First off let me say: DAMN, HOW BIG A WELDER YOU GOT? Anyway, now that that is out of the way, let me tell you a few things about me. I am presently not a licensed electrician, but in the past I held both a journeyman’s and a Master electrician’s license in a very large metropolitan city. A career change found me letting these licenses lapse. My knowledge of the National Electrical Code (NEC) is a little out of date, but it is sound. Although some things I have done in my own and friends shop might not follow the exact letter of the NEC as now written, I have never done anything I worried about, or in my opinion would jeopardize anyone’s safety.
That being said, I have questions:
1. Where your garage service is fed from, the primary dwelling, I presume. Does the panel there have bottom or “flow through” lugs at the bottom of the buss to accept wire lugs? Most residential panels don’t, but it would be cool if it did.
2. Do you have a space for a 100 amp breaker in this box, possibly replacing the 40 amp, two pole you have now?
3. Is the garage separated from the house, or attached? If it is detached, the NEC now says it must be treated as a separate structure for the matters of grounding. (a good thing)
4. You need to get yourself a book. “Ugly’s Electrical References” Distributed by Burleson Distributing Co, 3501 Oak Forest Drive, Houston, Texas 77018. Your local electrical distributor might have one. Great electrical book for laymen.
5. Now the important stuff: your 1” PVC conduit will hold two # 2 or 3 #3 THW or THHN (THHN recommended) wire. This will give you a rated 100 amp or 115 amp capabilities. (all NEC ratings are based on THW) using the #2 will require you setting a new ground rod at the shop location for Neutrals and grounds (do it)
6. My final suggestion would be to install a new 100 amp panel with about 20 or 24 spaces, feed your welder and compressor from it, and still have 16 single spaces for everything else.
7. Keep in mind that the NEC allows for de-rating of conductor size and branch circuit breaker size for welders, based on duty cycle, but the nameplate amps should take priority over that. Unless you have a real Hog, #8 wire should run it. (55 amps)
8. I don’t like “piggy back” or tandem breakers. (but I don’t like fat chicks, either, but some guys are alright with them. It’s a personal thing, I guess).
9. If you even consider using anything other than copper wire, I will personally send my dog, Bubba to bite you.
10. Remember, electrical outlets are never as cheap as they are during construction of a new structure. Put in plenty.
11. Make certain all of your connections are tight. Loose connections build heat, which is not good.
12. If you have any concerns about anything I have written here, ask an electrician locally. Shop fires that burn down your wife’s house can be hard to explain to her. After all, I might be a crackpot, or just stayed at a Holliday Inn Express last night.
13. I hope I have been of some help. Gotta go now, my fingers are starting to cramp. (just the two I type with).