Space requirements for first subpanel and wiring preference

Question

I would like to install two subpanels and my main panel is full of breakers. Would it be better to feed both subpanels directly from main or have 1st subpanel off main and 2nd subpanel off 1st subpanel?

1st subpanel will hold the breakers that are moved out of main to make room for subpanel breakers and any future electrical expansion. The 2nd subpanel will feed four 20 amp duplex receptacles about 17 feet away. A E-mon Class 1000 meter will be installed on the feed to 2nd subpanel.

Looking at the picture can I put both subpanels side-by-side along the green line using 2005 NEC? The subpanels are 14" wide.

I plan for 1st subpanel to have a 100 amp breaker and 2nd panel 80 amp breaker. Current plans do not exceed 40 amps to either.

Answer 1

The NEC requires 30" of space for any Panel, but it does come with a lot of exceptions. One is door swings you must be able to the door 90 degrees to access the breakers. Seconfd it requires 30" but the panel does not have to be center of the space. Third panels can share the 30" space.

Also there are requirements for clearances. See attached.

Good luck

Answer 2

First, the space you plan is just fine for this use. Wise to mount a 3/4" sheet of plywood (even better: something nonflammable) to the joists, simply to give them freedom to mount the panels, conduits and attachments as you please, instead of being forced to line it up to joists.

Most panels are 14.5" wide because joists are on 16" centers and 2x4's are 1.5" wide. This does not include the cover, which will overhang too much to put two panels in adjacent joist bays. I don't like in-joist mounting anyway, as it wastes the dozens of side knockouts and forces you to cram everything in the top or bottom. Ugly.

The subpanels will come with neutral buses. You will need separate ground buses - think about that when buying panels, some cheapie panels sell you the ground buses separately - costs more in the end, worst of both worlds! If that main panel is a "CH", that's a fine type, you can reuse your breakers.

First Subpanel

As I mentioned, don't be bashful about making the first subpanel absolutely huge in number of spaces. 42-space is not excessive. The problem is right now, buying 10 more spaces will cost you maybe $30. Later, 10 more spaces will cost you a world of labor and finding a location for another panel. Wildly oversizing the first panel means not ever needing a second. As a bonus, larger panels come with more "bonus breakers".

Do not worry about the subpanel "amp rating" It needs to be greater or equal to the breaker it's fed from, but it's totally fine to feed a 200A subpanel from a 100A breaker!

Another trick you can do with "subpanel #1" is groom it to be your future main panel. In that case you'd buy a subpanel with a main breaker. It's OK for the subpanel's "main breaker" to be larger than the breaker it's fed from.

Your subpanels do not need their own main breaker -- they are in the same building as the panel they're fed from.

I would run nice, big, fat, 1-1/2" or 2" EMT metal conduit between the main and subpanel #1, probably from the bottom. For two reasons - ease of pulling and room for expansion. Use "conduit bodies" or prefab bends to round corners. The metal is the ground which means you only need to pull 3 wires :)

I would install #1 AWG aluminum which is good for 100A. Given the very short distance, I wouldn't object to the cost of #3 copper, but the lugs you'll be attaching to will be aluminum. Why create a dissimilar-metal problem when you don't have to?

Second subpanel

I gather this power is for a server cluster or something that will draw most of its load continuously.

You want four 20A@120V circuits. Most people wire their subpanels as 2-pole 240V/120V. That would allow you to wire the four circuits evenly on two poles, so it would be 40A @ 240V.

Wiring the subpanel as 120V only @ 80 amps is not a great plan. It will create 80A of imbalanced load, which will create high stresses in everything in your electrical system including the power company's feed. Note the thing ThreePhaseEel says about downsizing neutrals - it breaks that. You will need twice as many spaces in your panel, as every other row will be dead. And your breakers will be more expensive.

Since you see this as a very limited use panel, I'm ok with at least a 12-space panel so you have some breathing room. As discussed, if you wire it as a 240V panel balanced, a 40A feed will suffice.

This can be wired with #8/3 AWG copper wire, but given the short distance, run #6/3 and you'll be able to bump it to 60A later. I would just use cable rather than conduit in this case, since you have an intermediate device to attach to. If running conduit through that makes sense, then sure, do EMT and let it be the ground :) But 3/4" conduit will limit you to #6 wire.

Subpanel #2 off main or subpanel #1?

I'm perfectly OK with feeding subpanel #2 off of subpanel #1. However, this will add 40 (-80?) amps of demand to subpanel #2 and let's look at breaker prices for a moment.

• 40A breaker - $9
• 60A breaker - $9
• 100A breaker - $45
• 125A breaker - $85 and 4 spaces
• 150A breaker - $135 and 4 spaces

Yikes. Feeding off subpanel #1 will raise the cost of its breaker quite a lot, because it has to passthru an extra 40A guaranteed, so its breaker will be $80 more. (now were you just mentally arguing with me about a 42-space panel being a needless expense? ;-)

I prefer to stay in "cheap breaker land" and feed sub #2 with a $9 breaker and sub #1 with a $9-45 breaker. (do you really need 100A if they are separately fed)? I would still run the #1Al/#3Cu wire so you are 100A-ready.

