Electrical – How to determine the correct capacity for a sub-panel for a shop

electricalsubpanelwiringworkshop

I plan to install a sub-panel for my woodworking shop, and would like to determine the total capacity that should supply that sub-panel. These are the main items that will likely be running in the shop, possibly simultaneously:

  • table saw, 240v, 14 amp
  • dust collector, 120v, 9 amp
  • freezer, 14 cu ft (I don't have the amperage)

(The lights are run on their own circuit.)

What is an appropriate capacity for the sub-panel?

Best Answer

It's all about Volt Amperes.

NEC 2008 gives us an easy way to do things in residential.

220.82 Dwelling Unit.

(A) Feeder and Service Load. This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater. It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in Part III of this article. The calculated load shall be the result of adding the loads from 220.82(B) and (C). Feeder and service-entrance conductors whose calculated load is determined by this optional calculation shall be permitted to have the neutral load determined by 220.61.

(B) General Loads. The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:

(1) 33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit. The calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

(2) 1500 volt-amperes for each 2-wire, 20-ampere small appliance branch circuit and each laundry branch circuit covered in 210.11(C)(1) and (C)(2).

(3) The nameplate rating of the following:

a. All appliances that are fastened in place, permanently connected, or located to be on a specific circuit

b. Ranges, wall-mounted ovens, counter-mounted cooking units

c. Clothes dryers that are not connected to the laundry branch circuit specified in item (2) d. Water heaters

(4) The nameplate ampere or kVA rating of all permanently connected motors not included in item (3).

So we can use 220.82 (B)(2) to figure for the dust collection, freezer, and an additional circuit for receptacles.

1500VA * 3 = 4500VA / 120V = 37.5 Amperes

You'll then have to use the values from the nameplate on the table saw to figure for that (A Volt-Ampere value should be listed on the nameplate, use that number for more accurate calculations). You could also use this method for the dust collection system and freezer since they are both "permanently connected, or located to be on a specific circuit".

3360VA / 240V = 14 Amperes

Now we'll add them up.

37.5A + 14A = 51.5A

So This is what our subpanel will look like.

  • 60A double pole breaker in the main panel.
  • 6 AWG feeder cable for a run up to 75 ft., 4 AWG feeder cable for a run up to 150 ft.
  • 60A main breaker in the subpanel.
  • 20A double pole breaker for table saw.
  • 20A single pole breaker for dust collector.
  • 20A single pole breaker for freezer.
  • 20A single pole breaker for convenience receptacles.

Notes:

Don't forget to balance your loads between the two legs in the subpanel.

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