Why a motor circuit is protected twice
An ordinary branch circuit uses one device for everything. A motor circuit cannot, because starting current and overload current are separated by an order of magnitude and no single device can be set for both. A typical squirrel-cage motor draws six to eight times its full-load current for the first few seconds of acceleration. A device set to trip on a sustained 25% overload would trip on every start; a device set to ride through the start would never notice a 25% overload.
So NEC Article 430 splits the job. The overload device — a thermal relay, an electronic overload, or in small cases a fuse — protects the motor and its conductors against sustained running overcurrent. It has a long time delay, it is set just above the nameplate current, and it does not interrupt short circuits. The branch-circuit short-circuit and ground-fault device — a breaker or fuse — protects against faults. It is set far above starting current so it does not trip on inrush, and it gives no overload protection at all.
That division is why the code's percentages look strange in isolation. A 250% inverse-time breaker on a motor drawing 27 A looks recklessly oversized until you remember it is not protecting against overload; the overload relay set at 31.9 A is doing that. A single 70 A breaker with no overload relay would let the motor burn out at 60 A quite happily.
The third element is the disconnecting means, required by 430.102 to be in sight of the motor and its driven machinery, and rated under 430.110(A) at not less than 115% of the full-load current. Its purpose is safe isolation for maintenance, not protection.
Two currents: the table value and the nameplate value
The single most common error in motor circuit design is using one current where the code asks for the other. NEC 430.6(A)(1) is explicit: for sizing conductors, ampacity, ground-fault protection and short-circuit protection, use the values in Tables 430.247 through 430.250 — the horsepower-and-voltage tables — and not the nameplate current. NEC 430.32, by contrast, requires overload protection to be based on the motor nameplate full-load current.
The reason for the split is that the table values are deliberately conservative generic figures for a given horsepower and voltage, so a conductor sized from them suits any motor of that rating and a replacement motor does not invalidate the installation. Overload protection, on the other hand, has to match the specific machine's thermal capability, and only the nameplate knows that. A modern high-efficiency motor commonly draws several percent less than the table figure, and that difference is exactly what the two rules are designed to handle.
Two exceptions to the table rule are worth knowing. Motors marked with a NEMA design letter and torque characteristics per 430.6(A)(1) still use the table. Torque motors and low-speed or multispeed machines have their own provisions, and a motor supplied by a variable-frequency drive is handled under 430.6(C), which requires the current to be the rated input to the drive rather than a table value.
The overload percentage itself has two tiers. NEC 430.32(A)(1) permits 125% of nameplate FLA for a motor with a marked service factor of 1.15 or greater, or a marked temperature rise of 40 °C or less. Every other motor gets 115%. That is a nameplate check, not an assumption — and where the sized device will not allow the motor to start or carry the load, 430.32(C) permits an increase to 140% or 130% respectively, but no further.
Worked example: a 20 hp, 460 V three-phase motor
A 20 hp squirrel-cage motor runs on a 460 V three-phase supply. NEC Table 430.250 gives its full-load current as 27 A. The nameplate reads 25.5 A and carries a 1.15 service factor. It will be protected by an inverse-time circuit breaker.
- Branch-circuit conductors. NEC 430.22 requires at least 125% of the table full-load current: 1.25 × 27 = 33.75 A. Choose a conductor whose ampacity after any temperature correction and bundling adjustment is at least 33.75 A — that is the ampacity derating step, and the answer is commonly 8 AWG copper.
- Overload device. The motor has a 1.15 service factor, so 430.32(A)(1) permits 125% of the nameplate: 1.25 × 25.5 = 31.875 A. That is a maximum setting. If the motor trips during a legitimate start, 430.32(C) permits going to 140% of nameplate, 1.40 × 25.5 = 35.70 A, and no higher.
- Short-circuit protection. Table 430.52 allows an inverse-time breaker at 250% of the table full-load current: 2.50 × 27 = 67.5 A.
- Round to a standard size. 67.5 A is not a standard rating from 240.6(A). Exception 1 to 430.52(C)(1) permits the next higher standard size, which is 70 A.
- Disconnect. NEC 430.110(A) requires at least 115% of the table full-load current: 1.15 × 27 = 31.05 A, so a 60 A switch or a 30 A switch only if its rating actually meets 31.05 A — it does not, so 60 A is the practical choice.
Notice how far apart the numbers are. The conductor carries 33.75 A of required ampacity, the overload trips somewhere above 31.9 A, and the breaker is 70 A. A fault current of 500 A opens the breaker; a sustained 40 A running current opens the overload relay while the breaker never notices. Neither device covers the other's range, which is precisely the design intent.
Change the protective device and only step 3 changes. Dual-element time-delay fuses are permitted at 175%: 1.75 × 27 = 47.25 A, rounding up to 50 A. Non-time-delay fuses get 300%: 3.00 × 27 = 81 A, rounding up to 90 A. The time-delay fuse gives by far the closest protection, which is why it is preferred where fuses are used at all.
Reading the results and choosing within them
Everything this calculator returns for short-circuit protection is a maximum. NEC 430.52(C)(1) permits a device rated up to that value; it does not require one. The correct practice is to select the smallest device that will reliably hold during starting, because a smaller device clears faults faster and lets through less energy. Starting from the maximum and working down is the right direction of travel.
The overload figure is a maximum too, and here the margin is much tighter. Modern electronic overloads are set in amperes directly and are adjustable across a range; thermal relays are selected by heater element, and the tables the manufacturer supplies already incorporate the code multipliers, so read the manufacturer's selection table rather than applying 125% twice. Overload relays also carry a trip class — 10, 20 or 30 — describing how long they permit locked-rotor current before tripping, and a high-inertia load may need a class 30 relay even though the setting in amperes is unchanged.
