MICC Cable (Pyro): The Fire-Survival Cable Explained

An emergency lighting circuit goes in at first fix on standard fire alarm cable. Two years later the building's fire risk assessment flags it: the circuit was specified to survive a fire for 30 minutes, but the cable carries no fire-integrity rating. Ripping out a buried circuit through finished plasterboard costs several times what the right cable would have cost. Knowing when MICC is genuinely required, and when it is overkill, saves that mistake.

What it is and what it's for

MICC stands for Mineral Insulated Copper Clad cable. On site almost nobody calls it that. It is "pyro", a name borrowed from Pyrotenax, the original brand that dominated the market for decades. You will also hear "MI cable" or "fire-survival cable".

The construction is unusual and worth understanding, because it explains everything else about the cable. Solid copper conductors run down the centre. Around them, packed tight, is magnesium oxide (MgO) powder, a white inorganic mineral. The whole lot is sealed inside a drawn copper tube, which is then drawn down so the powder is compressed to a dense, rock-hard mass. There is no plastic anywhere in the core of the cable.

That matters because plastic insulation, the PVC or cross-linked polyethylene found in ordinary twin-and-earth or armoured cable, melts and burns. Once the insulation goes, the conductors short or open and the circuit dies. Magnesium oxide does neither. It does not burn, melt at any temperature a building fire reaches, or break down. A MICC circuit keeps conducting at temperatures above 1000°C, long after every plastic-insulated cable around it has failed. That single property, circuit integrity under fire, is the entire reason the cable exists.

In the UK, circuits that must keep working during a fire are governed by the wiring regulations BS 7671 alongside the fire detection standard BS 5839 for alarm systems. The fire-resistance performance of the cable itself is proven against BS EN 50200 (the test that subjects a live cable to flame and mechanical shock) or the older, more demanding BS 6387 categories, of which Category CWZ is the toughest, combining fire, water spray and mechanical impact.

Types, sizes and specifications

MICC is sold by conductor size and by the number of conductors in the cable. The common copper-conductor sizes are 1mm², 1.5mm², 2.5mm², 4mm² and 6mm², the same cross-sectional areas you would recognise from ordinary cable. The conductor count runs single, two, three or four cores, written as 2L1 (two-core, light duty), 3L2.5 and so on, where the number before the L is the core count and the number after is the conductor area.

Two outer-sheath options exist, and choosing the wrong one causes problems.

The copper sheath doing double duty as the circuit protective conductor (the earth) is a real practical advantage. It means a two-core MICC cable already has its earth built in, so it carries the same circuit as a three-core ordinary cable in a smaller package.

What the size tells you in practice is current capacity, the same as any cable. A 1.5mm² MICC will run a lighting-load emergency circuit comfortably. You step up to larger sizes for smoke extract fans or fire suppression pumps that draw real current. The electrical designer specifies the size from the load and the run length, exactly as with standard cable, with one twist: MICC carries more current for its size than PVC cable because the mineral insulation tolerates far higher temperatures, so the designer may be able to use a smaller conductor than the equivalent twin-and-earth.

How to work with it

This is where MICC stops behaving like normal cable and where a homeowner needs to understand why it is a specialist job.

MICC is stiff. It is a solid copper tube, so it holds whatever shape you bend it into, which makes for tidy, self-supporting runs. But that stiffness has a hard limit: bend it too tight and the magnesium oxide core cracks. Once the MgO is fractured, the compression that holds the powder dense is lost, moisture can get in, and the cable's insulation resistance collapses. A cracked run looks identical to a good one from the outside and only shows up on testing. The accepted minimum bending radius is around six times the cable diameter, and bends are formed slowly and deliberately, often with a bending tool, never yanked round a corner.

Termination is the part that catches people out. You cannot strip MICC and push it into a terminal like twin-and-earth. Every single connection point needs a proper pot-and-seal assembly. The process: the copper sheath is cut and ringed, a brass pot is screwed onto the threaded sheath, the loose MgO powder inside is cleaned out, and the pot is filled with a sealing compound and capped with a disc that the conductors pass through as insulated tails. A specialist crimping tool, the pot plier, swages the seal. Done correctly the termination is moisture-tight and the insulation resistance reads as it should. Done badly, it leaks moisture and fails.

Storage is simpler. Bare-copper MICC keeps fine in the dry, but the cut ends must be sealed or temporarily capped to keep moisture out before termination. It does not degrade in sunlight the way some plastics do, and it is mechanically tough. None of this is homeowner work. Terminating MICC is a skill a competent electrician either has or sends to someone who does.

