# of 15 Ampere Ccts from NEC Exam Prep

[mrford]

Hey everyone,

Maybe someone can enlighten me on this one here out of one of Mikes Books 2008 NEC Exam Prep:
pg 254 if you have the book
* Number of 15 Ampere Circuits Example
question: How many 15A circuits are required for a 2,000 sq ft dwelling unit?
step 1: general lighting VA = 2,000 sq ft x 3 VA
general lighting VA = 6,000 VA

Step 2: General Lighting Amperes
I = VA/E
therefore I = 50VA
Step 3: Determine the number of circuits
number of circuits = General Ltg Amps / Circuit Amps
number of circuits = 50A / 15A
number of circuits = 3.3 or 4 circuits rounded up.

That is what the book says. But can you really have 15A loaded on a 15A breaker because the 15 amp breaker in the house is not going to be rated for continuous duty therefore it would be really rated for only 12 amps of which could be occuring on a circuit no problem for more than 3 hours if someone plugged in a portable heater etc etc.. thefore it would be:
number of circuits = 50A / 12A
number of circuits = 4.16 or 5 circuits rounded up.

The other thing I find interesting is you de-rate a range oven over 12 KW. If you are cooking large items in a large oven for more than 3 hours wouldn't that be an item that would need a rating of continuous duty?

Ideas?

 

[Lcdrwalker]

First thing... receptacle circuits aren't rated continuous. Ranges are derated due to the fact that even if you are cooking a large meal, all oven elements and top elements wouldn't be on at the same time for any appreciable amount of time.

 

[iwire]

Continuous load?

Continuous load?

Mr Ford, I strongly suggest a careful look at the definition of continuous load in Article 100. :smile:

 

[mrford]

It's probably just a Canadian code thing vs American code thing. Canadian code we only load 12 amps on to a 15 amp circuit because we don't know what will be plugged into the receptacle in case it's an item that someone does use continuously such as a plug in heater. I know lots of seniors that plug in those stand up oil heaters everywhere and let them run on all day. But really from us it comes from counting each receptacle as 1 amp and adding them up to 12 if you were to say their would be a 1 amp item plugged into each outlet. That would account for the 80% loading.

Continuous Load: A load where the maximum current is expected to continue for 3 hours or more.

So if the breaker is only rated for 15 amps short term load and 12 amps continuous loading and you don't know what will be plugged in...

Is their a rule that actual states that receptacles in a dwelling unit shall not be considered to be a continuous load?

cheers

 

[iwire]

But really from us it comes from counting each receptacle as 1 amp and adding them up to 12 if you were to say their would be a 1 amp item plugged into each outlet.

That is the difference, we do not do that, the receptacles in a house under the NEC do not count for any load, I can put 5 receptacles per circuit or 500 receptacles per circuit. The load is determined by square feet. (For Dwellings)

Continuous Load: A load where the maximum current is expected to continue for 3 hours or more.

How can you 'expect' any load on a receptacle?

As soon as you have more than one receptacle on a single circuit all bets are off, the circuit could easily be overloaded.

Is their a rule that actual states that receptacles in a dwelling unit shall not be considered to be a continuous load?

No, but more importantly there is no rule requiring us to.

You can always choose to go beyond the code but when taking a test on the NEC you have to stick to the NEC.

Cheers to you as well. :smile:

 

[radiopet]

lol....and considering I took part in that book....lol......It's Correct