Does the 2:1 rule guarantee coordination at fault current?

No. The 2:1 ratio only covers the overload/low-fault region. At currents above the upstream instantaneous trip threshold, only series-rated combinations from the manufacturer's published tables or current-limiting devices can guarantee full selectivity.

Protection Coordination Calculator — IEEE 242 + NEC 700.32 / 701.32 / 708.54 / 517.31 Selectivity

How the Protection Coordination Calculator works

Selective coordination is the ability of a downstream overcurrent protective device (OCPD) to open before its upstream device, isolating only the faulted circuit. This tool applies the baseline ratios from IEEE 242 — Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book) and flags NEC 700.32 / 701.32 / 708.54 / 517.31 jurisdictional requirements.

Two regions of the time-current curve

Overload region — currents up to ~10× rating. Coordination here is driven by thermal-magnetic curve shape; the 2:1 ratio is generally sufficient for MCCB/MCCB and 1.5:1 for fuse/fuse same class.
Fault region — instantaneous and short-time. Coordination depends on the upstream device's instantaneous pickup vs. the downstream device's clearing time. Often requires series-rated combinations from manufacturer tables or current-limiting fuses to coordinate at high available fault current.

NEC 700.32 / 701.32 / 708.54 / 517.31 — When full selectivity is required

For these systems, the upstream OCPD must not trip for any downstream fault, up to the highest available fault current:

NEC 700 — Emergency systems (egress, fire alarm, life-safety)
NEC 701 — Legally required standby
NEC 708 — Critical operations power systems (COPS)
NEC 517 — Healthcare facility essential electrical systems

For these designations, IEEE 242 baseline ratios are not enough — the design must include a documented selective coordination study with manufacturer time-current curves.

Frequently asked questions

What is selective coordination per NEC?

Defined in NEC Article 100 as "localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings". NEC 700.32 / 701.32 / 708.54 / 517.31, 700.32, 701.32, 708.54, and 517.31 mandate it on specific systems.

What is the 2:1 rule for MCCB coordination?

IEEE 242 §15.6.5 recommends a minimum 2:1 ampere ratio between upstream and downstream molded-case breakers to achieve coordination in the overload region. At currents above the upstream instantaneous pickup, the ratio is insufficient — manufacturer published tables must be consulted.

Are series-rated combinations the same as coordination?

No. Series rating allows a downstream breaker to be applied at a higher interrupting rating than its individual marking, but during a fault both devices may trip. Series rating is forbidden on emergency / legally required / COPS systems by NEC 240.86(A) and the selectivity Articles.

How does ground-fault protection affect coordination?

Where two levels of NEC 230.95 ground-fault protection are required (i.e., service and feeder), they must be coordinated per NEC 230.95 informational note and 517.17 for healthcare. A typical solution is fully-adjustable LSIG settings on the upstream device with at least 100 ms time delay.