Residential Subpanel Calculations

[vw55]

I understand the standard and optional service/feeder calculations for residential services, however, I am struggling to clearly understand how to attack the subpanel load calculations. I have several high end residential projects that I am currently involved in and this issue arises on every one of them. For example:

I can justify a 400 amp main service for the house using the service/feeder calculations, so I specify a 400A, 120/240V 1ph 3W meter/main assembly. These units can only accommodate up to two 200/2 subfeed breakers, so I specify two 200A, 120/240V 1ph 3W subpanels.

Then, when I assign individual branch circuits in the house to the two 200 amp subpanels I've chosen, the connected loads exceed the panel rating. I cannot justify a 200 amp panel that shows 230 amps of connected load. The local AHJ is requesting panel schedules and I don't believe they will approve such an installation.

Is there a diversity/demand calculation that can be applied to the subpanels as well? Please include code references, links, etc. that I can use. Thanks.

 

[kwired]

I understand the standard and optional service/feeder calculations for residential services, however, I am struggling to clearly understand how to attack the subpanel load calculations. I have several high end residential projects that I am currently involved in and this issue arises on every one of them. For example:

I can justify a 400 amp main service for the house using the service/feeder calculations, so I specify a 400A, 120/240V 1ph 3W meter/main assembly. These units can only accommodate up to two 200/2 subfeed breakers, so I specify two 200A, 120/240V 1ph 3W subpanels.

Then, when I assign individual branch circuits in the house to the two 200 amp subpanels I've chosen, the connected loads exceed the panel rating. I cannot justify a 200 amp panel that shows 230 amps of connected load. The local AHJ is requesting panel schedules and I don't believe they will approve such an installation.

Is there a diversity/demand calculation that can be applied to the subpanels as well? Please include code references, links, etc. that I can use. Thanks.

How are you determining the load for each branch circuit? Use nameplates of fixed equipment, and 1500 VA for laundry or SABC's. But for most general purpose lighting and receptacles in a dwelling, you have to realize they will seldom be loaded to branch circuit capacity for any significant length of time. If you have a circuit supplying a bedroom do not use 15 or 20 amps, or even 180VA per outlet, you will come up with a high load really fast. Use the 3 VA per square foot for general lighting calculations. Throw in the actual value of any expected heavy loading as a safety net, though you often will be fine without doing so.

 

[vw55]

How are you determining the load for each branch circuit? Use nameplates of fixed equipment, and 1500 VA for laundry or SABC's. But for most general purpose lighting and receptacles in a dwelling, you have to realize they will seldom be loaded to branch circuit capacity for any significant length of time. If you have a circuit supplying a bedroom do not use 15 or 20 amps, or even 180VA per outlet, you will come up with a high load really fast. Use the 3 VA per square foot for general lighting calculations. Throw in the actual value of any expected heavy loading as a safety net, though you often will be fine without doing so.

I am using the nameplates for the fixed equipment and 1500VA for the required laundry and small appliance circuits. If I take the 3VA per square foot for general lighting and receptacles, as 220.14 (I) allows, this will cut down the connected load significantly. However, I'm not sure how to parse that calculation among the branch circuits in such a way that would be acceptable to the AHJ reviewing the panel schedules. Would it be acceptable to simply divide the total VA by the total number of branch circuits and show the same load on each branch circuit? This won't necessarily correlate with the power and lighting plans that I've generated either.

 

[suemarkp]

First, what is you calcualted load overall (how close are you to 400A)? There is a basic load in each panel that has to be 100% (10KVA in the Optional Cal case), so if you have a load calc of 390A there is no way you can spread that among two 200A panels because of that 100% initial load. You could use just a 400A meter base and then put in 3 panels if you have to. But the meter sockets only allow two conductors per lug so you'd need to tap off someplace to implement a 3rd panel.

If you know what areas the general lighting and general purpose branch circuits serve, you should be able to assign a square footage to them. It seems reasonable to me that if you have a 5000 square foot house, the square footage served in panel 1 plus the square footage in panel 2 should be 5000. There may be no way to make this perfect if one circuit is serving multiple floors and common walls of rooms, but I don't tihnk it is really going to matter. If three circuits serve a 150 square foot room, then assign 50 square feet to each for that room. You can have the minium required number of general purpose branch circuits or have 10X as many -- the overall calculated load for them is the same in either case.

 

[kwired]

I don't know exactly how to go about it, and never have had to worry about calculating load (to a precise level anyway) for dwelling unit general purpose circuits. Like Kent had mentioned, if you know the square footage supplied by a particular circuit I guess you could go with that. Circuits that supply lighting only you usually know how much load the lighting is and can assume a worst case of all being on at one time. Receptacle outlets in rooms like living rooms, bedrooms, dens, etc. have such a load diversity I don't really worry about assigning a specific load to each circuit, and seldom have trouble with overloading the branch circuits, feeders or services because of the load on these type of circuits. The SABC's, laundry, and fixed equipment you need to pay more attention to, and if you account for them you are usually fine. Never have had an inspector want to know how much load was on a particular branch circuit in a dwelling either, outside of maybe fixed equipment.

 

[kwired]

I don't fully understand why your inspector is evaluating panel schedules - especially when talking loading for a dwelling unit.

For the general purpose areas of the dwelling all the NEC gives us to go with for service and feeder calculations is 3 VA per square foot. I guess that would mean you will need at least 1 20 amp circuit for every 800 square feet or one 15 amp circuit for every 600 square feet. But beyond that NEC could care less if you put every receptacle on an individual branch circuit, the load calculation is still 3 VA per square foot.

 

[Smart $]

....

Is there a diversity/demand calculation that can be applied to the subpanels as well? Please include code references, links, etc. that I can use. Thanks.

.... Would it be acceptable to simply divide the total VA by the total number of branch circuits and show the same load on each branch circuit? This won't necessarily correlate with the power and lighting plans that I've generated either.

You calculate subpanels the same way as you do the service (though you are actually calculating for service and feeder conductor ampacity). The same loads that go into the service cal' get apportioned among the feeder conductors and subsequently the subpanels. The same demand factoring applies. However, when apportioned, the permitted demand may not be the same as the whole.

For example, a demand factor applies to having four appliances on the service, but if they are apportioned two to each subpanel you cannot use that demand factoring for the feeder. It would be better to apportion all four appliances to only one of the subpanels.

Another example is lighting load demand factor. If all the lighting load is in one panel, only the first 3000VA is subject to 100% demand. However, the up to 6000VA could be subject to 100% demand.

The first question needing asked here is where are the subpanels located. If they are next to each other, it doesn't really make much difference which load ends up in which panel from a "closest to load" perspective. If on the other hand these subpanels are at opposite ends and different floors of the house, it may make a difference. Then it becomes a design vs. NEC requirement enigma.