Wiring a subpanel

[Prashun Patel]

I'm thinking to add a subpanel to service my basement shop.
What's a good size for the subp? 50A? 100A?

All subpanel wiring I've seen uses a doublepole breaker to feed from the main to the sub. As an elec engineer by training, I'm embarrassed to admit that I can't understand what benefit DPST's offer over SPST's... As long as the wiring and bus can support the current, why do you need 2 contact points?

Shawn.

[Rod Sheridan]

Hi Shawn, the double pole breaker is required because home single phase services are an Edison circuit.

The transformer secondary feeding your home has a center tap, connected to ground. This is your neutral.

The transformer has two 120 volt feeds (the ends of the transformer secondary) which are 180 degrees apart.

Thus you have 120 volts from each line to neutral, and 240 volts line to line.

Obviously for the 240 volts you need a breaker in each line, so a 2 pole unit is required.

Regards, Rod.

P.S. If you don't have anything large in your shop, a 50 or 60 ampere feed would be plenty.
I have a 1.5HP cyclone, and my shop machinery is 3 HP, so I run the shop from a 30A feeder. Note that lighting in the shop isn't on the 30A feeder. If it was 40A would be fine for my shop.

The approach to use is look at your usage, probably a cyclone and one machine at a time plus some lighting. Calculate that out and see what you arrive at.

[Scott T Smith]

Shawn, the DPST allows you to bring in 240 to your panel, and if there is a short both legs of power will trip.

If you used two SPST breakers, if a short occured on one leg and tripped the breaker you'd still have live power on the other leg - causeing a potential safety issue.

Most small sub panels are rated for 125A, and if it were me at a minimum I'd size my wire for 125A and feed it with a 100A breaker, in order to provide excess capacity for start up currents. 100A of power would provide a lot of lattitude for anything that you may wish to add in the future. As an example, when I start my 3 hp dust collector, the start up current surge is 60A for around 3 seconds.

If you only have 60A available, and have lights, a shop fridge, stereo, cordless tool chargers plugged in, a compressor running, etc, and turn on a dust collector and a table saw at the same time you will certainly exceed 60A, especially if you have a lot of 120V equipment and have not balanced the load across both legs of your incoming power.

You didn't mention how many pieces of equipment and what hp sizes that you'll have in your shop, so it's difficult to be more specific, but when it comes to subpanels I'd rather have more capacity than not enough.

Regards,

Scott

[Rod Sheridan]

Breaker time/current curves take start up currents into account.

If a motor draws 12 amperes full load, a 15 ampere breaker will handle motor inrush current and full load current.

Regards, Rod.

[Mike Henderson]

I have a small shop and put in a 50 amp subpanel. Never had a problem.

Mike

[John Coloccia]

I would bring in 100A. It's easy to run the wire now than rip it out and rerun it later. If you ever add a 5HP dust collection with another 5HP tool (shaper, TS etc), a 50A feed is getting marginal.

Also, if you ever want to run a welder, or something like that, you're going to want a larger panel as well.

FWIW, I have a 100A panel in my shop and have no regrets.

[Prashun Patel]

Thanks all. By the time I'm ready for 5HP, I'll be moving out of my basement Fo Sho!

I didn't account for the dual phases. Of course. Thank goodness my township allows homeowners to do their own elec work. Like w/ all my elec work, I'm getting a permit.

Curious: how many of you get permits on stuff like this?

[Rob Russell]

I wouldn't do the work without a permit and having it inspected.

One of the downsides to doing the work without a permit is that, when you sell the house, if the buyer's house inspector discovers that the subpanel was put in without a permit, you could be required to remove the entire subpanel and wiring assembly or bring it up to the then current code.

In terms of a subpanel size, 60A is plenty. Would you be running conduit or NM-Cable for the wiring to supply the subpanel?

[Rob Russell]

I would bring in 100A. It's easy to run the wire now than rip it out and rerun it later. If you ever add a 5HP dust collection with another 5HP tool (shaper, TS etc), a 50A feed is getting marginal.

Also, if you ever want to run a welder, or something like that, you're going to want a larger panel as well.

FWIW, I have a 100A panel in my shop and have no regrets.

I used to feel the same way. Now, given the cost of wiring to support a 100A panelboard, I'd run a 50A or 60A subpanel unless I really knew that the continuous, concurrent loads would require the larger subpanel.

[Rod Sheridan]

I agree with Rob, always pull a permit, I do.

