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How to calculate total current requirements for a sub-panel?

How to calculate total current requirements for a sub-panel?


Old 12-02-13, 07:20 AM
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How to calculate total current requirements for a sub-panel?

Hi all, New here -

I'm getting ready to add a sub-panel to my original panel to expand a basement. My basic question is; is there a formula that is typically used to calculate how many circuits you can place on that sub-panel? Since everything will never be on at the same time, what allowance factor in amperage is generally acceptable? I'm initially thinking 100-amp.

BTW, this is not your typically basement as I will have a media room with lots of equipment, a vintage audio room (lots of gear), computer equipment room (servers, modem, NAS boxes, etc.), a small electronics workshop, rec room and a mini-split ductless heat-pump. Including more lights and wall sockets than the minimum required. IOW, I'm planning on 4 dedicated 20-amp circuits, and at a minimum, 4 15-amp circuits. Is all that too much to place on a 100-amp subpanel?
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Old 12-02-13, 08:01 AM
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There is no hard and fast rule for the load in a subpanel but what comes closest is a Load Analysis using the rules in the National Electric Code and considering the portion of the home served by the subpanel.

Included are such things as 3 watts per square foot of space, taking 80% of the wattage of intermittent loads such as a washing machine, taking 100% of the wattage of continuous loads including heating and air conditioning. (There's more.) I would consider the video equipment and any vacuum tube audio equipment to be continuous loads since they would be on for hours at a time. But non-tube audio equipment qualifies as intermittent because it draws large wattage only during loud or heavy bass passages.

2. There is a limit to how many breaker handles may be in the subpanel, if you are thinking of using tandem breakers (single wide double breakers). Also a maximum feed amperage for each make and model of panel.

3. My super rough crude formula. The total amperage of the breakers in the panel may be one and one half times the panel or feed amperage. Don't forget that for 100 amps as a standard 120/240 volt feed, you have two allotments of 100 amps of 120 volt usage, for example, given four 15 amp breakers and four 20 amp breakers, you put two 15's and two 20's on each leg for a total of 70 amps per leg that is far below the 100 amp feed you are proposing. Or plenty of headroom (amperes, not ceiling height) for an additional 240 volt breaker for the heat pump if that is 240 volts.

Last edited by AllanJ; 12-02-13 at 08:32 AM.
Old 12-02-13, 11:13 AM
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Join Date: Feb 2012
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Thanks AllanJ for that lucid answer. More questions.... If there are no more open breaker slots in my main panel how would I connect the sub-panel? Normally the sub comes off a breaker in the main - correct? Even if there were slots, what size breaker would I use? 60? 100? I believe #4 copper would be used to the sub-panel too? Can you ever come right off the hot bus-bars in the main to a 100-amp breaker in the sub? If my sub will be located > 30' from the main should I increase wire gauge?
Old 12-02-13, 11:43 AM
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You need to remember that circuits (and panels) are sized based on their anticipated load. So that 20A receptacle circuit could have one 20A device on it (window AC), or may have 20 little-used receptacles.

I don't know much about requirements for a mini-split, but the rest of it you could probably be fine with a 60A 120/240v subpanel. But to be sure, add up the amperage of each device and map it out per circuit and for the panel.

You'll size the breaker in the main panel to the size of the wire. I'd recommend using a 100, 125 or 150A main-lug-only panel in your basement with double the number of slots you think you need. (You can use one with a main disconnect, but it's not required). You'll notice, you can use a 60A breaker and corresponding wire connected to a 100A panel... the 100A panel will just give you more available slots than a 60A panel.
Old 12-02-13, 03:02 PM
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Maybe I'm not communicating clearly.. How do I connect the wiring to the sub to the main panel if I only have one slot open? Don't I need a 2-pole/ganged breaker to bring in 110v to each bus-bar of the sub? I've never wired in sub-panel before, but I know what results I want. Maybe I'm unclear on the correct path. I do want everything safe and to local code. I think I do want a breaker at the sub to easily disconnect my entire basement circuit when needed. I'm planning on putting the sub-panel in an equipment room at the other end of the basement.

When it comes to the load, I'll always error on the conservative side. I need and want the expansion capability of the sub-panel to allow for more breakers if needed in the future. I'll always double the slots I think I need! I have a very good idea of where and why I want all receptacles, switches, double-gang boxes, can lights, etc. I'm tired of the every 8' minimum rule and pretty much loath extension cords. Especially since I can never find one around my house! I now have a chance to 'over' wire my basement space and that's what I intend to do.
Old 12-02-13, 04:03 PM
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Yes you need to connect to a 2 pole breaker for your sub. You may need to make provisions to run one of the existing circuits off of your sub in order to accommodate the 2 pole breaker.

NEC may vary, but CEC says each receptacle counts as 1A and you can put 12 on a 15A circuit. I agree with you on overkilling, so go nuts with your receptacles.

You have the luxury of knowing what equipment you will be running. Check the power requirements and plan accordingly.

Lights are easy to figure out. Once you have your lights picked out, tally up the wattage of the bulbs and divide it by 120V. This will be the current drawn.
Ex: you add 10 new pot lights with 75W bulbs in them.
10 x 75W = 750W
750W / 120V = 6.25A

On the topic of overkilling, have you considered a patch panel for your data and adding a buttload of jacks?
Old 12-02-13, 06:05 PM
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Join Date: Feb 2012
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I was hoping I wouldn't have to move a circuit from the main panel to the sub, just to accommodate the necessary feeder breaker. That would involve a lot of extra cabling unless I can find an isolated circuit close by where I'm adding the sub. The fridge comes to mind, but that's an extra load and possibly could add noise on circuits I plan to you for audio only equipment. We'll see.

The patch panel is already planned, and I do intend to add, as you say, 'a buttload' of jacks everywhere I think I will need them. From network port, to speaker cabling, to HDMI ports, etc.
Old 12-02-13, 06:22 PM
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NEC may vary, but CEC says each receptacle counts as 1A and you can put 12 on a 15A circuit
Yep--- that's one difference between NEC & CEC .
Actually it's not limited to 12 receptacles but rather 12 devices --- which could include a mix of lights and recpetacles. There is some talk that may be changed to 10.
Unlike in the US --- no limit

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