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Adding a subpanel - load calculations

Discussion in 'North America' started by GlmnAlyAirCar, Jan 3, 2017.

  1. GlmnAlyAirCar

    GlmnAlyAirCar Member

    Joined:
    Mar 19, 2015
    Messages:
    726
    Location:
    Gilbertsville, PA
    I recently decided it's time to update my current setup in the garage. I currently have a NEMA 14-50R installed, wired directly to my main panel in the basement, into which I plug my UMC. Additionally, my garage has a single 120V outlet and two single light bulbs on the ceiling. Since these are both inadequate, I figured I would add a subpanel and run a dedicated circuit or two to supply additional outlets and lighting. So, while I'm at it, I might as well add a HPWC. I only have a single charger in my car, so 50A would be enough. However, if I am going to go to the trouble of adding a subpanel, I might as well run a big enough circuit to allow me to run an HPWC at 80A if I ever add another charger.

    I have read FlasherZ's FAQs and other posts about adding the subpanel. I plan to do the work myself but have it fully permitted and inspected.

    Before getting started, I thought I better do some load calculations to make sure my current 200A service is up to the task. I am using one of the online calculators (http://www.nojolt.com/load_calculations.shtml) but am a little unsure of a couple of things.

    1. How are "lighting" and "small appliance" branch circuits defined? If I understand correctly, the lighting load is accounted for by the square footage of the dwelling. So, these circuits shouldn't be added to the total. However, "small appliance" circuits should. The problem is that my house's wiring has, shall we say, evolved over the years and there are many circuits that drive both overhead lighting and individual outlets in the same room. So, technically, nearly all of my circuits not accounted for elsewhere could be considered "small appliance" circuits. How should these be counted?

    2. Under my current calculations, with my UMC at 40A, the load calculator says I need 245A service. As I mentioned before, I have 200A service. Everything is working fine. I have never had any voltage sags, my UMC never drops back to 30A no matter what else is running, and I have never tripped a breaker. From this, I assume my load is OK and I am doing my calculations incorrectly. Can anyone offer some tips on doing load calculations on an existing installation?

    I am starting to get familiar with the NFPA 70 but it is no small task to digest this thing.

    Any help would be appreciated!
     
  2. Craig-Tx

    Craig-Tx Member

    Joined:
    Aug 1, 2016
    Messages:
    58
    Location:
    Fort Worth, TX
    DISCLAIMER: I AM NOT A LICENSED ELECTRICIAN.

    I went through this recently when I added a HPWC and a NEMA 14-50. Here is my understanding.
    1: I believe that lighting and general wall outlets are all counted by square footage. I believe small appliance circuits are dedicated circuits for locations such as garage, laundry, bathrooms. I don't believe they include general outlets. I could be wrong though.

    2: There are two load calculations described by the NEC. One for calculating the needed service size for a new service. The second for adding a new load to an existing service. IIRC NEC 220.83 is the one you want to look at. I can't explain the full difference, but it was substantial.

    3: It sounds like you will be running a new feed from the Basement to the garage. Will you be running conduit with THHN cables, or are you planning on running NM-B (Romex type) cables. The reason I ask, is that NM-B must be rated at a 60 degree ampacity capability. With this a 4 AWG cable can only support at most a 70 AMP circuit. I doubt you will find anything larger than 4/3 NM-B cable. If you want to go higher than 70 A, you will likely have to run conduit. If you're already planning conduit with THHN, you should be set.

    If you haven't found it yet, the NEC is available for free on nfpa.org. You will have to create a free account, but then you can see the full NEC (several versions in fact)

    DISCLAIMER: I AM NOT A LICENSED ELECTRICIAN
     
    • Helpful x 2
  3. GlmnAlyAirCar

    GlmnAlyAirCar Member

    Joined:
    Mar 19, 2015
    Messages:
    726
    Location:
    Gilbertsville, PA
    Thanks for the response. I do have an account on nfpa but haven't dug into the details yet. Thank you for pointing me to the appropriate sections.

    I am planning to run conduit. It's a little pain up-front but makes future runs _much_ easier.
     

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