How to choose NH fuse size NH00 to NH3 correctly?
The right NH size decides current capacity, fit in the fuse base and heat loss inside the cabinet. This guide sorts NH00 to NH3 by rated current and mounting space and explains selection to IEC 60269-2.
View NH fusesWhich NH sizes exist and what for?
NH fuses (low-voltage high-breaking-capacity fuses) are graded to IEC 60269‑2 in sizes from NH000 to NH4a. Each size has a fixed blade contact spacing and fits only its matching fuse base. The size therefore sets the maximum possible rated current, not the desired value on its own.
In everyday distribution work NH00, NH1, NH2 and NH3 dominate. NH000 and NH00 share the same blade spacing but differ in body height and current range. Larger sizes offer more contact area and breaking capacity but need more space and higher rated currents so that the power loss can be dissipated.
How do you pick a size by rated current?
The starting point is the rated current of the equipment and the permissible current of the cable to be protected. The fuse is chosen so its rated current sits above the operating current but below the current-carrying capacity of the cable. Only then does the size follow that can actually carry this rating.
- Set the utilisation category: gG/gL for full-range cable protection, aM for motor short-circuit protection.
- Choose the rated current In so that Ib ≤ In ≤ Iz (operating current, rated current, cable capacity).
- Take the smallest size that safely covers the required In - this saves space and cost.
- Check selectivity to the upstream fuse, usually a ratio of 1.6 between the rated values.
- Mind the rated voltage (400/500/690 V AC) and breaking capacity (up to 120 kA).
What matters about mounting space?
The size sets body height, reach and the required contact area of the terminals. NH switch-disconnector strips for busbar systems (60 mm system) usually accept NH00 to NH3, but not every strip carries every rated current. Adequate clearance and ventilation are mandatory because every fuse gives off power loss as heat.
- The marking strip shows rated current and utilisation category - check before replacing.
- Prefer matching base, link and strip from the same manufacturer for reliable compatibility.
- Only choose a reserve size if strip and cable can carry it thermally.
Frequently asked questions
Can I fit an NH2 link into an NH1 base?
No. NH1, NH2 and NH3 have different blade contact spacings and body heights and are not interchangeable. Only NH000 and NH00 share the same blade spacing.
Which size for 250 A?
250 A sits at the top of NH1 and within the NH2 range. NH2 offers more reserve and lower heating, while NH1 is more compact. The choice depends on the strip, cable cross-section and heat removal.
What does utilisation category gG mean?
gG (formerly gL) is a full-range fuse for general cable protection. It guards against both overload and short circuit. aM by contrast only protects the short-circuit range, for example on motors.
Why not always take the largest size?
An oversized body costs space and money and can spoil selectivity. Choose the smallest size that safely and thermally carries the required rated current.
Looking for the right NH fuse?
We supply NH fuse links and bases in sizes NH00 to NH3 in gG and aM - tested to IEC 60269-2.
Standard compliant
All NH links meet IEC 60269-2.
Full range
NH00 to NH3 in gG and aM from stock.
Safely matched
Bases, links and strips coordinated with each other.
Expert advice
Specialists help with size and selectivity.