SizingKitElectrical · overcurrent
Fuse size calculator for the circuits where the device beats the wire
Pick a motor or a transformer and this page returns the largest fuse the code allows on it, which on both of those circuits is legally far above the conductor’s own ampacity. Motors run through Table 430.52(C)(1) — 175% of table full-load current for a dual-element fuse, 300% for a non-time-delay one — and take a second, much smaller overload device sized on the nameplate under 430.32. Transformers run through Table 450.3(B), and the page states only the band of that table it can source and refuses the rest.
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- NEC 430.52 and 450.3(B)
- 5 fuse-only ratings
- Overload device too
Both are 240.4(G) circuits, where the device is allowed to sit above the conductor’s ampacity. A lighting, receptacle or appliance circuit is not one of them.
The motor
Horsepower and voltage read the code table; the nameplate figure is asked for separately because it sizes a different device.
The motor’s rated voltage, not the system’s: the 460 V column serves 440–480 V systems and the 230 V column serves 220–240 V.
Sized at 75 °C on 75 °C terminations, 30 °C ambient, three current-carrying conductors.
Off the plate on the motor, not off the table above. It sizes the overload device only.
Maximum branch-circuit device
20 A
175% of 14 A is 24.5 A — Table 430.52(C)(1)
Maximum overload device
16.5 A
1.25 × the nameplate 13.2 A — 430.32(A)(1)
| Table full-load current — 430.250 | 14 A |
|---|---|
| Branch-circuit conductor at 1.25 × — 430.22(A) | 14 AWG · 2.08 mm² |
| That conductor’s ampacity | 20 A |
| Round-up permitted by Exception No. 1 | 25 A |
Why this looks wrong and is not. The device above is 1.33 times the 15 A that 240.4 would allow on this same conductor anywhere else in the building. 240.4(G) is what moves the circuit to Article 430, where the device answers for short circuits and ground faults alone and the sustained overload is the separate device on the right. Take either one away and half of what can go wrong with a motor is unprotected.
These are the code's table values, not the motor's nameplate current, and the two are different on purpose. NEC 430.6(A)(1) requires the values in Tables 430.247 through 430.250 to be used — instead of the nameplate — whenever sizing conductors, ampacity, switches and branch-circuit short-circuit and ground-fault protection. The nameplate governs exactly one thing: the running overload device, under 430.6(A)(2) and 430.32. The table figures are deliberately conservative and run above what a modern high-efficiency motor actually draws, so substituting a lower nameplate current under-sizes the conductor and the disconnect. The two exceptions are a listed motor-operated appliance and a multispeed motor, which use their nameplate for both.
The synchronous-motor row here is the full-voltage, resistor and reactor starting case; low-torque, low-speed synchronous motors used for compressors and similar duty take 200 percent for a nontime-delay fuse, and the table has separate columns for autotransformer and high-reactance starting that are not carried here. Two exceptions move these numbers. 430.52(C)(1) Exception No. 1 permits the next higher standard rating of 240.6(A) where the computed value does not land on one — which is why the round-up is offered below. Exception No. 2 permits, where the computed value is not sufficient for the starting current, a nontime-delay fuse up to 400 percent, a time-delay (dual-element) fuse up to 225 percent, an inverse-time breaker up to 400 percent for 100 A or less and 300 percent above 100 A, and an instantaneous-trip breaker up to 1300 percent on a Design B energy-efficient motor. An instantaneous-trip breaker is only permitted at all as part of a listed combination motor controller — 430.52(C)(3).
Fuse classes, and what each one is an envelope for
The interrupting-rating column is the one that decides whether a fuse belongs in a particular panel at all. A class fixes the body, the voltage and the ampere range; the curve is the manufacturer’s.
| Class | Amperes | Volts AC | Min. interrupting | Body and use |
|---|---|---|---|---|
| CC | 1/10–30 A | 600 V | 200 kA | 13/32 × 1-1/2 in rejection ferrule; control circuits, small motors |
| H | 0–600 A | 250 and 600 V | 10 kA | One-time and renewable; not current-limiting, and the class to stop specifying |
| RK5 | 0–600 A | 250 and 600 V | 200 kA | R rejection; dual-element time-delay, the general motor fuse |
| RK1 | 0–600 A | 250 and 600 V | 200 kA | R rejection, same envelope as RK5, markedly lower let-through |
| J | 1–600 A | 600 V | 200 kA | Smaller than Class R and not interchangeable with it; own holder |
| T | 1–1200 A | 300 and 600 V | 200 kA | The most compact body; fast-acting, so not a motor-starting fuse |
| L | 601–6000 A | 600 V | 200 kA | Bolt-in blades; where the 601 A rating of 240.6(A) starts |
UL 248, Low-Voltage Fuses — the multi-part standard with one part per class, which fixes each class's ampere range, voltage rating, minimum interrupting rating and dimensional envelope. NEC 240.60(B) requires that a fuseholder for current-limiting fuses not accept a fuse that is not current-limiting, which is what the R, J, T and CC rejection features implement, and 240.61 requires a fuse to be applied within its voltage and current classification. A class is an envelope, not a fuse. A particular catalog number is listed at or above its class minimum interrupting rating, and its time-current curve, let-through energy and any time-delay marking are the manufacturer's published data for that number alone. This table is declared on this page rather than in lib/standards/ and belongs there; nothing in it is a substitute for the data sheet. The five ratings that exist only as fuses and have no breaker equivalent are 1, 3, 6, 10, 601 A — NEC (NFPA 70) 2023, 240.6(A), second sentence: additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601.