- it's required by law in my area

- if I ever have a fire that's caused by an electrical failure, that inspection is the difference between my fire insurance paying or not.

When I installed my panel, it was $48 for inspection, including the starter and push button station for the cyclone, and all the devices connected to the panel.

It was $25 to have my dishwasher wiring inspected.

Regards, Rod.

[Chris Padilla]

I didn't account for the dual phases.

Shawn,

It is still single-phase feeding your house...not dual (or 2-phase). Sometimes, it is better to think of it as a "split-phase" system or even a "3-wire single phase" system.

[Prashun Patel]

My main panel is already in my shop. The subpanel will be literally next to the main. I assumed I could just run the feeder NM out/in the tops of the respective boxes. The feeder's only exposed above the 'reach line' and where it can suffer damage, so I didn't think I needed to run it thru conduit.

I'm planning on running 110v/20A throughout and 220/20A to the bigboy locations.

What's yr opinion of overhead outlets?
Any other lessons learned/design advice from yr own shops?
Any advice for shop lighting?

[Prashun Patel]

Sorry, Chris, I meant split; mixing terms. The reason appears to be quite simple:

Your house only gets 110v service. But you get 2 lines, so to realize 220v, you gotta gang them in series. Combining the lines with SP breakers might leave 1 energized in the event of a fault; DP's make the lines trip together.

Codes notwithstanding, I COULD use a 50A SPST breaker to feed the subpanel, but that subpanel would only ever be able to deliver 110v to anything it feeds.

[Chris Padilla]

Codes notwithstanding, I COULD use a 50A SPST breaker to feed the subpanel, but that subpanel would only ever be able to deliver 110v to anything it feeds.

I suppose you could but that would be an imbalanced system and overall poor design, me thinks. :)

Plus, you are then stuck with only being able to use 120 V (no 240 V available) and potentially only half of your open breaker positions...unless you pigtail over.

Bad, bad, bad.... :)

[Chris Friesen]

I used to feel the same way. Now, given the cost of wiring to support a 100A panelboard, I'd run a 50A or 60A subpanel unless I really knew that the continuous, concurrent loads would require the larger subpanel.

Around here a home panel kit (with a 100A panel with a main breaker and a bunch of circuit breakers) was far cheaper than a smaller panel with main lugs.

There's nothing that says you need to feed a 100A panel with a 100A circuit. Mine just has 40A coming to it currently--the previous owner had left the cable in the wall, and I didn't think it was worth pulling thicker stuff until and unless it becomes necessary.

[Rob Russell]

Putting in a 100A rated panel is fine, but I wouldn't pull the wire for 100A unless you need it.

Shawn,

If you want to run NM, the cheapest is probably SER 2-2-2-4 aluminum. Even though it carries a higher amperage rating than you need, it's typically cheaper than even 6-6-6-4 copper NM.

[Bruce Wrenn]

In my shop, I have a 60 amp sub panel. (I believe 60 amps is the minimum required by the NEC.) I have two table saws, two RAS's, jointer, 15" planer, cyclone, window AC unit, drill press, bench grinder, mig welder, and stick welder, air compressor, along with general purpose lighting circuitts. In a one man shop, how often do you run more than one tool at the time. One power tool and DC maybe. And occassionally the air compressor.

[Rollie Meyers]

Putting in a 100A rated panel is fine, but I wouldn't pull the wire for 100A unless you need it.

Shawn,

If you want to run NM, the cheapest is probably SER 2-2-2-4 aluminum. Even though it carries a higher amperage rating than you need, it's typically cheaper than even 6-6-6-4 copper NM.

#2 aluminum is only good for 90 amperes, it's use for 100 amperes is not allowed for a subpanel and if your jurisdiction has adopted the 2008 NEC #2 SER is only good for 75A. because in the 2008 SER is treated like NM cable and the 60° column is to be used to determine ampacity.

[John Morrison60]

Shawn
I just completed installing a side panel.
I had a 200 amp main panel that was full of circuits.
I installed a 16 circuit side panel that will cover my shop.
(my shop is in the basement)
I moved my existing shop circuits to the new box, and installed two new
circuits for anticipated new "requirements"

While I was playing electrician, I also installed a 40 amp circuit for a garage "charging station".
I figured that with all the talk of electric vehicles, I would beat the rush (and cost)
of that installation.
I used a 60 amp 220 breaker in the main box to feed the side panel.
Conduit was easy to install since I was only going a couple of feet.
I used #6 copper to supply the side panel, and #6 aluminum for the garage circuit.
since the side panel was within 5 feet of the main panel, I did not need a main breaker in the side panel.
I would also suggest that you use the same brand of box as your main panel.
That way you will have interchangeable breakers.

