Money Market Funds vs. Money Market Accounts: FDIC vs. SIPC, Breaking the Buck, and Where Your Business Should Park Idle Cash

Your operating account holds $180,000 earning 0.01%, and every month you watch it sit there doing nothing while your brokerage dangles a "money market" paying over 4%. So you move the cash — and only later realize the two products sharing that name answer the question "is my money guaranteed?" in opposite ways. One is a bank deposit backed by the federal government up to $250,000. The other is an investment that can, in rare and memorable cases, hand you back 99 cents on the dollar. Confusing them does not just cost you yield; it can put payroll money in a vehicle that was never designed to be failure-proof.

This guide untangles the pair: what each product is, how FDIC insurance differs from SIPC protection, what "breaking the buck" actually means and how the rules have changed since it last happened, and a practical framework for deciding where each tier of your business cash should live.

The Difference in One Paragraph​

A money market account (MMA) is a deposit account at a bank or credit union. Your balance is FDIC-insured (or NCUA-insured at a credit union) up to $250,000 per depositor, per bank, per ownership category, and you can usually write checks or use a debit card against it. A money market fund (MMF) is a mutual fund you buy through a brokerage. It pools your cash with other investors' to buy very short-term, high-quality debt — Treasury bills, commercial paper, repurchase agreements — and aims to keep its share price at exactly $1.00. It is not FDIC-insured. If your brokerage holds it, SIPC protection may apply, but SIPC covers a failed brokerage — not a fund that loses value. Same first two words, different legal species.

What a Money Market Account Actually Is​

Think of an MMA as a savings account with checking-account privileges. You open it at a bank or credit union, earn a variable interest rate quoted as an APY, and get limited transaction access — typically checks and a debit card, sometimes with a monthly transaction cap the bank sets itself. (The Federal Reserve suspended Regulation D's six-transfer limit in 2020, but individual banks may still enforce their own.)

Three features define it:

• Federal deposit insurance. At an FDIC-member bank, your MMA balance is insured up to $250,000 per depositor, per ownership category. Joint, single, and business accounts in different categories each get their own $250,000 ceiling. If the bank fails, the FDIC makes you whole within those limits — usually within days.
• Tiered, variable rates. Banks commonly pay higher APYs on larger balances, and the rate floats with the market. Top-yielding accounts recently paid APYs above 4% while the national average on standard savings languished under 1% — the spread between shopping around and staying put is several hundred dollars a year on a six-figure balance.
• Direct spendability. Unlike a brokerage fund, you can pay a vendor straight from the account. For a quarterly tax reserve or an equipment-replacement fund, that means no transfer delay between "saved" and "spent."

The trade-off is access friction in the other direction: moving money from your MMA to a brokerage for investing takes a standard bank transfer, and yields reset at the bank's discretion rather than tracking the market mechanically.

What a Money Market Fund Actually Is​

A money market fund is a mutual fund regulated by the SEC under Rule 2a-7, which restricts it to short-maturity, high-quality paper — securities maturing in 397 days or less, with the portfolio's average maturity capped at 60 days. Roughly $7.5 trillion sits in these funds, making them one of the largest cash-parking vehicles in the world. You buy shares through a brokerage or fund company, often as the default "sweep" for uninvested cash.

Funds come in three flavors, and the flavor matters more than most buyers realize:

• Government and Treasury funds hold U.S. government debt — T-bills, agency paper, repos backed by Treasuries. They keep a stable $1.00 share price and are the conservative end of an already conservative category.
• Prime funds also buy high-quality corporate commercial paper and bank obligations to squeeze out extra yield. Institutional prime funds must float their NAV like any mutual fund; retail prime funds may keep a stable $1.00 price but can impose liquidity fees in stress.
• Municipal (tax-exempt) funds hold short-term state and local debt, with income generally exempt from federal tax — and sometimes state tax if you buy a single-state fund for your home state.

Yields are quoted as a 7-day SEC yield — the income earned over the last seven days, annualized — rather than an APY, and the fund's expense ratio is already subtracted before you see it. A fund yielding 4.5% with a 0.40% expense ratio earned 4.9% on its paper and kept the difference; comparing two funds' published yields already accounts for that skim, but comparing a fund's yield to a bank's APY is approximately apples-to-apples only over short stretches, since both float.

FDIC vs. SIPC: The Protection Gap That Matters Most​

This is the section to get right, because the acronyms sound interchangeable and are not.

FDIC insurance protects deposits against bank failure. If your bank collapses, the FDIC covers your MMA (plus your checking and savings at the same bank, aggregated by ownership category) up to $250,000. Coverage is per depositor, per insured bank — spread cash across three banks and you carry $750,000 of protection. The guarantee is about the institution failing, and it covers principal plus accrued interest to the limit.

SIPC protection protects securities against brokerage failure — not against losing value. If your brokerage goes under and your assets are missing, SIPC steps in to recover up to $500,000 per customer, including up to $250,000 in cash. But if your money market fund's holdings fall in value — the "break the buck" scenario below — that is an investment loss, and SIPC explicitly does not cover it. A fund share bought at $1.00 and redeemed at $0.99 leaves you with no claim against anyone.

The practical upshot: cash in an MMA at a failed bank comes back (within limits); cash in a money market fund at a failed fund does not, even though the brokerage holding your fund account is an SIPC member. Many business owners discover this distinction only when it matters. Check which one covers any account holding more than a month of operating expenses.

