Portable Quad box

[Gary G™]

Someone broke the ground off the portable quad box used around the shop.
My BIL originally built it using some random cord with 12 gauge strand.
Since the molded plug has a broken ground and it’s a little short, I’m replacing the wire.

I have some leftover 10 gauge Romex from when I built and wired the shop.
Is there any harm in using 10 gauge for the cord for a 20 amp quad box?
Sure, it’s overkill but it’s mine, here and ain’t doing anything anyway.

Typical use is running 2 15 amp tools such as Planer and ShopVac. Or, vac and router.

[MstrCarpenter]

Your Romex has solid copper conductors. the cord cap will have to have a straight in clamp type connection (vs. wrap around a screw) and it that style for 120v, 20a that the 10 ga. will fit will be the more expensive (and yes, better) ones at your local big box store. If you're going to be moving it around and/or wrapping it up , I would suggest S.O. cord and save the romex for a future machine connection.

[TDKPE]

NM isn't made for flexing and can be relatively easily damaged.  I wouldn't use it as an extension cord.  Use SE, SEO, SJEO, SO, or SJO* Hard Service cord (the "J" is for "Junior", with 300V rating instead of 600V, and thinner jacketing).  Even the BORGs sell SO and SJO by the foot.  Or just replace the plug.

*One "O" means the outer jacket is oil-resistant, two "O"'s means the actual insulation is also oil resistant, though that's overkill for a wood shop extension or tool cord.

[Gary G™]

Y’a convinced me not to be a cheap bastage.

[AnHonestOpinion]

Y’a convinced me not to be a cheap bastage.

If the cord itself is not damaged why not just cut the molded head off and replace with a new cord cap.  The cord cap can be purchased in either straight or right angle configurations.  They also make replacement cord caps with built-in GFCI protection.

Good luck