Good Luck.
John

[Rob Russell]

#2 aluminum is only good for 90 amperes, it's use for 100 amperes is not allowed for a subpanel and if your jurisdiction has adopted the 2008 NEC #2 SER is only good for 75A. because in the 2008 SER is treated like NM cable and the 60° column is to be used to determine ampacity.

Rollie,

I wasn't suggesting that he run the 2-2-2-4 aluminum SER for a 100A subpanel, rather that he use for for a 50/60A subpanel. In my area, the 2-2-2-4 aluminum SER is cheaper than the copper NM he'd need to run.

[Rollie Meyers]

Rollie,

I wasn't suggesting that he run the 2-2-2-4 aluminum SER for a 100A subpanel, rather that he use for for a 50/60A subpanel. In my area, the 2-2-2-4 aluminum SER is cheaper than the copper NM he'd need to run.

One would have to be careful w/ that, the breaker lugs may not be able to accept larger size cable.

Siemens QP frame breaker, 40-50 ampere wire range #8 to #4 AWG 60-70A is #8-#2 AWG.
SQ D QO is the same.
Cutler-Hammer type CH 40-50A & 60-70A #14 to #2 AWG.
Cutler-Hammer type BR no information avail.

GE THQL 35-60A #8-#2AWG.

Murray would be the same as Siemens as they both use the same breaker branded differently, but they cannot be used interchangeably.(UL listing issues).

I chose not to list SQ D HomeLine.

As you can see from the list of the most popular residential circuit breakers, some will work, some won't.

[Rob Russell]

OK - good point.

Looking through the list, it appears that using a 60A breaker would allow the use of the 2-2-2-4 aluminum SER for any of the brands listed.

[Prashun Patel]

Now I'm thoroughly confused, though.

Here's what I'd LIKE to do. Can someone advise whether it's permissible?


Install 100A main lug panel (no onboard main breaker).
From the main panel, I'll connect to the sub with a 50A dp breaker.
From the main to the subpanel, I'll use 6-3g copper (which seems to support up to 50A).

I only want to use the 100A panel because it has more slots. However, I don't anticipate exceeding 50A on this panel. My choice of 50A / 6-3g is based on availability locally.

If I ever need to upgrade, I'd just have to swap the 50A breaker for something higher and rewire to the panel with 3 or 4 ga, right?

The subpanel will be about 1 foot from the main panel, if that makes any difference.

[Rob Russell]

#4 copper NM is rated for up to 70A. 70A is a standard size breaker, so you can't upsize to a breaker larger than that and you can't swap in an 80A breaker later.
#2 aluminum is rated for 75A. I think you'll find that the #2 aluminum SER is cheaper than the #4 copper NM. If you use the AL SER, make sure you use some Ox-Gard on the AL conductors to prevent corrosion.

[Tom Veatch]

Rob, if I'm remembering correctly, isn't there some maximum number of breakers you can have in a panel without having a main disconnect? Been a while, but the number "6" hangs in my memory. Something to do with the maximum number of operations necessary to completely kill power from the panel. If so, that could impact the OP's desire to use a 100 amp panel to have more breaker spaces.

[Rob Russell]

I believe the 6-disconnect rule is for service equipment. For example, if you have a multifamily dwelling and multiple services, you need to be able to kill power to all of the services in 6 or less actions ... or something like that. I'd have to go digging into the code to find the specifics.

[John Coloccia]

This is what confuses me. You're only going 1', so the price difference between wire for 100A and 50A is minimal. I had to go about 40 feet, so we used aluminum. IMHO, it's a slam dunk to just run copper wire, sized for 100A. If you really want to use a 50A breaker for some reason, OK. It just seems like this doesn't need to be anywhere near this complex.

"Life is not an optimization problem". I keep that note stickied in my workshop to remind me to stop fussing and get on with it :D

Just my opinion.

[Rob Russell]

John,

Running conductors sized for 100A will add cost and potentially make it more complicated installation.

100A means running either #2 copper NM-cable (rated for 95A, but can round up in this case to 100A) or 1/0 aluminum SER. An alternative is to run #3 copper THHN/THWN in 1 1/4" Schedule 40 PVC conduit. #2 copper NM and 1/0 aluminum SER are both going to be pretty stiff to work with. That's actually one of the reasons I keep suggesting the #2 SER - the aluminum conductors in that will be easier to work with than #4 copper because they bend easier.

