Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

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?

share|improve this question

2 Answers 2

This is not an answer related to code but simply doing the layman's math.

First off the Freezer should be on a dedicated circuit. Not because of any excessive amperage draw, but because if you fire up the table saw and the dust collector at the same time and it pops the breaker, you don't want the freezer being shut off for potentially an undetermined period of time, ruining whatever's inside.

If you figure 20A breaker for the table saw (another dedicated circuit would be wise), 15 for the dust collector and 15 for the freezer, plus another 15 for assorted outlets, you're looking at a 100A sub panel to allow for the 65A you're needing, plus to allow room for expansion (required by code in CA, not sure about US but you generally don't want to max the panel out from the beginning).

Which raises the question - what kind of amperage does your MAIN panel carry - you're adding a 100A sub, which means even if your house panel is 200A it may not be enough.

---- addendum ----

Please note - I'm adding up breakers in this answer as a quick sanity check.

I am not trying to say that if you add up all the breaker limits on a panel you need that much incoming coverage.

It is really about what your peak load will be at any given time - but with a 14A saw, 9A fan and we don't know what A freezer will take but the initial cycle start can take up to 10-12A - that means you'll be looking at periodic peaks (when all three are running) of 36A. 50A would be plenty for this - but I'm assuming you're going to install other outlets as well.

share|improve this answer
    
A 100A subpanel is fine, even if the main panel is only 100A. Overdrawing at any point beyond the main panel will simply trip the main breaker.... –  Matthew Sep 6 '12 at 15:09
    
Even if the total house draw is going to exceed 100A when he's out in the workshop working? –  The Evil Greebo Sep 6 '12 at 15:10
1  
There isn't a safety or regulatory issue. He'll just trip his main breaker.... same as if I draw the maximum from all the household circuits... a 100A main panel could certainly have 10 20A breakers in it. Try to draw them all to full load and you'll just trip the main. –  Matthew Sep 6 '12 at 15:11
1  
Regularly tripping the breaker is not "fine". It may not be unsafe, but it's not fine. If you're going to regularly need that much power (and with a dust vac, table saw and freezer they're not going to run individually) then the right thing to do is upgrade the panel to meet the need. –  The Evil Greebo Sep 6 '12 at 15:15
    
We completely agree. If he regularly intends to draw even close to his main panel rating then he should upgrade it. –  Matthew Sep 6 '12 at 15:17

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 feeder cable for a run less than 75 ft., #4 feeder cable for a run less than 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.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.