What the Electrical Fuse Size Calculator does
This calculator picks a fuse rating for a cable or a motor circuit, chooses between the gG and aM utilisation categories, and runs the three checks that matter: that the fuse is at least the design current, that it does not exceed the cable's derated capacity, and that it still provides overload protection under the 1.45 x Iz condition. It also checks discrimination against an upstream fuse.
The third check is the one that catches people moving from breakers to fuses. A gG fuse's conventional fusing current is 1.6 times its rating, not 1.45, so In <= Iz is not enough on its own - the real overload ceiling is 0.906 x Iz, and above it the fuse still clears short circuits but no longer protects the cable against a sustained overload.
How to use it
- Enter the design current and the cable's capacity after derating, from the cable size calculator.
- Say whether you are protecting a cable or a motor circuit. A motor circuit needs a fuse oversized enough to survive starting, which is why it defaults to a 1.6 multiplier.
- Leave the category on automatic to get gG for a cable and aM for a motor, or force gG if you want a full-range fuse on a motor circuit.
- Enter the upstream fuse rating for the discrimination check, or leave it blank if there is none.
- Read every finding. A rating that passes the first two checks and fails the third is short-circuit protection only, and the cable needs protecting some other way.
Reading the results
gG is the general-purpose full-range class: it protects against both overload and short circuit, and it is what a cable or distribution circuit normally gets.
aM is a back-up class: it protects against short circuit only and will not clear a modest overload at all. It is used on motor circuits alongside an overload relay, which does the overload job.
The gG overload ceiling, 1.45/1.6 x Iz, is the largest gG fuse that still protects this cable against overload. Above it the circuit needs another means of overload protection.
The discrimination ratio is upstream rating over downstream rating. 1.6:1 or better is the long-standing rule of thumb for gG fuses, but the definitive check is the two devices' let-through energy curves.
Worked example: a 40 A circuit on a cable derated to 57 A
Design current 40 A, cable capacity 57 A, general cable circuit, so the multiplier is 1 and the target is 40 A. The IEC gG series holds 40 A exactly, so that is the rating.
Check one: 40 >= 40. Passes. Check two: 40 <= 57. Passes.
Check three, the one that is specific to fuses: the gG overload ceiling is 1.45 / 1.6 x 57 = 51.66 A. At 40 A the fuse is well inside it, so it gives full overload protection to the cable. Equivalently, I2 = 1.6 x 40 = 64 A against 1.45 x 57 = 82.65 A.
Now push the design current to 52 A on the same cable. The next gG rating is 63 A, which is above both the 57 A capacity and the 51.66 A ceiling, so two checks fail: 63 A does not protect a 57 A cable at all. The cable has to grow, or a 50 A fuse has to be accepted with the knowledge that it will not carry 52 A indefinitely.
With a 100 A upstream fuse the discrimination ratio is 100 / 40 = 2.5:1, comfortably above the 1.6:1 rule of thumb - so a fault on this circuit should clear on the 40 A fuse without disturbing the upstream one.
Formulas and scoring rules
- The coordination rule
Ib <= In <= IzThe same rule as for a breaker: design current, fuse rating, cable derated capacity.- Conventional tripping condition
I2 <= 1.45 x IzIEC 60364-4-43.- gG conventional fusing current
I2 = 1.6 x InFrom IEC 60269-1. This is what makes the fuse case different from a breaker's 1.45 x In.- gG overload ceiling
In <= (1.45 / 1.6) x Iz = 0.906 x IzThe two conditions above, combined.- Motor circuit sizing
In >= 1.6 x motor full-load current (IEC practice), or per NEC 430.52 for the fuse class in useThe overload relay, not the fuse, protects the motor against overload.- Discrimination rule of thumb
upstream rating / downstream rating >= 1.6For gG fuses. The definitive check is total I²t downstream below pre-arcing I²t upstream.
Why fuses are still used
A fuse has no moving parts, no mechanism to maintain and nothing to go out of adjustment. On a heavy fault it is faster than almost any circuit-breaker: an HRC fuse can clear within the first quarter cycle, before the current has reached its prospective peak, so the let-through energy that reaches the cable and the equipment is a small fraction of what a breaker would allow.
That current-limiting behaviour is why fuses still protect semiconductor converters, large feeders and anything where a high fault level would otherwise demand very expensive switchgear. The breaking capacity of a common HRC fuse - 80 kA or 120 kA - is far beyond what most moulded-case breakers manage.
