Ask Me Help Desk - Sub Panel Wiring

Ok... I am wanting to run a 60 amp subpanel in my attached garage. My main breaker panel, located in my basement is 200amp. I am planning on having a Siemens 60amp 2-Pole Breaker in my main panel (Basement)- then run #6 THHN Cooper Wire to my subpanel in 1-1/4" Conduit. Will be running 2 Hots (Black/Red), 1 Neutral (White)and Ground Wire. I will be using A 125 main lug panel, and put a "Main 60amp 2-Pole Breaker" into two slots (Panel has 12 slots total). I will wire the hots to the breaker, which will feed the hot bus bar- and act as a main breaker for the rest of the breakers I add later. Some of my questions. In my plastic conduit,can I and do I use a bare solid copper ground, what size....6 or 4? Or can I run ground wire that is covered/insulated #6? The subpanel will be approx 35feet away from main. The reason I am hooking up a "Main Breaker" in the subpanel,is to be able to shut off power to the subpanel without going downstairs. Any insite will help.

Greeting and welcome to this forum.

Your plan in general is a reasonable plan, however there are things you need to be aware of. The very first thing to do is to submit your plan to your Local Authority Having Jurisdiction and to obtain an electrical permit.

1) #6 THHN copper conductors are used in two ways. A) the 60 degree column. B) the 75 degree or column "B" of table 310.15 in the NEC.

Why the difference? Although the THHN is also listed in the "C" column, the answer lies in the equipment you are connecting to.

If either the main panelboard or the panelboard you select for the garage are listed at 60 degrees, then you must use the data from Column "A", which is 55 amps. However, if both panelboards are listed at 75 degrees, you can use the information in column "B", which is 60 amps.

The column "C" (90 degree) information of 65 amps, can only be used if both panelboards are listed for 90 degrees. Normally this is not found in a residential environment.

2) Be careful about your choice of PVC. First you must use either Schedule 40 or Schedule 80 electrical PVC.

3) Schedule 40, trade size 1.00" will hold six (6) THHN conductors. Trade size 1.25" will hold 11 THHN conductors.

Schedule 80, trade size 1.00" will hold 5 THHN conductors and trade size 1.25" will hold 9 THHN conductors.

4) If you are going to use a Main Lug Only panelboard in the garage, then you should install a disconnect switch in front of the garage panel board. The 60 amp flow from the main panelboard will be protected by the circuit breaker in the main panelboard.

5) According to table 250.122, a # 6 AWG ground would be fine. The # 6 AWG (Copper) is listed for 60 amps. A #4 (Copper) ground is listed for 100 amps.

It is critical that Neutral and Ground be isolated from each other inside the garage panelboard. That also means that Neutral and Ground conductors cannot be attached to the same bus bars.

Thanks for the reply, this I can tell will be a great site to discuss electric, projects, etc. A little about myself, a home owner, works in the fire service, understands the improtance of building codes, and doing "the right thing" when completing projects. I donot cut corners, which I appreciate your insight on code referral. I do submit all my work to my local authority. Just got done wiring a 50amp hot tub, sitting on a finished paver patio. That was a new experience with all the bonding requirements which have changed locally... but needless to say, all inspections have gone fine. Anyway, will look at the fine print on the sub panel I will be purchasing. Do you have an opinion on Copper vs. Aluminum (I assume- silver) hot bus bar in new panels. Is one better for my application? I am just looking to add a 220 outlet off the subpanel for my 60 gallon air compressor, and a few additional outlets in my garage. Just want the abililty to expand a little in the future if needed. All the equipment, including the main panel is fairly new... new home construction. I will be using electrical rated PVC conduit. Also, I researched a cutoff box for the subpanel, would it be OK just to backfeed a 60amp breaker in the panel, and label it as the main? Thanks for the insight so far.

Unless you have a very long distance (cost) I would recommend Copper. Copper is a better conductor and it handles the heat generated by electrons moving through the media (current) much better.

Current Aluminum, I believe it is called Series 8000 is much better the older solid conductor Aluminum from the late 50s to early 70s.

I would install a main disconnect switch in front of the panelboard rather than put the cut-off breaker inside the panelboard. Again, the external disconnect is an immediate shutdown of the panelboard. A breaker will do the same thing, but it will take up two slots that you may need later.

Don, can you provide a text book that states this explanation:?

Quote:

Originally Posted by donf

Copper is a better conductor and it handles the heat generated by electrons moving through the media (current) much better.

Copper has a lower resistance than aluminum when comparing same size wire.

Upsizing aluminum one size usually will then equal the copper for ohms per foot.

Installing a circuit breaker in two slots of branch circuit breakers and backfeeding to use as a Main breaker is fine, and allowed by Code as long as a retaining kit is used to secure this breaker into the panel, and labelling this breaker as "MAIN". Then a disconnect ahead for this panel is not needed.

Simple answer, the lower the resistance, the lower the heat caused by collisions of the electrons passing through the conductor.