Unless Shawn really thinks he's going to need 100A, putting in a 60A subpanel should serve his needs just fine and the smaller conductors will be easier to work with. If something happens in the future such that he really needs 100A of juice in the subpanel, he can replace the breaker and wiring at that point in time.

[Prashun Patel]

"Unless Shawn really thinks he's going to need 100A, putting in a 60A subpanel should serve his needs just fine and the smaller conductors will be easier to work with. If something happens in the future such that he really needs 100A of juice in the subpanel, he can replace the breaker and wiring at that point in time."

The savings (albeit marginal) comes from the smaller breaker and my not having to curse as much trying to work with 2, 3, or 4 ga wire - it's not the price of the wire itself that I'm concerned with. Anyway, like you suggest, I'm on the fence and might just go for 100a for the sake of never having to regret it later.

[Rob Russell]

One thing you could do is install the 1 1/4" PVC conduit and then just pull smaller THHN/THWN. #8 copper THHN/THWN is good for 50A, #6 is good for 65A. The conduit shouldn't be a big deal - (2) 90 degree bends, a piece of straight and (2) connectors for the ends where the conduit hits the panelboard.

That way, if you ever decide to go to a 100A subpanel, all you have to do is pull #3 copper or #1 aluminum conductors through the conduit which would be easier than messing with a cable.

[Prashun Patel]

One thing you could do is install the 1 1/4" PVC conduit and then just pull smaller THHN/THWN. #8 copper THHN/THWN is good for 50A, #6 is good for 65A. The conduit shouldn't be a big deal - (2) 90 degree bends, a piece of straight and (2) connectors for the ends where the conduit hits the panelboard.

That way, if you ever decide to go to a 100A subpanel, all you have to do is pull #3 copper or #1 aluminum conductors through the conduit which would be easier than messing with a cable.


I just mght do this!

[Kendall Landry]

Thanks all. By the time I'm ready for 5HP, I'll be moving out of my basement Fo Sho!

I didn't account for the dual phases. Of course. Thank goodness my township allows homeowners to do their own elec work. Like w/ all my elec work, I'm getting a permit.

Curious: how many of you get permits on stuff like this?

Even if nothing ever happened, I couldn't look at myself in the mirror knowing I didn't get electrical work I'd done inspected. My family lives in that house! Got to remember whats important.
Can't change a toilet here without a permit.
Can't replace more than 25' of privacy fence without a permit.
Inspections are important. Even the best make mistakes.
I have 100A in my shop. Wouldn't have any less.
I build overhead into everything I can. That's how I roll. :D

[John Coloccia]

Can't change a toilet here without a permit.
<snip>
I build overhead into everything I can.

Maybe you went a touch overboard on the toilet if it required a permit :p

::::duck::::

Seriously, all my work is permitted and inspected. It doesn't cost much, and it just takes a few minutes for the inspection if you do everything to code. The last thing you need is a rejected insurance claim because the work was not permitted and not inspected.

[Kendall Landry]

Maybe you went a touch overboard on the toilet if it required a permit :p

::::duck::::

Seriously, all my work is permitted and inspected. It doesn't cost much, and it just takes a few minutes for the inspection if you do everything to code. The last thing you need is a rejected insurance claim because the work was not permitted and not inspected.

hehehe

You've prolly seen some code violations yourself. I've got some pics of some absolutely scarry things poeple have done to their electrical systems! Way beyond violation.:eek::eek::eek:

I've also seen many shop pics with romex exposed all over.Going to receptacles under benches and the like.:eek:

[Prashun Patel]

Is there a way to convert a main breaker type panel to a main lug type?

[Rollie Meyers]

Is there a way to convert a main breaker type panel to a main lug type?

Some are convertible mains but there is no reason to remove the main breaker from the panel, if you use a 100 or 200A main (as long as the lugs are rated to accept the smaller wire) & smaller wire is protected at the source you just have a over rated disconnect switch and there is no safety hazard.

[Mike Lipke]

In my town, I need to pull a permit to even use the terlet.
Then, I have to report its usage to another agency as an EPA reportable.
My wife pulled a permit for the bathroom, saying it was a toxic waste dump.
In spite of all this, the neighbors call 2-3 times a month to report violations of the noxious emissions statute here.