"Breaking the Buck": What It Means and Why the Rules Changed​

Money market funds target a constant $1.00 net asset value per share. "Breaking the buck" means the fund's holdings have lost enough value that a share is worth less than a dollar — investors redeeming get 99 or 98 cents back instead of the full amount. In the fifty-plus-year history of these funds, it has happened exactly twice, and the famous case is instructive.

In September 2008, the Reserve Primary Fund — then one of the largest money market funds in the country — held a large position in Lehman Brothers commercial paper. When Lehman failed, that paper became nearly worthless overnight, the fund's NAV slipped below $1.00, and redemption requests flooded in. The Treasury had to backstop the entire industry temporarily to stop a system-wide run. Investors eventually recovered roughly 99 cents on the dollar, but the episode proved the $1.00 target was a convention, not a guarantee.

Regulators responded in three waves:

1. 2010 reforms imposed liquidity minimums, shorter maturities, and stress testing.
2. 2014 reforms forced institutional prime funds to a floating NAV, and gave fund boards the power to impose redemption fees and gates during stress.
3. 2023 reforms raised daily and weekly liquid-asset minimums to 25% and 50% of the portfolio, removed gates entirely, and replaced them with mandatory liquidity fees when a fund's weekly liquid assets run thin — redeeming shareholders pay the cost of their own exit instead of being locked in.

The result: government and Treasury funds are about as close to cash as an investment gets, while prime funds now carry explicit, mechanical costs for running at the worst moment. None of this creates insurance — it just makes the buck harder to break and the damage more contained if it does.

Side-by-Side Comparison​

Where Your Business Should Park Each Tier of Idle Cash​

Stop asking "which one is better" and sort your cash by job instead. Most small businesses hold three tiers, and each has a natural home:

Tier 1 — next 30 days of outflows (payroll, rent, payables). Keep it in checking or an MMA at your primary bank. This money's job is availability, not yield; a same-day transfer beats an extra half-point of return. An MMA works well here because you can pay directly from it while earning something.

Tier 2 — reserves with a known purpose (quarterly estimates, a slow-season cushion, an equipment fund). An MMA or high-yield business savings account under the FDIC limit is the default. If the balance exceeds $250,000 at one bank, use an insured sweep network (IntraFi-style ICS/CDARS programs split your deposit across many banks while you see one balance) rather than stacking uninsured dollars in a single account.

Tier 3 — cash with no near-term job that still is not investment capital. Here a Treasury money market fund often wins: yields track T-bill rates closely, the interest inherits the state-income-tax exemption of the underlying Treasuries (a real edge in high-tax states), and same-day or next-day liquidity is plenty for money with no deadline. Prime funds' extra few basis points are not worth the complexity and remote NAV risk for operating cash — leave those to investment accounts.

Two traps to avoid: treating your brokerage's default cash sweep as automatically optimal (sweep options often pay far less than the same firm's flagship government fund — check what your idle cash actually sits in), and assuming "money market" on a brokerage statement means FDIC-insured (some brokerages do offer FDIC-insured bank-sweep programs through partner banks, which is exactly why the label confusion persists — verify the insurance line on your statement, not the product name).

Bookkeeping: How to Record Each One​

Your books should reflect the legal difference. An MMA is cash — record it as a bank account, and book its interest as interest income when credited. A money market fund is an investment position, even when it behaves like cash: most small businesses carry it at cost ($1.00 per share makes this painless) and book its distributions as interest or dividend income depending on how the 1099 reports them. If you keep your books in plain text, the Beancount documentation covers account setup, and you can watch the reserve balance trend in Fava once the accounts exist:

2026-10-01 open Assets:Bank:MoneyMarket USD
2026-10-01 open Income:Interest:Bank USD
 
2026-10-19 * "Sweep operating reserve into money market account"
  Assets:Bank:MoneyMarket    50,000.00 USD
  Assets:Bank:Checking      -50,000.00 USD
 
2026-10-31 * "October money market interest"
  Assets:Bank:MoneyMarket       182.50 USD
  Income:Interest:Bank          -182.50 USD

Reconcile both monthly like any bank account — sweep statements in particular, where a single line can net purchases, redemptions, and reinvested distributions that each need their own entry. And at year-end, confirm the 1099-INT/1099-DIV split matches your books: fund distributions sometimes arrive partly as dividends, which flow to a different tax line than bank interest even though both felt like "interest" all year.

Common Mistakes to Avoid​

• Assuming every "money market" is FDIC-insured. The brokerage fund is not. Read the insurance disclosure, not the product name.
• Parking payroll money in a prime fund for extra yield. A handful of basis points cannot compensate for liquidity fees or settlement timing on money with a hard deadline.
• Ignoring settlement. Fund redemptions typically settle next business day; bank transfers on top of that can push availability to day two or three. Never fund Friday payroll from a Wednesday redemption without confirming the timeline.
• Forgetting state tax. Treasury-fund income is generally exempt from state income tax; bank interest is not. In a 9% bracket, that exemption can outweigh a visible yield gap.
• Stacking over $250,000 in one bank "temporarily." Temporary has a way of becoming permanent. Set up the sweep network or second bank before the balance lands, not after.

Your Next Step​

This week, pull up every account holding business cash and answer two questions about each: what is it legally — deposit or investment — and what insurance actually covers it. If any Tier 1 or Tier 2 money sits in a prime fund, move it to an insured account; if any balance tops $250,000 at a single bank, set up a sweep network or open the second account now. The yield difference between "good enough" and "optimal" is a few hundred dollars a year — the cost of learning the FDIC/SIPC distinction during a crisis is the lesson itself.