A transfer between two accounts at the same bank can be instant, delayed, capped or refused. The reasons come from different systems, and they are worth separating.
Savings accounts were built for a different purpose
Deposit accounts are categorized by regulators according to how they function. Transaction accounts are designed for payments, savings accounts for holding balances.
That distinction historically carried limits on certain kinds of withdrawals and transfers from savings accounts, enforced by the institution rather than chosen by it.
Regulatory treatment in this area has changed, and some banks retained limits as policy after the requirement changed. Current terms are set by each institution's account agreement.
Internal and external transfers use different machinery
Moving money between two accounts at the same bank is a bookkeeping entry. Nothing leaves the institution, and the update can be immediate.
Moving money to an account at another bank generally uses the automated clearing house, which processes in batches on business days and settles over one or more days.
Same-day options exist on that network, and instant payment rails now operate continuously, but which options a given bank offers to consumers varies considerably.
Holds protect against reversal, not against the customer
An incoming transfer can be reversed within a defined window if it was unauthorized or if the originating account lacked funds.
Banks hold newly received funds partly to cover that risk, which is why money can appear in a balance while remaining unavailable for withdrawal.
The length of the hold generally depends on the source of the funds and the account's history, and it shortens as a relationship establishes.
Caps are risk controls
Daily and monthly transfer caps limit the damage from a compromised login. They are set by the institution and are frequently adjustable on request.
Newly linked external accounts often carry lower caps for an initial period, which is why a first transfer behaves differently from later ones.
These caps apply per rail, so a limit on external transfers may not apply to a wire, and the alternatives usually carry a fee instead.
What this means for keeping a buffer
Money held in a separate savings account is not immediately available in the way a checking balance is, and the gap can run to several business days.
A household planning around a cash buffer needs to know which of its accounts can actually fund a payment today and which cannot.
Testing a transfer once, and noting how long it took to become available, answers that more reliably than the timing described in marketing material.