Subpanel breaker sizing in detached shed with electrical feed from breaker in main house panel

Question

I want to feed 240V 30A from a new 30A breaker in my home main electric panel to a new subpanel in a adetached shed. The electric feed I want to use for the new subpanel is (4) 10awg THWN wires (R/B/W/G) to be fed from a new 240V 30A circuit breaker in my home main load center, and then through 60' of conduit across my basement and underground to the new shed subpanel. The subpanel would have (2) 120V 20A breakers for light and outlet circuits - as well the capacity for a 240V 20A future breaker for a compressor outlet. I will separate neutrals and grounds in the subpanel, and will ground the shed subpanel and shed circuitry devices with a new ground rod at the shed.
I have (4) questions :

1. Will a new 8-slot, 125A Square D 125A subpanel with a 125A main breaker that I want to use for a subpanel and disconnect in the shed satisfy NEC code requirements ?
   (I want 2nd opinion on early advice saying SuI'll have to swap out 125A main for 30A main)
   And if so, can you please point me to the applicable NEC code section(s) ?

2. Will I need two ground rods at the shed ? or just one ?
   And if so can you point me to the applicable NEC section(s) ?

3. Wiring in the shed will be with (3) 12awg THHN wires (B/W/G) in 1/2" EMT. IF I run the three wires for power from a 20A breaker in the subpanel to a light switch near a cornner shed door, can I run the switched wires part way back to the light in the same EMT conduit ? And if so, the applicable code section(s) ?

4. Any other advice anyone can offer on this plan to provide electricity in my shed ?

Thanks for any help.

Answer 1

Will a new 8-slot,

You seem very wrapped up chapter and verse of complying with NEC. You're missing the point. We are not serfs to electricity. We are the masters of this technology. Electricity works for us. Bare compliance is for slumlords. Make electricity wonderful!

#1 mistake is getting the smallest panel that will serve immediate needs. This creates a nightmare later when you want to add more stuff. More spaces are a trivial amount of money, so while we certainly are interested in saving you money, chintzing out on panel spaces is not the place to do that. Go BIG or go home. Bye bye 8-space. Hello 30-space.

And if you want to save some money, avoid "QO". Eaton CH is just as good and just as compact at 2/3 the price for the breakers.

125A Square D 125A subpanel with a 125A main breaker that I want to use for a subpanel and disconnect in the shed satisfy NEC code requirements ? (I want 2nd opinion on early advice saying I'll have to swap out 125A main for 30A main)
And if so, can you please point me to the applicable NEC code section(s) ?

Yeah, people get sucked into this mistake all the time. They get told to use a main-breaker panel as an outbuilding subpanel, but don't understand why they're doing that, and they make too much out of the main breaker, like "shouldn't that match or something?"

You do not need any main breaker at all in the subpanel. (NEC nothin'.nothin').

The only purpose of using a main-breaker panel is to provide a "main disconnect" as required in outbuildings per (NEC 225.31).

225.31 Disconnecting Means. Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.

However... if you plan to carry the feeder some distance within the building before it gets to the subpanel, you will need a separate disconnect near the entry point. If you do need that separate disconnect, then this satisfies 225.31 fully, and you do not need a main breaker at all at the panel.

225.32 Location. The disconnecting means shall be installed either inside or outside of the building or structure served or where the conductors pass through the building or structure. The disconnecting means shall be at a readily accessible location nearest the point of entrance of the conductors.

Needless to say, saying you need to downgrade to a 30A breaker is utterly laughable advice, the person is just "making stuff up".

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Will I need two ground rods at the shed ? or just one ?
And if so can you point me to the applicable NEC section(s) ?

Two needed unless you can get free services from an electrician.

250.53 Grounding Electrode System Installation.
(2) Supplemental Electrode Required. A single rod, pipe, or plate electrode shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8).

You can evade this requirement if your electrode tests out as having good conductivity to earth; however the cost of doing this test is a great deal more than driving a second ground rod. Other code calls out a minimum 6' distance, but more is better.

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Wiring in the shed will be with (3) 12awg THHN wires (B/W/G) in 1/2" EMT. IF I run the three wires for power from a 20A breaker in the subpanel to a light switch near a cornner shed door, can I run the switched wires part way back to the light in the same EMT conduit ? And if so, the applicable code section(s) ?

You can't use green wire!

250.119 Identification of Equipment Grounding Conductors. Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for hot or neutral circuit conductors.

I would suggest "red". Green is reserved for safety ground, but you don't need to wire that if it's EMT pipe.

358.60 Grounding. Grounding and bonding EMT shall be installed in accordance with 358.60(A) and (B).
(A) EMT shall be permitted as an equipment grounding conductor

As far as how many wires / can you do that switch, again can't prove a negative. Certainly any number of wires or circuits are allowed in conduit, if you're willing to pay the derate in 310.15(B)(3)(a)...

However all related conductors must be together in the same conduit.

300.3 Conductors.
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the neutral and all safety grounds shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord...

Answer 2

Lots of questions (supposed to limit to 1 I think I hit most of them) All of your receptacles in the shed will require GFCI protection. Because the GFCI’s only come in full size I would go with a larger panel, get at least a 20 full sized breaker panel. Yes you can use GFCI outlets and the lights of hard wired do not require GFCI protection.

Go big in some cases you may find the panel cost is no different or maybe 10$ for more than double the circuits. My best example is a customer that became a friend started with a 60 amp spa panel I told him go bigger , within a short time he wander more ampacity than that little panel could give with the #10 wire. He then jumped to the 100 amp 8/16 but only put 55 amp direct bury in again I told him to go bigger. , now at year 3 he wants a welder and the 240v air compressor to run. Time for conduit the ground was not two bad from the prior 2 trenches. Put in a 20/40 or 24/28 150 amp panel in and fed it with #1 copper 125 in the main panel, we were talking a few weeks later and he found my original quote and mentioned he now realized why he did not have a big aluminum boat and I did. Because he paid me 3 times to do what I suggested the first time.

Put in at least 1” conduit if not 1.5” trenching cost,,, conduit is cheap. Get a bigger panel if you compare costs the bigger panel is not much more and you won’t run out of space. Saving $ on wire at the start is a good idea with conduit.

Ground rods 250.53. I have only been asked 1 time to measure a single ground on a second location it passed, the grounds at the home create the secondary rod. But ground rods are cheap & inspectors can be jerks to home owners they know you won’t have an earth resistance meter, just drive a second rod.

You mentioned EMT it is legal in the new code for direct burial but I would suggest thinking about PVC , yes you need to run a separate ground conductor but counting on thin wall for a ground is not the greatest in my opinion , PVC is cheaper and easier even pulling a separate ground.

Can you run a set of wires to create a 3way ? From the house to the shop. Yes this is allowed and the number of hot conductors would require derating but you can use the 90 degree table for derating and this won’t affect the wire size until you have 7 or more current varying conductors so you will be fine.

The main in the shed is there to protect the panel as long as it is larger than the feeder you are good. The 30 amp breaker with #10 supplying your panel via conduit is a smart way to go as long as you over size your conduit.