TL;DR: US equity markets have two kinds of circuit breakers. Market-wide circuit breakers (MWCB) halt every stock and options exchange when the S&P 500 falls 7%, 13%, or 20% from the prior day’s close. Single-stock circuit breakers, run under the Limit Up-Limit Down (LULD) plan, briefly pause an individual security when its price would trade outside a percentage band around a rolling five-minute average.
Both systems exist to give humans, algos, and the market itself a chance to catch a breath during dislocations. Neither one prevents a crash — they just slow it down.
Why circuit breakers exist
The idea came out of the aftermath of Black Monday. On October 19, 1987, the Dow Jones Industrial Average fell 22.6% in a single session. A presidential task force — the Brady Commission — recommended coordinated trading halts as a way to interrupt cascading sell orders and let liquidity return. The SEC and exchanges implemented the first version in 1988. The current rules were rewritten in 2012 and took effect in April 2013 after being permanently approved by the SEC.
The animating idea is simple: when prices move faster than participants can process them, forced or panicked orders can create feedback loops. Pausing trading briefly gives market makers and the underlying auction process time to rediscover price. It is a governor on the engine, not a repair shop.
The two flavors of halt
Circuit breakers come in two forms, and they answer different questions.
- Market-wide circuit breakers (MWCB) answer “is the whole market falling apart?” They are triggered by the S&P 500 alone and, once triggered, halt trading across every US equity and equity-options exchange simultaneously. NYSE Rule 7.12 and Nasdaq Rule 4121 codify the levels.
- Limit Up-Limit Down (LULD) pauses answer “is this one stock printing prints that don’t make sense?” They apply per security and pause only that ticker when quotes would push through a price band. The LULD plan is a national market system plan; it was made permanent by the SEC on April 11, 2019.
Market-wide levels: 7%, 13%, 20%
The MWCB thresholds are measured off the prior trading day’s regular-session closing S&P 500 price. If the index falls by the trigger amount during regular hours, all listed venues halt trading. The halt duration depends on which level trips and the time of day it happens.
| Level | S&P 500 decline vs. prior close | Halt duration | Time-of-day rule |
|---|---|---|---|
| Level 1 | -7% | 15-minute halt | Only if triggered before 3:25 PM ET |
| Level 2 | -13% | 15-minute halt | Only if triggered before 3:25 PM ET |
| Level 3 | -20% | Market closes for the day | Triggers at any time during regular hours |
Two subtleties often catch people out. First, Level 1 and Level 2 halts skip if the trigger prints after 3:25 PM ET — the market keeps trading into the close even if the S&P is down 8% or 14%, on the theory that a short pause with only 35 minutes left does more harm than good. Second, Level 3 is different: a 20% decline closes the market for the day at any time. There is no second chance and no reopen.
Single-stock pauses: how LULD works
An MWCB trip is rare — most days, a stock will never come close. What is not rare is a single security printing an obviously bad tick: a fat-finger order, a mispriced sweep, an ETF quote gap. The LULD plan handles those.
LULD sets a symmetric price band around a rolling reference price, which is defined as the arithmetic mean of eligible trades over the prior five-minute window. If a quote would push a trade through that band, the exchange enters a 15-second limit state. If no trade happens back inside the band within 15 seconds, the security enters a 5-minute trading pause. When trading resumes, it does so via an auction on the security’s primary listing exchange.
The bands themselves depend on the security’s tier and its price. Tier 1 covers S&P 500, Russell 1000, and selected exchange-traded products — the deepest, most-liquid names. Tier 2 covers everything else on NMS.
| Segment | Reference price | Band (9:30 AM – 3:35 PM) | Band (3:35 PM – 4:00 PM) |
|---|---|---|---|
| Tier 1, price > $3.00 | 5-min rolling mean | ±5% | ±10% (doubled) |
| Tier 2, price > $3.00 | 5-min rolling mean | ±10% | ±20% (doubled) |
| Any tier, $0.75–$3.00 | 5-min rolling mean | ±20% | ±40% (doubled) |
| Any tier, < $0.75 | 5-min rolling mean | lesser of $0.15 or 75% | doubled |
The reference price refreshes every 30 seconds if it has moved by at least 1% versus the current band, so the bands themselves are moving with the market — they are not fixed to the day’s open.
What the market-wide levels look like intraday
The MWCB thresholds are horizontal lines below the prior day’s close. On a chart, they behave like this:
Worked example: March 2020
The circuit breaker system had a full stress test during the early Covid-19 sell-off. Level 1 halts fired on four separate trading days over an eight-session window as the S&P 500 fell 7% intraday on each occasion.
Two things to notice. Three of the four halts fired within minutes of the open — the overnight news drove futures into limit-down before the cash session began, and the sell orders arrived at 9:30 AM. On March 18, the halt didn’t fire until 1:00 PM, which is why the bar reaches higher on the chart. In all four cases the market resumed after the 15-minute halt; no Level 2 halt fired that month.
Common misconceptions
- “Circuit breakers stop the market from crashing.” No. They pause trading. Prices can — and often do — reopen lower after the halt. The point is to slow the cascade, not to prevent price discovery.
- “A Level 1 halt happens once and then trading is normal.” Only Level 1 reopens. If the market immediately drops another 6 percentage points after the reopen, Level 2 can fire on the same day. And Level 3 can then close things entirely.
- “LULD is a halt like MWCB.” LULD is much lighter — it is a 15-second limit state that becomes a 5-minute pause only if the price doesn’t come back inside the band. Most limit states expire without ever triggering a pause.
- “Circuit breakers apply to futures too.” Sort of. Equity index futures have their own overnight and intraday price limits set by the CME, distinct from NYSE’s MWCB, but the mechanisms are related and often move together.
- “MWCB uses the Dow.” Not anymore. Since 2013 the trigger is the S&P 500. The old Dow-points system is gone.
What to watch when volatility picks up
If you want to see the same feed the exchanges use, three places are worth bookmarking:
- The NYSE MWCB trigger page publishes the day’s Level 1, 2, and 3 threshold values in index points — computed off the prior close each morning.
- The CME index-futures price-limit page shows overnight and intraday limits for E-mini S&P, Nasdaq, Dow, and Russell futures.
- Nasdaq’s trading halt feed and NYSE’s LULD feed list individual securities that have entered limit states or pauses during the day.
None of these are trading signals on their own. They are diagnostic — they tell you the plumbing is being stressed, not what to do about it.
Related concepts
- Moving Averages Explained — the everyday chart-based signals traders watch.
- The VIX Explained — how implied volatility spikes typically precede circuit-breaker activity.
- Dark Pools Explained — off-exchange venues that also halt when MWCB fires.
Sources
- NYSE — Market-Wide Circuit Breakers and Trading Info
- Limit Up-Limit Down (LULD) Plan — Official Site
- SEC Investor Bulletin — Measures to Address Market Volatility
- Wikipedia — Trading Curb (secondary reference for historical event dates)
Disclosure: This article is for informational purposes only and is not investment advice.