Load center, power distribution

[Julio Navarro]

Thanks Rob, all good points

[Rob Russell]

I have a closet built around the main panels ...

If you decide to build a closet around your panel, be sure that the space is large enough and meets the NEC minimums. Remember also that this is a required "working space", so the closet (or space in front of any panelboard) needs to be kept clear.

One thing I have not seen addressed is how do you decide the max capacity of the sub-panel?

If, by "max capacity" of a panelboard, you mean the maximum number of amps that can be distributed by the panelboard, that's part of the manufacturer's label and specifications. In terms of the circuit breakers you put in the panelboard, those can add up to more than the rated capacity. The breaker that feeds the subpanel can't be more than the panelboard's rated capacity.

As an example, let's say you were installing a 100 amp subpanel and put in a 40-slot panelboard rated at 125 amps, fed by a 100 amp breaker in your main panelboard. Assume the 40 slots have (5) 15-amp/240v breakers, (5) 20-amp/240v breakers and (10) 20-amp 120v breakers evenly balanced across the 2 hot legs. If you were running ALL of those circuits at 50% of rated load, you'd be attempting to draw 137.5 amps and the 100 amp breaker feeding the subpanel would trip.

The reality of the amperage draw in a really serious home workshop is that you might draw a simultaneous max of:

• 25 amps @ 240v for a 5 HP tablesaw
• 25 amps @ 240v for a 5 HP dust collector
• 25 amps @ 240v for a 5 HP compressor that happened to cycle on
• 20 amps @ 120v in lighting to make your shop as bright as day (running at 80% amperage, that's 4 dozen 4' flourescent tubes)
• 20 amps @ 120v for your kick-butt shop stereo to make you as deaf as a post

All of the above is only 95 amps of current draw and wouldn't trip your 100 amp breaker.

[Tom Jones III]

My question is how do you decide if you need a 100 amp subpanel or a 150 amp subpanel or 60 amp subpanel? Is it as simple as adding up the watts from every thing you can imagine running and making certain you've got a few amps above that? Maybe calculating your max expected load then multiplying by 2?

I've got the panel for the house in my detached garage, so when I wired my garage to be my workshop, I simply used the empty slots in the panel. Now I'm thinking about making a dedicated sub-panel for the shop and placing it next to the main panel so that I can have more circuits.

[Rob Russell]

My question is how do you decide if you need a 100 amp subpanel or a 150 amp subpanel or 60 amp subpanel? Is it as simple as adding up the watts from every thing you can imagine running and making certain you've got a few amps above that?

For a subpanel to feed a workshop, that's pretty much it. For a house, there are a lot of additional rules, but the minimum service requirements still don't come up to be all that large. So, add up your possible concurrent loads and give yourself some headroom. Assume that you don't want to load things up past 80%. There is an NEC rule that says continuous use loads are supposed to draw no more than 80% of a circuit's capacity. Continuous is defined as 3 hours or more, but there is really little in our residential shops that would approach that. So, using the 80% rule is just a nice conservative way of loading your circuits.

Since you're putting the subpanel next to your main service panel, you're not going to spend huge amounts of money on conductors. That's a little different than the subpanels I ran for our addition which were 80 feet or so from our main panel. Anyway, the smallest panel I'd put in is a 100 amp panel with 20 slots. That allows for (4) 240v machinery circuits and (4) 120v general purpose receptacle circuits and leaves 8 slots open for expansion. Run any additional lighting out of your main service panel.

Even if you only think you'll run 40-50 amps worth of stuff at a time, the relative cost of putting in the larger subpanel just isn't that great and gives you flexibility in the future. If you add up your loads and find that you could be regularly drawing close to 100 amps, I'd go with a larger panel. If that turns out to be the case, there's a chance you'll have to go to an electrical supply house to get the breaker. The local HD near us doesn't carry individual breakers larger than 100 amps.

That's my story and I'm sticking to it!

[Bill Lewis]

If you decide to build a closet around your panel, be sure that the space is large enough and meets the NEC minimums. Remember also that this is a required "working space", so the closet (or space in front of any panelboard) needs to be kept clear.

Well, I did keep 36" clear in front of the panels, in part becasue I know that that is was the minimum space required, but it also served to box in the water main shutoff as well.
OTOH, I have seen "closets" that are nothing more than a door on the wall that has the panels directly behind it. My neighbors house is this way. This makes sense regarding the code, because once the door is open, you immediately meet the clearance requirement.

[Kent Fitzgerald]

According to the product manual, that HF motor is 230 V, single phase.