Does an exit sign need a battery?

Only if the building has no other emergency source. NEC 700.12 allows a two wire sign fed from a generator, inverter, or separate service that restores power within 10 seconds and carries the load for 90 minutes.

Can an exit sign be on a switched circuit?

No. NEC 700.12(I)(1) requires unit equipment to be connected ahead of any local switches so the egress lighting cannot be turned off.

How long must emergency lighting stay on?

90 minutes, at an average of 1 footcandle and a minimum of 0.1 footcandle along the path of egress.

What is a two wire exit sign with no battery, and does the NEC allow it?

A two wire exit sign is a sign fed by a single unswitched hot and a neutral, with no internal battery and no separate charging conductor. It lights whenever that circuit is energized and it goes dark the instant the circuit loses power. A self powered exit sign, by contrast, has a nickel cadmium or lithium battery inside and stays lit for 90 minutes after the supply drops. The NEC does not prohibit the two wire type, but it only accepts it when the branch circuit feeding it is itself an emergency circuit backed by a legal emergency source.

What NEC 700 actually requires

Section 700.12 requires that current supply be such that in the event of failure of the normal supply, emergency lighting is available within the time required for the application but not to exceed 10 seconds. Section 700.12(A) lists acceptable sources, including storage batteries, generator sets, uninterruptible power supplies, separate service, fuel cell systems, and unit equipment. Section 700.12(I) covers unit equipment, which is the battery pack style fixture, and requires the branch circuit feeding it to be the same branch circuit serving the normal lighting in the area and connected ahead of any local switches.

So a two wire sign is code compliant in a building with a standby generator or a central inverter feeding the emergency lighting panel. In a small building with no generator and no inverter, a two wire sign has no source to fall back on and fails the 10 second requirement, which is why most small commercial jobs use self powered signs.

Illumination levels and duration

Section 700.4 requires the emergency system to have capacity and rating for all loads to be operated simultaneously. Section 700.10(D) addresses fire protection of the wiring. NFPA 101 and the International Building Code set the performance side: the means of egress illumination must average 1 footcandle at the walking surface, with a minimum of 0.1 footcandle at any point, and must be maintained for at least 90 minutes.

Wiring practice in the field

Feed the sign from an unswitched hot. Section 700.12(I)(1) says unit equipment must be connected ahead of any local switches so a janitor cannot kill egress lighting with a wall switch. On a two wire sign fed from an emergency panel, tie the hot to the emergency circuit and land the neutral on the emergency panel neutral bar, not on a normal power neutral. Section 700.10(B) requires emergency circuit wiring to be kept entirely independent of all other wiring and equipment, with limited exceptions, so do not share a raceway or a box with normal power branch circuits.

Also keep in mind that Section 700.6 requires signaling for the derangement of the emergency source, and Section 700.3 requires periodic testing under load. A two wire sign inherits its testing regime from the source feeding it.

Field takeaway

Two wire means no battery. It is legal only when the circuit behind it is a real emergency circuit fed by a generator, inverter, or separate service that picks up within 10 seconds and carries 90 minutes. If the building has none of that, install self powered unit equipment, feed it from the unswitched local lighting circuit ahead of the switches, and label the circuit at the panel.