' is a feeder and not a service.

the second question is the solution doesnt use the demand factor table of 220.42 for the 4000VA load. Is this because its not specified as a lighting load?
Click to expand...


"Single family dwelling unit" sounds like a standalone house, not an apartment, to me.

What is the solution that they gave?
 
I would have guessed the #6 setup without doing the math.

What was the correct answer?

JAP>
 
TheSmoothestCriminal said:
"A single-family dwelling unit has a net load of 4000VA. A 6kW range will also be included in the design. The dwelling unit is supplied by a 120/240V, three wire, single phase copper feeder of type THW. According to the NEC, what size feeder is required? - AWG3/AWG4/AWG6/AWG8."

The solution references NEC 230.79 - Rating of Service Disconnecting Means and NEC 310.15(B)(7) Single family dwelling services and feeders.

The way this question reads, it sounds like an apartment unit, not a stand alone house. NEC 100 definitions back me up here. (uses term 'unit')

Im thinking that this solution is wrong because the 'feeder' is a feeder and not a service.

the second question is the solution doesnt use the demand factor table of 220.42 for the 4000VA load. Is this because its not specified as a lighting load?
Click to expand...

So, to go in to teaching mode on this. What is the NEC code definition of a single apartment unit? If the tenant has an individual utility meter, then what are the wires between the meter and the first means of disconnect called? I think you will discover that a stand alone house has a feeder and a Service in most cases and there is one wire difference between them, but the size of the wires is the same. What wire is the difference?
 
jap said:
I would have guessed the #6 setup without doing the math.

What was the correct answer?

JAP>
Click to expand...


The correct answer is #4AWG. First they calculate the load to be 68.3A, but then reference NEC 230.79 to come up with the 100A minimum size, and then they use NEC 310.15(B)(7)(1) for 83% of the disconnect - 83A for AWG#4
 
Strathead said:
So, to go in to teaching mode on this. What is the NEC code definition of a single apartment unit? If the tenant has an individual utility meter, then what are the wires between the meter and the first means of disconnect called? I think you will discover that a stand alone house has a feeder and a Service in most cases and there is one wire difference between them, but the size of the wires is the same. What wire is the difference?
Click to expand...


A single apartment is a dwelling unit.

If the meter is the service point, then the load side wire would be called the feeder.

My confusion is that if this is not a stand alone unit (apartment), then the service sections of the code being referenced wouldn't apply because its a feeder and not a service.

NEC 100- Service: the conductors and equipment for delivering electric energy from the serving utility to the wiring of the premises served. Wouldnt the service end at the meter? Unless the code would refer to the MCB in the panel feeding this apartment unit as the service disconnect.
 
TheSmoothestCriminal said:
A single apartment is a dwelling unit.

If the meter is the service point, then the load side wire would be called the feeder.

My confusion is that if this is not a stand alone unit (apartment), then the service sections of the code being referenced wouldn't apply because its a feeder and not a service.

NEC 100- Service: the conductors and equipment for delivering electric energy from the serving utility to the wiring of the premises served. Wouldn't the service end at the meter? Unless the code would refer to the MCB in the panel feeding this apartment unit as the service disconnect.
Click to expand...

Good answers. But even though I don't do many myself, many apartments are done with meter banks, others are still done with individual service drops, there is nothing against it except the Utility company may protest.
 
david luchini said:
I'd be curious how they come up with a load of 68.3A.
Click to expand...

Not sure.. I came up with 58.3 myself... using 50 amps for stove... which becomes 25 amps because it is 240 volts...

hmm...

but, isnt the USA requiring a 100 amp box for residential now as a minimum? Which explains the 83 amps for the feeder...
Still say my math looked pretty good considering grandsons were fighting over pokeman... and i left all my books upstairs...lol...
Maybe the studying is sinking in...
 
Ok Jumper... used 6000 watts /240 = 25

plus 4000 watts /120 = 33.333
gave me 58.3

but... lessee something... How about...nope, math not working... You are dropping the amount of power for stove...

50 watts derated 80 percent equals 40 watts
4000 watts derated same way equals 26.67
equals 67 watts...

So.. if they took same thing I do for stoves.. 50 amp breaker before derating... then my math comes close to theirs but only if I de rate the house circuits of 80 percent as well...

If my math is right... that is...

But again, since minimum panel for residence is 100 amps if I remember as well, unlike Jamaica where I could put i a 70 amp... the rest of the question works out...
so question really a trick because you have the minimums you must have- 50 amp cooker circuit, and the minimum panel size...

Unless I really know nothing about the codes, which is totally possible with me trying to learn 3 codes at one time..lol
 
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