The part that surprises people
The deduction attaches to the equipment being placed in service, not to how it was paid for. Buying with a loan does not reduce it.
That produces the effect the industry advertises: a deduction on the full price in year one while the cash leaves over several years.
'Placed in service' is the operative phrase. Equipment delivered but not yet in use by year end generally does not qualify for that year, which is why December deliveries need care.
The limits, and why we are not quoting them
There is an annual deduction cap and a spending cap above which the deduction phases out. Both are indexed and change every year.
Any figure written into a lender's web page dates within twelve months, and a stale number in this area is worse than no number. IRS Publication 946 carries the current ones.
There are also rules on business-use percentage, on vehicles above and below certain weights, and on how the deduction interacts with bonus depreciation. None of that is something to take from a funding site.
What to do about it
Talk to your accountant before the purchase rather than after, particularly near a year end. Timing is the part that is still in your control.
Do not let the deduction drive the decision. Buying equipment you do not need to reduce tax leaves you with equipment you do not need and a payment you do.
Keep the invoice, the finance agreement and the in-service date together. It is the documentation, not the intention, that supports the deduction.