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Thread: Load Calculations for Workshop Sub-Panel

  1. #1

    Default Load Calculations for Workshop Sub-Panel

    I have a detached 350 square foot workshop about 70 feet from my main service panel. I've buried 3-#4 Type USE-2 for 240v and 1-#6 USE-2 for ground. I've also driven a ground rod near the workshop's sub-panel and tied that into the ground-bar. The neutral and ground are not bonded in the subpanel.

    I've been using this configuration for a couple of years, with double 60a breaker on each end to protect the feed. The service to my home is 200 amps, so I think I have plenty of room there.

    I plan to add a hot tub, in the backyard, about 30 feet away from the workshop. It requires a 60a GFCI and breaker.

    I currently have a 3kw electric heater in the workshop, which runs less than 3 hours at a time, a 2.5 hp table saw (seldom used), 320w of flourescent lights, and a shop vac, as well as some low power items in the workshop. The max current on the hot tub will be 46 amps. It has about 5hp worth of motors in it, in addition to a 5.5kw heater. It will continuously pull 1/2 amp for the circulation pump. It is conceivable that the heater (12.5a) , table saw (13a), lights (1.5a), shopvac (11a) and hottub (46a) would all be at full tilt at the same time (84a).

    1) Would it be okay for me to upgrade my 60a breakers, currently protecting the workshop's subpanel feed, to 90a or 100a? The #4 USE-2 has an ampacity of 95 amps, according to NEC 310-16. I think the 2008 NEC 310.15(B)(6) code changed, so that sub-panels cannot be protected with the next higher amp breaker.

    2) Would I be overloading this 90amp circuit, is it borderline, or should I just run the hottub wiring directly to my main panel?

    Any help, advice, or opinions are all appreciated.


  2. #2
    Join Date
    Dec 2003
    Kent, WA


    You can't do the calculations in amps unless everything is 240V. It is best to convert to VA and then divide the total (after demand factors) by 240V to do the service load amp calculation. You may get to apply a demand factor for multiple appliances (depends on which calculation you're using), but some things need to be treated as continuous loads and shouldn't really have a demand in front of them (like the heaters, probably the tub, maybe the shop vac if you let it run continuously as a dust collector).

    You do have a problem in getting much more power out of your existing wires. Direct bury USE-2 is usually aluminum and not copper. So that is the first thing to verify. Second, you can't use its 90C ampacity because none of your terminations are rated for 90C. You can go 75C in most cases. So #4 at 75C is good for 85A if copper and 65A if aluminum.

    Finally, you may not need much more power, but I don't know the operating voltage of your equipment. Obviously the lights are 120V, and if the shop vac, table saw, heater, etc are all 120V you may have more power than you realize (there are two separate 120V legs in your 60A feeder rated at 60A each). But a 46A tub takes a lot of what you have.
    Kent, WA

  3. #3


    Thanks for the reply.

    You are correct. I have mistakenly mixed my 240 and 120 loads together in my calculations. See if this makes sense:
    Heater (240v) 3kw 12.5a
    Hottub (240v) 11kva 46a
    Lights (120v) 320va 1.33a (va/240)
    Saw (120v) 1560va 6.5a (va/240)
    Vac (120v) 1320va 5.5a (va/240)

    Here are my calculations:
    a)Loads that may run at same time 70.63a
    b)25% of largest motor not sure what to put
    c) 125% of largest motor 0
    d)Total of a+b+c 70.63a +
    e)Other loads 1a
    f)Total of d+e 71.63 +

    100% d) 70.63a
    50% e) .5a

    Minimum breaker & wire size: 71.13a

    Not sure what to put for 25% of largest motor. The hottub has three motors, two of them are 2.5hp. I'm not even sure if they are 120v or 240v, since I don't have the tub yet.

    It is most definitely copper wire. So I'm assuming I could safely move to an 80 amp breaker, but not a 90?

    It's not highly likely that I'll be in the shop, with the heat on, sawing with the vacuum on while the tub is running full blast, but you never know. I usually shut the vacuum off, due to the noise, when the saw isn't in use.

    If it looks too close to call, I can always run it to my main panel instead, it's just a little more of a hassle. If my numbers look right, though, then I think I can swing this on an 80a breaker. Please let me know what you think.

    Thanks again! I appreciate the feedback!

  4. #4
    Join Date
    Dec 2003
    Kent, WA


    If this is copper wire, you have 85A ampacity. You are allowed to round up to a 90A breaker as long as the load calculates to 85A or less. So your call as to whether you want an 80A or 90A breaker (80A would be safer).

    The largest motor is tough to call. It may be included in the hot tub nameplate if that is where you got the 46A. Otherwise, its your saw. And for this amp value, you have to use the applied voltage (120V). So 25% of 13A is 3.25A, and this gets added to you amp total. You don't do another 25% increase (125% is the same as adding 25% of the largest motor). The only thing that is 125% is continuous loads (defined as full load for 3 hours or more), and you typically don't have many those in residential calculations.

    So I think your load calculation is around 75 amps. An 80A or 90A breaker should work just fine.
    Kent, WA

  5. #5


    I think I will stick with the 80 amp breaker. I'd rather err on the safe side.

    Worst case, and the 80 amp breaker trips. If that happens, this particular spa can be re-configured, with jumpers to function at 40, 50 or 60 amp, which corresponds to 25a, 35a, or 46a total loads, according to the specs. With those jumpers in place, the spa onboard logic tells the spa not to run the heater when the jet pumps are running.

    Thanks for taking the time to help me with this. Your expertise is greatly appreciated.

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