How much do you need

You do not size a MICC run by guesswork, and there is no general "blocks per square metre" rule to apply. The quantity is whatever the fire-survival circuit needs: the cable follows the route from the consumer unit or fire panel to each emergency luminaire, sounder or fan, plus a sensible allowance for the drops, bends and the slack consumed at each pot termination.

A worked example. Suppose an emergency lighting circuit serves four bulkhead fittings down a single-storey extension and an escape route, with the cable clipped along the wall just below ceiling level. Measure the run from the supply to the first fitting, then fitting to fitting, then add the vertical drop to each. If that totals 38 metres, add roughly 10% for terminations and routing reality and order 42 metres. Round up to the drum or cut length the wholesaler offers. MICC is too expensive to over-order by a wide margin and too specialist to nip out and buy more of mid-job, so the design length wants to be right before you order.

Cost and where to buy

There is no getting around it: MICC is expensive compared with ordinary cable, and the terminations add labour on top of the cable cost. Where a 100m drum of 1.5mm² twin-and-earth is a modest spend, the equivalent length of two-core MICC, plus a pot-and-seal kit for every termination, runs to several times that. The exact figure depends on size, core count and whether you need the oversheathed version, so get a quote against the actual specification rather than a headline rate.

It is also a specialist supply. The big retail sheds and the trade counters at Screwfix and Toolstation stock what they label "fire alarm cable", which is a PVC-insulated, fire-resistant single-strand cable to BS 5839, an entirely different product that is fine for many alarm wiring jobs but is not MICC and does not have MICC's temperature performance. For genuine MICC you go to electrical wholesalers: CEF (City Electrical Factors), R&M Electrical, and similar suppliers who deal with commercial and fire-systems work. They sell it by the metre or on drums and stock the pot-and-seal kits and tooling alongside.

External resource

CEF - City Electrical Factors

National electrical wholesaler that stocks MICC cable, sealing pots and glands at trade counters. Useful for checking specialist availability against a fire-circuit specification.

cef.co.uk

Because it is specialist stock, lead time matters. A wholesaler may need to order in a particular size or oversheathed variant, so factor that into the first-fix programme rather than assuming it is on the shelf.

Alternatives

MICC is not the only way to build a fire-survival circuit, and for many domestic and light-commercial jobs it is not the one specified anymore.

The modern alternative is enhanced fire-resistant cable, sold under names like FP200, FP400 and similar. These use a fire-resistant insulation (commonly a mica-glass tape and an insulating polymer) inside a low-smoke sheath, and they carry the same BS EN 50200 or BS 8434 fire-integrity ratings. They are far easier to handle than MICC: flexible, terminated like ordinary cable with no pots or special tools, and considerably cheaper. For most extension-scale emergency lighting and fire alarm wiring, an FP-type cable is what an electrician reaches for first.

So why does MICC still exist? It outlasts the polymer cables at the highest temperatures and the longest fire durations, it is mechanically far tougher, and the all-copper construction gives it a service life measured in many decades. Where a circuit must survive a severe, prolonged fire, or in industrial and high-spec applications, MICC remains the benchmark. For a single-storey domestic extension, the designer will almost always be content with an FP-type cable unless something unusual drives the specification.

Where you'll need this

In a typical domestic extension you will not touch MICC for ordinary power and lighting. Standard twin-and-earth runs every socket, light and cooker circuit. MICC, or more often its FP-type alternative, only appears if a fire-survival circuit is required, and that is driven by your building control officer or fire risk assessment, not by choice. The usual triggers in a home extension are an emergency lighting circuit on an escape route, or a Grade D2 linked smoke alarm system with a maintained supply where the officer asks for fire-rated wiring.

• First fix electrics - where any fire-survival circuit is run, clipped and left ready before the walls are closed
• Electrical layout planning - the stage to confirm with building control whether any circuit needs fire-rated cable, before cable is ordered
• Building control final inspection - where emergency lighting and fire-alarm provisions are checked against what was specified

The need for fire-survival cable arises across any extension, conversion or renovation where a protected escape route or a linked alarm system is part of the fire strategy, not only in kitchen extensions.

Common mistakes

The classic error is the one in the opening: buying PVC "fire alarm cable" off the shelf and assuming it satisfies a fire-survival requirement. It does not. Fire alarm cable to BS 5839 and a genuine fire-survival cable rated to BS EN 50200 or BS 6387 are different products, and only the specification on the drawing tells you which the circuit actually needs.

The second mistake is specifying MICC out of caution when an FP-type cable would have been accepted, then paying several times over for the cable and the specialist terminations. Confirm what building control actually requires before ordering. The third is leaving the decision until first fix is underway, then discovering the specialist cable is not in stock and the programme slips while it is ordered in.