Where the calculated breaker will not hold on starting, do not simply move up a size. NEC 430.52(C)(2) Exception 2 provides specific higher limits — up to 400% for time-delay fuses at certain ratings and up to 225% for inverse-time breakers of 100 A or less, among others — and it is worth reading the exception rather than assuming. Repeated nuisance tripping on a correctly sized device more often indicates a starting problem: excessive load inertia, a jammed driven machine, low voltage at the terminals, or a soft starter set for too short a ramp.
Finally, remember that the conductor result here is an ampacity requirement, not a wire size. 33.75 A of required ampacity becomes a conductor only after the ambient correction, bundling adjustment and 110.14(C) termination check — and on long runs, after a voltage drop check, since motors are sensitive to terminal voltage and torque falls with the square of it. The voltage drop calculator is the second constraint on almost every long motor feeder.
NEC Table 430.52 maximum protection percentages
| Motor type | Non-time-delay fuse | Dual-element time-delay fuse | Instantaneous-trip breaker | Inverse-time breaker |
|---|---|---|---|---|
| Squirrel-cage, other than Design B energy-efficient | 300% | 175% | 800% | 250% |
| Squirrel-cage, Design B energy-efficient | 300% | 175% | 1100% | 250% |
| Synchronous, full voltage | 300% | 175% | 800% | 250% |
| Wound rotor | 150% | 150% | 800% | 150% |
| Direct current, constant voltage | 150% | 150% | 250% | 150% |
Where the calculated value does not correspond to a standard device rating in 240.6(A), Exception 1 to 430.52(C)(1) permits the next higher standard size. Exception 2 permits specific further increases where the device will not carry the starting current. Confirm the percentages against the code edition your jurisdiction has adopted.
Errors that show up on plan review
- Sizing conductors from the nameplate current. NEC 430.6(A)(1) requires the table value for conductors, ampacity, and short-circuit and ground-fault protection. Only overload protection uses the nameplate.
- Sizing the overload from the breaker. The two are independent. A 70 A breaker on a motor with a 25.5 A nameplate still takes an overload device set no higher than 31.9 A.
- Treating the Table 430.52 percentage as a required size. It is a ceiling. Selecting the smallest device that holds during starting gives faster fault clearing and less let-through energy.
- Assuming the 125% overload multiplier. It applies only to motors marked with a service factor of 1.15 or greater or a temperature rise of 40 °C or less. Everything else gets 115%.
- Going past 140% when a motor will not start. NEC 430.32(C) caps the increase at 140% of nameplate for the 125% class and 130% for the 115% class. Beyond that the problem is the starting duty, not the relay.
- Applying single-motor rules to a feeder. A feeder supplying several motors is sized under 430.24 at 125% of the largest motor's full-load current plus the sum of the others, and its protection under 430.62 — different rules entirely.
- Forgetting the drive. A motor fed from a variable-frequency drive is covered by 430.6(C) and Part X of Article 430, and the currents come from the drive's rated input rather than from the motor tables.
- Ignoring the trip class of the overload. Setting the current correctly does not help if a class 10 relay is protecting a high-inertia load that takes twenty seconds to accelerate.
Key terms
- Full-load current (FLC)
- The current from NEC Tables 430.247–430.250 for a motor of a given horsepower and voltage. Used for conductors, short-circuit protection and the disconnect.
- Full-load amperes (FLA)
- The current marked on the motor's own nameplate. Used only for overload protection under 430.32.
- Overload
- Operating current above rating that, if sustained, causes damage from overheating. It is not a fault; the distinction is what justifies two protective devices.
- Trip class
- The maximum time in seconds an overload relay permits at 600% of its current setting before tripping — class 10, 20 or 30. Chosen from the load's acceleration time, not from its running current.
Beyond a single motor
Everything above concerns one motor on one branch circuit. Feeders supplying several motors follow different arithmetic: NEC 430.24 requires the feeder conductor to be sized at 125% of the largest motor's full-load current plus the sum of the full-load currents of all the others, and 430.62 sizes the feeder protection at the largest branch-circuit device rating plus the sum of the remaining motors' full-load currents. The pattern is deliberate — only one motor is assumed to be starting at a time.
Combination starters bundle the disconnect, the short-circuit device, the contactor and the overload relay into one listed assembly, which simplifies compliance because the combination is evaluated as a unit. Manufacturers' selection tables for these already incorporate the Article 430 multipliers, so applying them again on top is a real and common error.
Variable-frequency drives change the picture substantially. The drive limits both starting current and running current electronically, so the enormous inrush that Article 430's percentages exist to accommodate largely disappears. NEC 430.6(C) directs you to the drive's rated input current for conductor sizing, and Part X of Article 430 governs the arrangement. Motor overload protection is usually provided by the drive itself, in which case the discrete relay may not be required — but that depends on the drive's listing and must be verified rather than assumed.
For the rest of the motor circuit, the motor full-load amps calculator gets you the current you start from, the ampacity derating calculator converts the required ampacity into a real conductor in real conditions, and the equipment grounding conductor calculator sizes the ground — noting that NEC 250.122(D) has its own rule for motor circuits protected by instantaneous-trip breakers. On the mechanical side, the motor torque calculator tells you whether the machine can accelerate the load in the first place, which is usually the real cause when a correctly sized device keeps tripping.