How to fuse a 10 hp 460 V motor
The example the motor pane opens on, and the reason the two numbers it returns differ by a factor of two.
Read the current off the code table, not off the motor
Set the phase, horsepower and rated motor voltage. 430.6(A)(1) requires the value from Table 430.248 or 430.250 for everything you are about to size — conductors, the disconnect, and this device — and forbids substituting the nameplate, which on a modern high-efficiency motor is lower and would under-size all three. A 10 hp three-phase motor at 460 V is 14 A in the table whatever its plate says.
Pick the device type, because the percentage moves with it
A dual-element time-delay fuse gets 175% of that current, a non-time-delay fuse 300%, an inverse-time breaker 250%, and an instantaneous-trip breaker 800% — and the last is only permitted as part of a listed combination motor controller under 430.52(C)(3). The ceiling is a maximum, so the standard rating at or below it is the answer, with Exception No. 1 offering the next rating up when the arithmetic lands between two.
Size the overload device separately, off the plate
Type the nameplate full-load amperes into the second field and tick the box if the motor is marked with a service factor of 1.15 or more or a temperature rise of 40 °C or less. That gives 125% of the nameplate under 430.32(A)(1), or 115% without the marking — a far smaller number than the fuse, because it is the device that answers for a motor that is running hot rather than one that is faulted.
Technical specifications
| Motor tables carried | Table 430.248 (12 single-phase rows, 1/6 through 10 hp) and Table 430.250 (27 three-phase rows, 1/2 through 500 hp). Table 430.247 for DC motors and the synchronous block are not carried, so those motor types are absent from the selector rather than approximated. |
|---|---|
| Protection percentages | 6 motor types × 4 device columns from Table 430.52(C)(1). Wound-rotor and DC sit at 150% across the board; everything else is 175% dual-element, 300% non-time-delay, 250% inverse-time. |
| Cells deliberately blank | Two, both in the 575 V column of Table 430.250: 40 hp and 350 hp. The recalled figures contradicted the 0.8 ratio the rest of that column obeys, so the page prints a refusal there instead of interpolating between the rows either side. |
| Fuse-only ratings | 1, 3, 6, 10, 601 A. The first four matter on control and transformer primaries where no breaker that small exists; 601 A exists because the UL fuse classes change above 600 A. |
| Overload multipliers | 125% of nameplate for a motor marked with a service factor of 1.15 or greater or a 40 °C rise, 115% for everything else — NEC 430.32(A)(1). Both are on the nameplate current, which is the one place in the article the plate beats the table. |
| Transformer band stated | Rated primary current of 9 A or more only: 125% primary-only, or 250% primary with the secondary held to 125%. The 2–9 A band, the under-2 A band and the whole supervised-location column of Table 450.3(B) are not reproduced here and the page says so rather than guessing. |
| Fuse classes listed | Seven — CC, H, RK5, RK1, J, T and L — with the ampere range, voltage rating and minimum interrupting rating each UL 248 class fixes. Class H is the only one of the seven at 10 kA; the other six are 200 kA. |
| Where it runs | Entirely in the browser. Motor nameplates, kVA ratings and voltages you type are never uploaded, which is the point when the plate you are reading belongs to a client. |
Frequently asked questions
Why is my 175% fuse bigger than the wire it is protecting?
Because 240.4(G) says the wire is not what it is protecting. On a motor branch circuit the fuse answers for short circuits and ground faults only, and the sustained overload — the condition that actually cooks a conductor — is a separate device sized at 115% or 125% of the nameplate under 430.32. Article 430 splits one job into two devices because a motor draws six to eight times its running current for a few seconds on every start, and a device that will not open on that cannot also protect against a slow overload.
Do I use the nameplate amps or the table amps?
Both, for different devices, and mixing them up is the classic motor-circuit error. 430.6(A)(1) makes the table value of 430.248 or 430.250 mandatory for the branch-circuit conductors, the ampacity, the disconnect and the short-circuit device; 430.6(A)(2) and 430.32 make the nameplate mandatory for the running overload. The table figures are deliberately conservative and sit above what a modern motor draws, so a nameplate current dropped into the conductor calculation under-sizes the wire and the switch. The two exceptions are a listed motor-operated appliance and a multispeed motor, which use the nameplate for both.
