# Load calculation for sub-panel



## Briancraig81 (May 25, 2007)

So I've been studying to get my NC limited. Not taking a prep-course or anything, just going by the sample questions in the book the board gives out and I did get a bunch of study material from a guy who had been through the prep-course. Anyway I've gotten the hang of a good portion of it and I'm working on load calculations now. Though I don't think it will be on the test I want to know how to do load calculations for sub panels in new and pre-existing dwellings (Or buildings). Here are my examples:

Scenario #1:
Lets say you wanted a sub-panel in a kitchen. My house for instance has 2 20a circuits, 1 20a Laundry circuit, 30a dryer, 40a range and 2 switches that control an inside light and porch light. How would you calculate the load. Would it be 1500x3(Counter recepts and Laundry) and the nameplate KW rating on the Dryer and Stove? What about the light circuit?

Scenario #2:
This really happened but I was laid off before I found out what the load calculation came up to be or how he did it. Did a kitchen remodel, added 1 15a circuit for a disposal, 1 20a/240v circuit for small oven, 1 40a circuit for cook top an another 50a circuit for wall oven. Panel was a 100 Main lug in the basement and already fed 2 20a counter circuits, 1 disposal, 1 dishwasher, 1 20a and one 30a for the Heat and A/C and various lights and recepticles in the finished basement, including the bathroom. Inspector said it was overloaded. I know it's pushing it but wasn't sure if it would be pulling 100a or not. My two main questions are how do you figure in lights and receptacles when doing a load calculation like this esp when there scattered around and cant go by the sq.ft? Also, does the 80% rule come in to play here (That you can't load a circuit beyond 80% of it's capacity).

I know I should have learned this stuff years ago but I've just kinda been chugging along and haven't gotten serious about learning the calculations and all until about 6 months ago. Any help is greatly appreciated.


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## McClary’s Electrical (Feb 21, 2009)

Briancraig81 said:


> So I've been studying to get my NC limited. Not taking a prep-course or anything, just going by the sample questions in the book the board gives out and I did get a bunch of study material from a guy who had been through the prep-course. Anyway I've gotten the hang of a good portion of it and I'm working on load calculations now. Though I don't think it will be on the test I want to know how to do load calculations for sub panels in new and pre-existing dwellings (Or buildings). Here are my examples:
> 
> Scenario #1:
> Lets say you wanted a sub-panel in a kitchen. My house for instance has 2 20a circuits, 1 20a Laundry circuit, 30a dryer, 40a range and 2 switches that control an inside light and porch light. How would you calculate the load. Would it be 1500x3(Counter recepts and Laundry) and the nameplate KW rating on the Dryer and Stove? What about the light circuit?
> ...


 
Scenario #, perform an NEC load calc. Don't concern yourseld adding breakers up.

Scenario #2 Again on this one a load calc is needed, and it sounds like you were close. I would have tried to sell a service upgrade. Also, in certain situations, you can load a circuit to 100% of it's rating.


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## spdone (Dec 13, 2009)

Wow, I just posted about this very same lack of understanding on my part. I thought I was alone sorta. Never was showed exactly how it all goes down. I am gonna keep an eye on your thread for helpful responses.:thumbsup:


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## Cletis (Aug 20, 2010)

*Formula*

Here is formula I use

Main Panel = General Load Calc 

Subs

S1 + S2 +S3 + x +M1 = General Load Calc

S1 = General Load Calc of S1 etc...

S1 x S2 x S3 = General Load Calc / 3VA = GLC


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## Briancraig81 (May 25, 2007)

BTT

Thanks for the reply McClary's.

I understand the part about not going by the breaker size, in scenario #1. So would you just ignore adding any Kw's if you had a circuit for a light? How about feeding another sub-panel from that sub panel that feeds an outbuilding with 3 15a circuits that were just there, not for any dedicated equipment?

In scenario #2, what you are saying is the just ignore any general lighting and receptacle loads and just go by any nameplate ratings that is fed by the sub-panel, right?


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