The cost is obvious: a blown fuse has to be replaced, in the dark, by someone with the right spare. Which is why final circuits in most modern installations use breakers and the upstream distribution often does not.
gG and aM, and the mistake in between
The two-letter code in IEC 60269 describes both what the fuse can break and what it is meant to protect. The first letter is the breaking range: g means full range - it will clear anything from its conventional fusing current up to its breaking capacity - while a means partial range, clearing only currents well above rated. The second letter is the object protected: G for general, M for motor circuits, R for semiconductors, Tr for transformers.
The mistake is fitting an aM fuse and treating it as complete protection. An aM fuse will sit unbothered through a 50% overload for as long as you like, because it is not designed to interrupt it. On a motor circuit that is correct behaviour - the overload relay is doing that job - but without a relay, an aM fuse leaves the motor and the cable with no overload protection at all.
Limitations: what the result does not prove
- It applies the ratio rules. It does not read real time/current or let-through energy curves, so it cannot confirm how quickly a particular fuse clears a particular fault.
- Discrimination is checked by the rule-of-thumb ratio only. Real discrimination between fuses, and between a fuse and a breaker, is settled by comparing published energy curves.
- Motor circuit protection ultimately depends on manufacturer-tested coordination between the fuse, the contactor and the overload relay.
- The breaking capacity of the fuse against the prospective fault current is not checked here; use the short-circuit calculator for that figure.
- Semiconductor protection (aR and gR fuses) is a specialised case with its own selection method and is not covered.
- Nothing here is a compliance certificate. A qualified engineer must verify the design against the applicable standard and the local regulations.
Privacy: where your data goes
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Standards and sources
- IEC 60269-1 - Low-voltage fuses: general requirements - checked 19 Sep 2026
- NFPA 70 - National Electrical Code - checked 19 Sep 2026
- IEC 60269-2 - Supplementary requirements for fuses for use by authorised persons (gG and aM)
- IEC 60364-4-43 - Protection against overcurrent
Frequently asked questions
What is the difference between a gG and an aM fuse?
A gG fuse is full-range: it clears both overloads and short circuits, so it can protect a cable on its own. An aM fuse is partial-range: it only clears short circuits and deliberately ignores overloads, so it is used on motor circuits where an overload relay does that job. Fitting an aM fuse without a relay leaves the circuit unprotected against overload.
Why can a fuse rating not simply go up to the cable capacity?
Because a gG fuse blows at 1.6 times its rating rather than 1.45, so the condition I2 <= 1.45 x Iz means the rating must be below 0.906 x Iz. A 63 A fuse on a 65 A cable satisfies In <= Iz but will let the cable sit at 100 A for an hour before clearing, which is not overload protection.
What size fuse do I need for a motor?
Large enough to survive the starting current, which means roughly 1.6 to 2 times the motor's full-load current for an aM fuse under IEC practice, or the multipliers NEC 430.52 sets out for the fuse class in use. The fuse is short-circuit protection only; the overload relay, set from the motor's actual full-load current, is what protects the motor.
What discrimination ratio do fuses need?
A ratio of 1.6:1 or better between gG fuse ratings is the long-standing rule of thumb for full discrimination. It is only a rule of thumb: what actually decides it is whether the downstream fuse's total let-through energy stays below the upstream fuse's pre-arcing energy across the whole range of fault currents, which the manufacturers publish as curves.
Are fuses better than circuit breakers?
They are better at some things. A fuse is faster on a heavy fault, limits let-through energy far more sharply, and has breaking capacities of 80 to 120 kA that most breakers cannot match. A breaker resets, can be switched deliberately, can carry earth-fault protection, and cannot be replaced with the wrong rating by someone in a hurry. Most installations use both, in different places.
Can I replace a fuse with a higher-rated one to stop it blowing?
No. The rating is tied to the cable's capacity by the checks on this page, and raising it removes the protection rather than the problem. A fuse that blows repeatedly is reporting an overload, a fault, or a starting current the fuse class was never chosen for - and the right response is to find out which.
What does the I²t figure on a fuse mean?
Let-through energy in ampere-squared seconds: the energy the fuse allows to pass before it clears. It is the quantity that damages cables and semiconductors, and it is the input to the adiabatic equation when sizing a protective conductor. Manufacturers publish pre-arcing and total I²t curves, and a current-limiting fuse's figures are far below what the prospective current would suggest.
Do fuses need derating in a hot enclosure?
Yes. A fuse's rating is established at a reference ambient, and inside a warm enclosure or a densely packed switchboard it will operate at a lower current than its marking. Manufacturers publish correction factors, typically a few per cent per 10 degrees above reference, and they are not modelled here.
Last reviewed by the A2Z.Tools team against the sources listed above.
