NEC Exam Question - Apprentice

[evalencia01]

I'm still learning how to calculate loads in the NEC. Not going through an apprenticeship program, I'm simply studying it on my own working for a licensed contractor. (This is allowed where I live)

For the Single Dwelling Load question I'm expecting on the exam, I've found online and through some other people to simply add the general lighting loads, small appliance loads, laundry loads, appliance loads, dryer loads, A/C/heating loads, motor loads and kitchen equipment loads, apply demand factors and then size according to the total amps I get out of the total VA. This seemed pretty straightforward and what I was going to do.

But I'm having some trouble. I've recently found some stuff on balancing the load and THEN sizing the conductors. I'm aware of how it works in regard to which line gets what, but how does this work in conjunction with the demand factors? Do I reduce the load applied to each conductor according to it's demand factor? Also, Will I need to balance the load on an electrical exam and THEN size the conductors?

One last thing. I also understand that you can reduce the load by 83% from the service disconnecting equipment. Let's say the load's 116A needing a 125OCPD and #2 90C conductors. Then I reduce the OCPD to 83% = 104A which allows the conductors to be #3 90C conductors. The OCPD will now be 110A correct, and will I use this on an exam?

Sorry if it's confusing to understand what I'm asking, any help is appreciated!

 

[evalencia01]

I'm taking a strictly residential licensing exam, no commercial work will be on it.

 

[david luchini]

One last thing. I also understand that you can reduce the load by 83% from the service disconnecting equipment. Let's say the load's 116A needing a 125OCPD and #2 90C conductors. Then I reduce the OCPD to 83% = 104A which allows the conductors to be #3 90C conductors. The OCPD will now be 110A correct, and will I use this on an exam?

The 83% is not a reduction in load, it's a reduction in conductor size. The OCPD would stay 125A.

You could use #2 conductors, based on the 75C rating.

 

[evalencia01]

The 83% is not a reduction in load, it's a reduction in conductor size. The OCPD would stay 125A.

You could use #2 conductors, based on the 75C rating.

Thank you for clarifying that. I meant it to be phrased the OCPD will now be 110 correct? Thank you for answering that for me. Do you know if in the exam we have to balance the loads along with the standard method?

 

[david luchini]

Thank you for clarifying that. I meant it to be phrased the OCPD will now be 110 correct? Thank you for answering that for me. Do you know if in the exam we have to balance the loads along with the standard method?

If the load is 116A, the ocpd must be a minimum of 125A. 110A would be too small.

I'm afraid I don't have any knowledge of the exam.

 

[Little Bill]

Balancing a load wasn't on my exam or anyone else's that I know. I'm only stating what I know and can't answer for the OP as I have no idea what the exam will be or who wrote the exam.

 

[evalencia01]

Balancing a load wasn't on my exam or anyone else's that I know. I'm only stating what I know and can't answer for the OP as I have no idea what the exam will be or who wrote the exam.

Okay I understand. I kind of wanted to reach a consensus on how to approach this sort of problem, I assume that they will specify it on the exam as I'm taking it. Thank you for your response, I will study both methods closely just in case.

 

[Little Bill]

Just a thought............
Balancing loads would come into play more for a 3Ø system than a single Ø, especially for a dwelling. Most heavy loads in a dwelling will be 2-pole and will, by default, be spread across both legs. Unless you had a ton of heavy single pole loads you shouldn't have much of an imbalance in a dwelling. With all the LED lighting now it's even less loads.

 

[suemarkp]

I believe the intent with calculations is to just add up the VA, whether 240V or 120V loads and it is assumed the electrician will balance them accordingly. Reality won't be perfectly balanced, because you never really know which loads are going to be used when others are not. But there is plenty of margin in the calculations, even if you had no pure 240V loads, that you'd rarely overload one leg of a service due to imbalance.

Also note there are two different calculation choices for dwellings -- the Standard and the Optional. I find the Optional typically gives a smaller service requirement.

 

[evalencia01]

Just a thought............
Balancing loads would come into play more for a 3Ø system than a single Ø, especially for a dwelling. Most heavy loads in a dwelling will be 2-pole and will, by default, be spread across both legs. Unless you had a ton of heavy single pole loads you shouldn't have much of an imbalance in a dwelling. With all the LED lighting now it's even less loads.

That's very true, thank you. Whenever I'm studying new material and think I've got it, I come across other material that is seemingly relational and end up overthinking it shaking my head

 

[evalencia01]

I believe the intent with calculations is to just add up the VA, whether 240V or 120V loads and it is assumed the electrician will balance them accordingly. Reality won't be perfectly balanced, because you never really know which loads are going to be used when others are not. But there is plenty of margin in the calculations, even if you had no pure 240V loads, that you'd rarely overload one leg of a service due to imbalance.

Also note there are two different calculation choices for dwellings -- the Standard and the Optional. I find the Optional typically gives a smaller service requirement.

Thank you, I'm definitely going to be using the Optional Method whenever I'm doing any dwelling calculations in real life, but I do understand now how balancing the load is a different type of problem

 

[Strathead]

Most likely on any exam, the best approach is to go to the examples in the back of the book. Find the example that has the most types of loads in common with yours, and then one by one, replace the values in the example with your values doing the math. Don't over think it. Also, learning the material and passing the test are two completely different things. Time is your main factor in the testing. Don't dwell on ANY calculation until you have found every "look up" answer on the test.