Workshop Planning #2: Planning a subpanel installation

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Blog entry by joshtp posted 01-05-2017 01:11 PM 1529 reads 0 times favorited 8 comments Add to Favorites Watch
« Part 1: What a Mess Part 2 of Workshop Planning series no next part

My attached garage currently has two duplex receptacles in one corner, and they’re on the same 20A circuit as many other receptacles in the house. Thankfully the 20A lets me get by with a vacuum and table saw at the same time, and the lights are on a separate circuit so the lights won’t dim. However there is no doubt I need to upgrade the electrical, especially as I hope to upgrade to a cabinet saw this year.

Here’s how I think power consumption will go assuming I do the upgrades I hope to do in the next few years:
  • HF Dust Collector: 110V, 15A draw, 20A circuit
  • Cabinet Saw, 3HP: 220V, 15A draw, 20A circuit
  • General purpose: 110V, two 20A circuit
  • Electric Heater: 220V, ~20A, 30A circuit

I think the highest concurrent draw I can expect is DC, heater, and saw, which would be 50A, so I could probably get away with a 60A panel. I don’t think I necessarily need dedicated circuits for DC, saw, and heater, but if I did then I would need at least 7 spaces in a panel, with one space having a tandem breaker.

I think the best spot for the panel would be in the back wall of the garage, midway between the middle window and the side wall, allowing room for a workbench below the window, installing the panel just to the left of the rack in this picture:

I currently have a 200A Eaton Cutler-Hammer Type CH main breaker panel with 30-space/30-circuit capacity:

As you can see, every space is filled, so I can’t just add a breaker for the subpanel, and this panel doesn’t accept tandem breakers. My options are to replace the entire panel which is fairly costly and time consuming, or I can add a small subpanel next to the main panel:

I can then remove one of the 2-pole breakers from the main panel, add a 100A breaker in it’s place that feeds this subpanel, which itself has two 2-pole breakers, one to replace the breaker that was removed and one to feed the garage subpanel.

To provide 60A service, I can either use four #6 THHN wires, but that comes to $3.56/ft. 2-2-4-6 Aluminum Mobile Home Feeder (MHF) can support up to 90A and is only $1.79/ft, and would allow me to expand service if that was ever needed.

To run the cable I’ll have to go through conduit. The main panel is in the back wall of the walkout basement. The desired location for the garage subpanel would also be on the backside of the house. The easiest way to run the wire is via Schedule 40 PVC conduit. If I use MHF cable, minimum size is 1.5” but 2” will be easier to pull. I’ll also need two elbows, and at the end will need type LB conduit bodies to go through the wall. Here’s the approximate conduit run I’ve planned:

So here’s an outline of the installation procedure:
  • Install subpanel next to current main panel
    • Install Eaton 125 Amp 12-Space/Circuit Type CH Main Lug Load Center (CH12L125BP, $43.02) which comes with cover
    • Move 30A 2-pole water heater breaker from main panel to subpanel
    • Install 90A 2-pole breaker in main panel (CH290 $45.51) to feed subpanel
    • Install 60A 2-pole breaker for garage sub-panel (CH260 $15.37)
    • Run conduit between main panel and subpanel (scrap conduit, two term adapters $2.36, locknuts, $2.10)
    • Run spare 2-2-4-6 Al MHF from main to subpanel (~5ft MHF ~$10)
  • Install garage subpanel
    • Install Eaton 125 Amp 8-Space Surface Type CH Main Lug Load Center (CH8L125SP, $28.65) on back wall of garage
    • Install grounding bar in panel (GBK10CS, $3.81)
  • Connect subpanels
    • Install ~25ft 2-in Sch 40 PVC conduit (three 10ft $21.33, two elbows $6.60, two type LB conduit body $18.52) (1.5” is min for MHF, 2” would be easier to pull) on back exterior of house
    • Connect conduit to panels via male terminal adapters (two of E943JRR, $2.36) and locknuts (one of 96196, $2.05)
    • Run through conduit 2-2-4-6 AL MHF ($45) and connect to both subpanels

For a total cost of about $240 plus tax.

The only thing I’m not clear on is how to connect the conduit from the exterior to the panel. The exterior conduit should end in a Type LB conduit body I believe, so should this go straight through the back of the subpanels? The garage subpanel will be surface mount, so do I bridge the ~4” gap from sheathing to panel with a short section of conduit? For the main house panel it will be flush mount, so there would be little to no gap from sheathing to panel, so is no conduit needed here?

I plan to stop by either the county inspector or a local electrical supply to see if they can give me any input. But of course I’d welcome input from all of you!

8 comments so far

View Notw's profile


742 posts in 2359 days

#1 posted 01-05-2017 03:42 PM

My concern, and maybe i missed it in your write up, but how do you plan on getting power to your new subpanel with your house panel being full? Typically your subpanel is fed from breakers in your main panel. Do you have breakers at the meter outside?

View HerbC's profile


1801 posts in 3465 days

#2 posted 01-05-2017 04:05 PM

You will need to use conduit and / or threaded nipples to connect the LBs to the panels. You cannot just butt the LB body up to a hole in the panel body.

Notw, OP has already explained that he will add a small panel connected to the main panel which will allow him to move one existing circuit out of the main panel and to add a new circuit for the garage subpanel.

Good Luck and …

Be Careful!