What is the difference between an RK5 and an RK1 in the same holder?
Let-through, not rating. Both are Class R, both fit the same rejection holder, both are rated 200 kA, and swapping one for the other changes nothing you can see on the label — but an RK1 limits the peak current and the energy passed to the downstream equipment far more tightly than an RK5. That matters when the equipment's own short-circuit current rating was established with a specific fuse class, in which case the drawing names the class and substituting a cheaper one voids the assumption. Neither class fixes the time-current curve, which is per catalog number.
The percentage lands between two standard ratings. Which way do I go?
Down, unless you invoke Exception No. 1 to 430.52(C)(1), which permits the next higher standard rating of 240.6(A) where the computed value does not correspond to one. The page prints both figures side by side for exactly this reason: the round-down is always available and the round-up is a permission you are choosing to take. Note that the round-up applies to the standard ratings, so on a fuse it can land on 1, 3, 6, 10 or 601 A, which no breaker offers.
The fuse opens every time the motor starts. What now?
Exception No. 2 to 430.52(C)(1) is the answer and it is generous: where the value from the table is not sufficient for the starting current, a non-time-delay fuse may go to 400%, a dual-element fuse to 225%, an inverse-time breaker to 400% at 100 A or less and 300% above that, and an instantaneous-trip breaker to 1300% on a Design B energy-efficient motor. That exception is not applied automatically on this page, because it is a documented engineering decision about a specific motor rather than a step in the arithmetic — and the first thing to check is whether a fast-acting fuse was fitted where a dual-element one belongs.
What multiplier does a transformer primary fuse take?
125% of rated primary current where that current is 9 A or more and there is no secondary device, or 250% where the secondary is separately protected at 125% — Table 450.3(B), with Note 1 permitting the next standard rating up when 125% does not land on one. Below 9 A the table has two further bands and there is a whole supervised-location column besides, and this site does not reproduce any of the three: those figures could not be confirmed to the standard the rest of its tables are held to, so the page names the gap and sends you to the code book rather than printing a number it cannot stand behind.
Can I fit a fuse where the drawing calls for a breaker?
Not by substitution, because the two have different maximum percentages on the same motor and different physical listings in the same equipment. A dual-element fuse takes 175% where an inverse-time breaker takes 250%, so a fusible switch and a breaker on identical motors are legally different sizes. Beyond that, a fuseholder listed for current-limiting fuses must reject a non-current-limiting one under 240.60(B), and the equipment's own short-circuit current rating may have been established with one device and not the other.
About 240.4(G), and the two jobs one device cannot do
Every conductor in a building is protected at or below its ampacity, and then there are two circuits where it is not. 240.4(G) lists them — motor and motor-control circuits, air conditioning and refrigeration equipment, welders, audio amplifiers, fire alarm and a few others — and hands each to its own article. The reason is inrush. A squirrel-cage motor draws six to eight times its running current for the second or two it takes to come up to speed, and a device that rides through that cannot possibly react to a conductor sitting 10% over its rating for an hour. So Article 430 uses two devices: a large one against short circuits and ground faults, sized from Table 430.52(C)(1), and a small one against overload, sized from the nameplate under 430.32. A fused disconnect with a 40 A non-time-delay fuse and a starter with a 14 A heater in it is not a contradiction; it is the design.
Transformers are the other case, and Article 450 is protecting the transformer rather than anything attached to it. That distinction explains the number people find hardest to accept: a 45 kVA transformer at 480 V three-phase draws 54.1 A on the primary, 125% of that is 67.7 A, and because 67.7 is not a standard rating Note 1 takes it up to a 70 A primary fuse — above the winding’s own rated current by a third. Meanwhile the secondary conductors leaving it may be protected under 240.21(C) at their far end rather than where they originate. If you are sizing the transformer itself rather than its protection, the transformer size calculator rounds a computed kVA to what is actually manufactured, and the kVA to amps calculator gives the primary and secondary currents on their own.
One thing this page cannot do for you is prove the fuse will open. A rating is a promise about steady-state current; interrupting rating is a promise about a fault, and the difference between a 10 kA Class H fuse and a 200 kA Class RK1 in the same holder is invisible until the day it matters. Work out what is actually available at the equipment on the short circuit current calculator before choosing a class. For an ordinary lighting, receptacle or appliance circuit — none of which 240.4(G) touches — the breaker size calculator is the page you want, and the amperage calculator converts a shelf of nameplates into the load figure it starts from. Nothing on this page is a compliance finding: it reproduces code tables and the arithmetic over them, and the person who signs the installation is the one who decides.
Where the nameplate you type goes
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The fuse-class sheet is generated at build time from the table declared in the page, so it prints from a laptop with the network off — which is where most people are standing when they need it.