Fuse sizing equations and limits
Fuse selection is a constrained calculation: the fuse must carry the design current without exceeding the protected cable or PV module limit, and its breaking capacity must exceed the prospective fault current.
Id = Ib × KIb ≤ In ≤ IzId,pv = Isc × KpvIcn,fuse ≥ Isc,prospectivewhere:
- Ib
- Normal circuit design or load current[A]
- Id
- Corrected current after the selected design factor[A]
- In
- Fuse-link rated current[A]
- Iz
- Corrected conductor ampacity[A]
- Isc
- PV module short-circuit current or prospective system fault current, as identified by the subscript[A or kA]
- K
- Applicable current or design factor
These equations shortlist a rating. Exact motor-starting tolerance, clearing time, selectivity, peak let-through current, and I²t require the selected manufacturer's curves and coordination tables.
How to size a fuse
Choose the application first because gG, aM, and gPV fuse links do different jobs. Enter the real load or module current, apply the justified design factor, then check the rounded standard rating against the protected conductor or module limit.
- Choose gG, aM, or gPV duty.
- Calculate or enter the design current.
- Round up to a standard fuse rating.
- Check cable or module limits.
- Verify voltage, breaking capacity, holder, curve, and temperature.
gG fuse sizing example
A 32 A general load with a 125% factor creates a 40 A corrected current. A 40 A gG fuse is the first listed rating, provided the corrected cable ampacity is at least 40 A and the time-current requirements are satisfied.
Id = 32 A × 1.25 = 40 A
32 A ≤ 40 A ≤ cable ampacity
Motor and PV fuse selection
An aM motor fuse must tolerate the real starting-current profile while a separate overload relay protects the motor against sustained overload. A gPV fuse must carry the PV design current, remain within the module maximum series fuse rating, and interrupt DC faults at the array voltage.
| Category | Primary duty | Mandatory follow-up |
|---|---|---|
| gG | Full-range cable and feeder protection | Cable coordination and curve |
| aM | Motor short-circuit protection | Overload relay and start curve |
| gPV | PV string protection | Module limit and DC rating |
Checks after fuse calculation
A current rating alone does not specify a fuse. Confirm utilization category, AC or DC voltage, breaking capacity, physical size, holder compatibility, power dissipation, ambient derating, time-current curve, I²t, selectivity, and any tested combination rating before ordering.
Assumptions
- IEC-style utilization categories
- Standard current ratings are used for preliminary rounding
- PV reverse-current contribution is approximated from the other parallel strings
Important Warnings
- Do not treat the calculated rating as proof that a fuse will ride through a motor start; verify the actual aM curve at the stated current and time.
- The fuse voltage rating, DC capability, breaking capacity, holder, size, power dissipation, ambient temperature, and manufacturer data must all match.