Answer 3

Would it be better to feed both subpanels directly from main or have 1st subpanel off main and 2nd subpanel off 1st subpanel?

You should feed them separate not daisy-chained.

That way the second panel's load is not on the feeder to the first panel.

Also, follow RME and Harper's advice for working space and breaker capacity. It will never be cheaper than the initial installation.

BTW: plastic on basement walls will just trap moisture and promote mold.

Good luck!

Answer 4

Use two feeders, one per subpanel

Using separate feeders from the main to each subpanel has the advantage here that you can basically provision two smaller (100A and 40A) feeders instead of having to run a monster (140) feeder from a panel -- not all residential panels even have 150A breakers available for them!

However, you can run them together

Using a wide-sweep 1.5" elbow and some 1.5" conduit (at minimum -- if your wireway of choice has an end plate that accepts 2" conduit, by all means use that) from the main panel, you can fit 2 1/0AWG aluminum hots for the expansion panel (103*2 = 206mm2 fill) + 2 8AWG copper hots for the metered panel (24*2 = 48mm2 fill) + 1 1/0AWG aluminum neutral (103mm2 fill) for both feeders in, which leaves a bit of room left over if needed (526mm2 permissible vs 357mm2 used). This relies on NEC 215.4 authorizes sharing the neutral between two feeders in the same raceway/enclosure, and also on the unbalance across both panels being limited to 100A, which is reasonable enough.

The elbow then proceeds to exit into a wireway (this is like a conduit, only in the form of a trough with a lid instead of pipe) that is close-nippled to the two subpanels and to the power meter. This wireway can be a 4" square unit (smallest you can use for a 1/0 wire bent 90° as per 376.23(A) and table 312.6(A)) although I'd use a 6" square wireway to provide expansion and routing room, with 1.25" nipples to each subpanel and a 3/4" nipple to the power meter. This provides a home for the tap splices for the meter's voltage wiring (you'll be running 3x 14AWG for hot/hot/neutral to the meter) as well as the current sensors clamped around the feeder hots to the metered panel. Also, don't double-tap the lugs on either end of the metered feeder for this -- it's better to splice in the wireway instead, which is allowed by 376.56(A) and the tap rules in 210.19(A)(4) Exception 1. (Granted -- it's an odd tap because it's tapping branch conductors off a feeder, but the E-mon Series 1000 manual provides for this configuration, and you're talking with your AHJ about this whole setup up front anyway right?)

If you are not going to use a feeder tap (say, because you're running separate raceways for the two feeders), then a two-pole, 15A breaker in the subpanel being metered would suffice. Locking the breaker on is highly recommended here, though, to prevent the meter from being inadvertently shut off while the fed loads are still on. This also has the downside that you're including the meter's own powered consumption in the metered load, too.

Last but not least -- the supplemental fuse specifications in the E-mon series 1000 manual are a bit of an oddity, as 100mA is rather small for branch circuit fuses, and using fuses intended for supplemental protection for protecting a tap conductor is a violation of 240.10. This means that we must use 100mA, time-delay, type CC fuses (Littelfuse KLDR.100 or equivalent) in a matching type CC deadfront fuse holder (Littelfuse LPSC002ID or equivalent) mounted in a cutout box, such as a Wiegmann A080404 or equivalent, that is nippled to the wireway, to protect the meter tap. (What makes cutout boxes special is that they have a hinged front to allow access to the inside for changing blown fuses and such without the use of tools.)

If you are using the breaker-in-subpanel approach to this, this gets around the tap conductor issues, but poses a practical issue with fuse replacement. Having to take a panelboard deadfront off to change fuses isn't the most practical thing on the planet, so this means you will still need the aforementioned cutout box in this configuration.

Putting the panels next to each other in the space given is O.K.

The 30" wide clear working spaces for adjacent pieces of equipment are allowed to overlap and be off-center, so there is ample room in the 58" wide space given for 2 standard 14" wide panels. Just make sure both cabinet doors can open at least 90 degrees and you're set.

Go big or go home!

One last point -- get as many panelboard slots, especially for the expansion panel, as you can now because going back in later to fix short-sighted penny-pinching is much costlier than spending a few extra dollars to get more slots now. I'd use a 54 or 60 space (not circuit, the circuit count will be higher, but is useless) panel in your preferred make/model for the expansion panel if you can get one (main lug is fine since the feeder breaker's right around the corner anyway), or a 42 space panel if 54 or 60 space units are absolutely unavailable where you're at. The metered panel can be smaller if you must make it smaller, but I'd go no smaller than a 24 space 100A unit. Don't worry if the busbar ampacity of the panel is larger than your feeder -- you can always put a higher amperage panel on a small feeder/breaker, just not the other way around. (It's akin to using fat wire on a small breaker to control voltage drop on a long run.)

TORQUE ALL CONNECTIONS TO SPEC

The 2017 NEC added a requirement in 110.14(D) that all connections to equipment labeled with a tightening torque must be torqued to specification with a torque-indicating tool (torque screwdriver or torque wrench), just like the head bolts on your car are torqued to specification (only with less torque needed). This is important, especially with aluminum wire which isn't quite as forgiving of mistorqued lugs as copper is, to keep the connection from failing down the road, either from being too loose or too tight.