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Answers to Electrical Questions About Conduit and Wire Size for Sub-Panels and 240 Volt T-Stat Rating

January 10, 2011

DIY Electrical Wiring Help Doug asks:
I am running power to my detached workshop and need some advice.
Here’s some detail: Installing 90amp dbl breaker in main house panel for power-up at workshop. Distance: 100 ft. Plan to use #3 Cu stranded x 3 runs for power and neutral/ #8 solid Cu for ground from main panel to ground bar in sub panel. Wires to be buried 18? in conduit. Have a Seimens 125amp external sub panel with 8 possible circuits. Will use 4 total circuits: 2 for tool power @ 20 amp; 1 for workshop lighting@ 20 amp; one for loft and external security lights @ 20 amps.
My issue is this: I can run feeders from the main panel in conduit to my crawlspace: should I run conduit through my crawlspace as well or can I secure the wires to the floor joists? What would you do? Crawlspace is secure. Also: how high should the bottom of the sub panel be from the floor of the workshop? It will be INSIDE the workshop – I got an external box mainly for protection of the circuits and lockability to prevent curious fingers from being hurt. Do I have a workable plan? Suggestions are always welcome! Thanks for the help!

I would try to run conduit all the way; from the main panel to your sub-panel. However, this can sometimes be very challenging through wood framing. If you can’t run the conduit all the way, then you need to install a junction box to change over to NM (romex) or SER cable. Then you will need another junction box where you change back to conduit.

From what you have described, your plan looks good. If you install gas, phone, internet or CATV from your house to your workshop, then you need to install ground rods at your workshop.

Walter asks:
How many baseboard heaters (240 Volts) can you install on one wall thermostat?

A wall mounted t-stat is rated for 22 amps. However, it is rare that you will place this much load on one t-stat. Typically, there is one t-stat per room with only a couple of heaters controlled by it in a residential application.

D Stewart asks:
Need to know the size and type of wire needed to run less than 75 feet from main service (200 amps) to 100 amp panel in house—–under ground.

I’m going to assume that the 100 amp panel is a sub-panel. If you install direct burial cable, then you need #1 URD aluminum with a #6 USE aluminum ground wire. You will need to sleeve the wires in 1 1/4″ PVC schedule 80 from the main service box to 18″ below grade and from the sub-panel to 18″ below grade.

If you use conduit, then you need #3 THHN copper wire with a #8 THHN copper ground wire. Install this in 1 1/4″ PVC schedule 40 underground and schedule 80 where exposed to physical damage as indicated in the paragraph above.

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4 Responses to “Answers to Electrical Questions About Conduit and Wire Size for Sub-Panels and 240 Volt T-Stat Rating”

  1. Bob Raley on January 15th, 2011 9:38 am

    I too am getting ready to run power from my house to my work shop, a 36 x 36 metal building, I’ll need to run the wire just under 200 feet from the panel coming into my house out to the shop. I want to get 240 out there, and my question is, what guage and type of wire would you recommend I use. It will run in conduit under ground out to the shop. I’ll have a panel at the shop with a shut off in between the panel and the house at the shop. Thanks in advance for your guidance.

  2. chuck lapierre on July 9th, 2011 4:45 am

    I would like to know the code article that specifies what size over current protection to have on a snap switch used as a disconnect for an ac unit. Currently there is a 20A switch protected by a 30A breaker.

  3. steve on January 30th, 2012 6:40 am

    what do i do to wire a metal building

  4. Corey on March 8th, 2012 9:43 pm

    I need to run a 100 feet from a 30amp Bracker what size wire do i need ?