430.110
Motor Disconnecting Means — Ampere Rating and Interrupting Capacity
Every motor must have a disconnect — a switch, breaker, or other device that can be used to completely de-energize the motor for service. Section 430.110(A) says this disconnect must be rated at 115% of the motor FLC from the NEC tables (not the nameplate). The 15% safety margin accounts for the fact that switching a motor off under load creates an arc at the contacts, and the disconnect must be robust enough to handle that. For a 10 HP, 230V, 3-phase motor with 28A FLC, the minimum disconnect rating is 28 x 1.15 = 32.2A, which typically means you would select a 60A disconnect (the next commonly available trade size). There is an important alternative: a listed non-fused motor-circuit switch with an HP rating equal to or greater than the motor HP is always acceptable, because the manufacturer has already designed it to handle the 115% requirement internally. For fused disconnects, the fuse size inside is governed by 430.52, not 430.110.
When You Need This
- Selecting a disconnect switch for a new motor installation — this is often the first piece of hardware you order
- Replacing a motor and verifying the existing disconnect is still adequately rated
- Answering exam questions about motor disconnect sizing — the 115% multiplier is a common test item
- Designing motor control centers where each bucket needs a properly rated disconnect
Key Points
Common Mistakes
Sizing the disconnect at 100% of FLC instead of 115% — the code requires the 15% safety margin
Using the motor nameplate FLA instead of the NEC table FLC for the calculation
Confusing the disconnect ampere rating with the fuse size — for fused disconnects, these are separate calculations under different code sections
Selecting a disconnect based on HP rating alone without verifying the ampere rating meets the 115% requirement
Exam Tip
The disconnect calculation is straightforward: FLC x 1.15, then select the next standard trade size disconnect. Do not confuse this with the 125% conductor sizing rule (430.22) or the 250% breaker rule (430.52). Each motor circuit component has its own multiplier.
Frequently Asked Questions
Yes, if the breaker is within sight of the motor controller location, it can serve as the disconnect. Many installations use a circuit breaker at the panel as the disconnect, with a separate motor starter providing overload protection near the motor.
Table 430.250 gives an FLC of 28A. The minimum disconnect rating is 28 x 1.15 = 32.2A. Standard disconnect sizes are typically 30A, 60A, 100A, etc. Since 30A is less than 32.2A, you would select a 60A disconnect. Alternatively, you could use a motor-rated switch with a 10 HP or higher rating at 230V.
Inline Tools
Motor FLC Calculator
Look up full-load current for any motor HP and voltage
Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.