-- Herb, Florida - Here's why I close most messages with "Be Careful!"

View joshtp's profile


35 posts in 1484 days

#3 posted 01-05-2017 05:52 PM

HerbC, so ideally I’d want to use a knockout in the back of the panel and have a short section of conduit go from the LB body to a terminal adapter with locknut inside the panel. Just means some careful measuring I suppose.

I also plan to fully map out the electrical in the house, as perhaps I’ll get very lucky and could combine four circuits to two and free up two spaces, eliminating the need for the secondary subpanel.

View dday's profile


172 posts in 2036 days

#4 posted 01-05-2017 06:56 PM

I always wondered.. doing work like this yourself.. are you able to? I guess it depends on the state..
and you should do it to code, in case you sell later, right?

I ask because I have a garage shop that has everything (plugs and lights) on the same circuit.. and I need to fix it..

View dday's profile


172 posts in 2036 days

#5 posted 01-05-2017 06:57 PM


View magaoitin's profile


249 posts in 1555 days

#6 posted 01-06-2017 12:35 AM

Joshtp, Outstanding idea and I wish you the best. This is all fairly simple stuff for a wood butcher (like myself :)) to tackle.

As HerbC said, no need to use the LB between the Main and the Sub. LB’s are used to help you pull wire (and you are going to need it if you are pulling a Service wire through 30’ of conduit and (2) 90°’s. That is not an easy pull. Also it is a no no to “makeup” a connection inside an LB. Splices are usually not allowed in an LB (at least in my State, or the NEC for that matter…)

In your post you said that your Main doesn’t accept a tandem (double pole) breaker, but your write up says you are going to use the 30A double pole from your water heater for the new 90A. How it your water heater wired if not on a tandem breaker? To two different breakers?

I am sure you have done your research, but a word of caution with MHF. Most jurisdictions where I live (Washington State) wont allow this to feed a subpanel. We have to use a SE (Service Entrance) Wire, SEU or SER. I am not saying it wont work, just if you are having this inspected make sure you have crossed the T’s and dotted the I’s. Would hate to install something to then have to rip it out.

I am a little confused with your note that the 220v heater or a 220v saw would use a single pole breaker as well To get 220v you have to use both sides of a 110v main feeder, and this can only be done with a double pole breaker as far as I know.

The same goes for a lot of cabinet saws, they are typically not single pole breakers. A contractor style saw is a single pole 110v, but most cabinet saws jump up to a 3 or 4 wire 220v-240v plug and receptacle.

Please keep us updated. I love reading these kind of details in a shop upgrade/build.

dday, you do have to check with your City/county code or the AHJ (Authority Having Jurisdiction) since every one is different.

In my shop build, the City didn’t inspect or govern my electrical install, but it was my power company with a State Electrical inspector. Also where I live, if you are the Owner/Occupier you can do all the work yourself, but have to sign a document stating that you reside at the location the work is being done (prior to pulling a permit), and sometimes have to sign a statement that you will stay living in the house for a minimum of 2 years after the inspection. Both my Gas line installation and electrical had me sign the 2 year statement, since I was DIYing them both.

This is to keep house flippers and landlords from doing dangerous work where they would be putting someone else’s life in jeopardy. But it is completely acceptable to put your own life (and your family’s) at risk.

-- Jeff ~ Tacoma Wa.

View joshtp's profile


35 posts in 1484 days

#7 posted 01-06-2017 11:39 AM

dday, from what I understand there are very few localities that forbid homeowners from getting a permit to do their own electrical work. Sometimes they make it difficult by requiring a knowledge test before issuing a permit, but most just require you to sign a form acknowledging the risks, and sometimes they require you to sign a form that you will live in the house for some number of years. I don’t think it’s often required by law to get a permit at all, but then you supposedly have the risk of an insurance claim being denied if it’s found that unpermitted work caused the damage. As for selling your house, unless the work screams amateur, I’ve been told by real estate agents that no one ever asks to see the permits.
And I certainly think anyone on this site can do this work themselves if they read up a bit and get others to check their plan and their work. There are dangers, but they’re well known and there are straightforward, commonsense ways to be safe.

View joshtp's profile


35 posts in 1484 days

#8 posted 01-06-2017 01:17 PM

Magaoitin, I’ll only use an LB on the exterior where the cable enters the house.

My water heater is on a 30A 2-pole (220V) breaker that takes up two spaces. The meaning I understand for tandem is a 15A/15A or 20A/20A 1-pole (110V) breaker that takes up only one space. This would allow me to replace four regular breakers with two tandem breakers, which would free up a spot for another 2-pole 2-space breaker. I just got an email from my box manufacturer, and apparently I can use tandems, even though it’s not marked on the box. I’d have to check with the county if they’re ok with that however.

I had gone with MHF because the cost was half of copper. However I just looked up 2-2-2-4 Al SER and it is $1.69/ft, so slightly cheaper than the 2-2-4-6 Al MHF which is $1.79/ft. I believe both will allow up to 90A in my situation. If I really wanted the option for 100A I’d have to jump to 1-1-1-3 or 1/0-1/0-1/0-2 Al SER which would jump to $2.75/ft at big box prices. I’ll just go with the 2-2-2-4 Al SER.

Maybe I didn’t communicate that right, but I do understand the 220V saw and heater need 2-pole breakers, and have used that in my planning.

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