Mismatch between Theoretical and Actual load calculations

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Timona

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Graduate Electrical Design Engineer
NOTE: In my question, when I say receptacles, I mean normal household receptacles. I am am intentionally omitting loads for special appliance receptacles because I think they are not needed in my question.

Suppose I am trying to calculate the load of 100 sq foot dwelling. I use 3 VA/ ft as deemed by NEC for a total of 300VA.

1. From my little understanding, this means that both my receptacle and lighting loads are included and should I add them both manually, they should be less than or equal to 300VA, right? Basically, my theoretical load of 300VA should match my manually load, right?

2. Now, NEC calls for 12 ft separation between 2 receptacles along a wall. Being that I have 100 sq ft dwelling, I am bound to put atleast 2 - 180VA receptacles in this space so as to comply with this rule. These 2 alone will give me a manual total of 360VA, which is above my theoretical 300VA. Remember, I am yet to add the VA for the lights.

Number 2 above is where I get confused. Or am I not understating this altogether?
 
First, when I saw your subject line, I already knew that my initial reaction would be to tell you that, especially for dwellings, the NEC load calculation methods have next to nothing to do with what real consumption will be. They are a guideline to make sure that services aren't drastically undersized, and they tend to oversize for most dwellings.

As far as the specifics of your question, electrofelon beat me to it while I was typing. You don't use 180 for dwelling units, the 3VA per sq ft covers all non-dedicated outlets as well as lighting. It's just a rule of thumb assumption that a home needs 3VA per sq ft. The outlet spacing requirements are about keeping homes free of lots of extension cords, not because they will all draw a certain amount of power. In non-dwelling occupancies, the number of outlets installed is more likely to correspond to an actual level of need, and therefore each outlet must be accounted for in the load calc. At least, that's the way I understand it.
 
jaggedben said:
First, when I saw your subject line, I already knew that my initial reaction would be to tell you that, especially for dwellings, the NEC load calculation methods have next to nothing to do with what real consumption will be. They are a guideline to make sure that services aren't drastically undersized, and they tend to oversize for most dwellings.

As far as the specifics of your question, electrofelon beat me to it while I was typing. You don't use 180 for dwelling units, the 3VA per sq ft covers all non-dedicated outlets as well as lighting - Understood to mean that we completely do not use 180VA for unit dwelling but 3VA/ ft. Kindly correct if I am wrong. It's just a rule of thumb assumption that a home needs 3VA per sq ft. The outlet spacing requirements are about keeping homes free of lots of extension cords - Does this mean I should maintain my 12 ft spacing, which would give me 4 receptacles in my 100 sq foot unit? Or if you were me, how many would you put?, not because they will all draw a certain amount of power. In non-dwelling occupies, the number of outlets installed is more likely to correspond to an actual level of need, and therefore each outlet must be accounted for in the load calc. At least, that's the way I understand it- Understood
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You can have all the receptacles and lights you want. You just figure your general receptacles and lighting at 3va/sq ft. Don't know how to make it clearer.

Then you add to that your small appliance circuits, laundry, hvac, etc.

-Hal
 
I think I get it now....the 180VA per receptacle does not play a big role in dwelling units, but becomes a key player if we are talking about non-dwelling unit receptacles loads.

1. So I should just maintain the 12 ft between receptacles per NEC guidelines BUT
2. I should not manually try to match the VA (rec x 180VA + light wattage) to see if it matched with what I get from the 3VA/ ft BECAUSE the 180VA per receptacle does not play a big role here. So in short, I should only care about the 3VA/ ft

Sorry folks. I just graduated and trying to find my footing in this field. My apologies for any dumb question.
 
hbiss said:
You can have all the receptacles and lights you want. You just figure your general receptacles and lighting at 3va/sq ft. Don't know how to make it clearer. I get it now. This was where my problem was as I was trying to match my theoretical 3VA/ ft calculation with the 180VA per recptacle manual calculations. But I now understand it

Then you add to that your small appliance circuits, laundry, hvac, etc. Yap. I already knew how to do this

-Hal
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Timona said:
I think I get it now....the 180VA per receptacle does not play a big role in dwelling units, but becomes a key player if we are talking about non-dwelling unit receptacles loads.

1. So I should just maintain the 12 ft between receptacles per NEC guidelines BUT
2. I should not manually try to match the VA (rec x 180VA + light wattage) to see if it matched with what I get from the 3VA/ ft BECAUSE the 180VA per receptacle does not play a big role here. So in short, I should only care about the 3VA/ ft

Sorry folks. I just graduated and trying to find my footing in this field. My apologies for any dumb question.
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You do not need to have 12 ft between receptacles, the rule is you must have a receptacle within 6 feet of the starting point and ending point which often is a doorway and then no point along the floor line can be more than six feet from a receptacle. Which ultimately will allow up to 12 feet between receptacles, but you can always have less distance between them. In fact you could put one every six inches and the load calculation is still the same unless there are outlets intended for specific fixed equipment.
 
The Article 220 calculations result in a load that is at least twice the real world load.

The 50 kVA transformer outside of my house feeds 10 dwelling units without any issue. Most are larger than the 700 sq ft calculation shown post 6.
 
1. So I should just maintain the 12 ft between receptacles per NEC guidelines.

6/12 ft is minimum you can have as many receptacles as you want.

2. I should not manually try to match the VA (rec x 180VA + light wattage) to see if it matched with what I get from the 3VA/ ft BECAUSE the 180VA per receptacle does not play a big role here. So in short, I should only care about the 3VA/ ft

Correct. Only a waste of time, it's not supposed to match. It doesn't play any role in residential calculations, only commercial.

-Hal
 
Timona said:
I think I get it now....the 180VA per receptacle does not play a big role in dwelling units, but becomes a key player if we are talking about non-dwelling unit receptacles loads.

1. So I should just maintain the 12 ft between receptacles per NEC guidelines BUT
2. I should not manually try to match the VA (rec x 180VA + light wattage) to see if it matched with what I get from the 3VA/ ft BECAUSE the 180VA per receptacle does not play a big role here. So in short, I should only care about the 3VA/ ft

Sorry folks. I just graduated and trying to find my footing in this field. My apologies for any dumb question.
Click to expand...
Take the 180VA out of your mind and lock it in the closet. It plays NO part in a dwelling load calculation!
 
180 VA per outlet don't exactly mean a lot for non dwellings either. It is just something thrown in there to account for outlets with non known uses, those with known loads will often be more then 180 VA and you normally will use the known load for those.
 
kwired said:
180 VA per outlet don't exactly mean a lot for non dwellings either. It is just something thrown in there to account for outlets with non known uses, those with known loads will often be more then 180 VA and you normally will use the known load for those.
Click to expand...

This makes perfect sense!!
 
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