I need to drill 3/8" diameter holes, at a 45 degree angle, into my poured foundation basement walls to inject polyurethane, in order to prevent shrinkage cracks from leaking ground water. There is rebar in the walls, but I do not know to what extent. The drill bit needs to go 5" into the wall at that diagonal angle. What is the lowest amperage needed for a job like this?

I am looking at around 80 holes.

  • 4
    Disclaimer: Not an expert. At a hole diameter of only 3/8", I'd imagine that most any rotary hammer would do the job. A "lighter duty" one may require clearing chips more frequently and take a bit longer, but should most certainly do the job. Considering that they make battery powered ones that'll drill through concrete just fine, I'd imagine that anything corded (and not made of super cheap Chinesium) will do the job with no worries.
    – FreeMan
    Commented Sep 29, 2021 at 11:55
  • 4
    Horrible Fright Chinesium may be good enough if you do the project within their whopping 30 day (or whatever it actually is) warranty and it's not too far to the store to return it if it breaks. But a rental will be more reliable.
    – Ecnerwal
    Commented Sep 29, 2021 at 12:02
  • I'd go with the rental if you want a decent SDS. Mine cost £650, 15 years ago; still rockin' strong. Pointless getting a cheap one, just rent one.
    – Tetsujin
    Commented Oct 1, 2021 at 18:55

3 Answers 3


Any true concrete drill like this 4.2A Makita will work. Any drill with a chuck that takes an sds-plus bit should work. 3/8" holes are not particularly a big deal.

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Some "hammer drills" have a regular drill chuck that you can use a masonry bit, but you would likely be just as well off using your forehead. enter image description here


0 Amperes. Grab a star drill and a hand sledge and your safety glasses and ear muffs and have at it.

Now, if you'd like to do it faster, you might want more. If you are mainly doing this one project, then see what your rental center has for a hammer drill to rent, and use that, and return it when done. Other than that option, any hammer drill will do it, and most non-hammer drills will also do it, slower.

  • 1
    Wow! you're being cruel today. He's got a number of holes and you're gonna make him use a star drill. Glad I don't work for you.. :-) (unless at Gunflint Lodge)
    – JACK
    Commented Sep 29, 2021 at 12:24
  • He really needs a hammer drill. Drills without the hammer function are very slow to drill concrete. Commented Sep 29, 2021 at 14:25
  • 1
    Minimum amperage was requested, and number of holes or speed was not specified. The rental hammer-drill is probably the best answer, but the star drill is minimum amperage...and I've drilled plenty of holes in concrete both ways. For a few holes, slow with the drill you have is faster than a trip to the store to buy or rent a hammer drill and buy a bit for it.
    – Ecnerwal
    Commented Sep 29, 2021 at 14:57

It's not about amperage...

Hammer drills are garbage. In a hammer drill, the whole chuck moves back and forth, and that imparts some motion the the drill bit. Most of the power is wasted moving the chuck (which is heavy) back and forth, and very little power ends up doing useful work.

So instead you get a rotary hammer with a SDS chuck. This has a pneumatic hammer hitting the back end of the drill bit, so it is like hitting a chisel with a hammer. Most of the energy goes into the useful job of drilling the hole. It's much faster.

How fast a rotary hammer will drill depends on the impact energy (in Joules) which is mentioned in the product description. 2 Joules is fine for light work (up to 15mm diameter) and also not expensive. For big holes like 20mm or more, get more joules, but the tool will be heavy and more expensive.

With 2-3J tool and a 3/8" (10mm) bit in concrete you can expect about 15-20 seconds for your 5 inch hole. Maybe 30 seconds if the drill is a bit anemic. You don't need to buy the drill, you can rent it. Rebar is not a problem with these, you will feel a resistance when it hits, but it'll chew through it.

These kind of tools use less than a kilowatt, so that'll give you the amps, if you're worried about that.

If you've never used one of these:

  • Wear eye and ear protection

  • Always both hands on the drill with a firm grip! the bit will seize in the hole at some point and the drill will rotate. It has a clutch, which will do its job, so you will be able to hold it with two hands, but if you hold it with one hand, you might fall off the ladder or hurt your wrist.

  • To drill at a 45° angle in concrete, first drill a few mm at right angle to make a starting hole so the bit doesn't slip along the wall, then slowly angle the drill to the desired angle.

  • Yes, I planned on a rotary with a SDS chuck. Forget the rental as it is an excuse to buy a new tool. A masonry bit will get destroyed by the rebar won't it? It is way to early to try and convert Joules to Amps...
    – Evil Elf
    Commented Sep 30, 2021 at 11:26
  • 1
    lol, that's legit ;) so if you want to buy it, look at the joules and forget the amps. SDS masonry bit has tungsten carbide tip, it's made to go through rebar concrete.
    – bobflux
    Commented Sep 30, 2021 at 11:27
  • You might get through one piece of rebar with a drill bit, but you won’t get through many. If you hit rebar and absolutely have to get through, there are specialty bits available. (Most people opt to move the hole a bit.) And as a minor point, rotary hammers don’t use a pneumatic action: it’s a hammer/anvil affair. Commented Sep 30, 2021 at 12:25
  • Mine has pneumatic, but who cares, what matters is that the tool works!
    – bobflux
    Commented Sep 30, 2021 at 14:47

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