13.1 Inverter Specification and Location

A grid-connected inverter is not a box you hide wherever the DC cables happen to finish. City & Guilds 2922 handbook v1.3 learning outcome 4.3.2 treats inverter specification and location as a design decision: the unit must be installed in a location that is suitable both to its own design and to the manufacturer's specifications. Get that wrong and you invite thermal tripping, water ingress, unserviceable isolators, unnecessarily long DC runs, and fire-spread arguments that an MCS audit or a fire investigation will not treat as a minor paperwork miss.

Why location is on the 2922 paper

The inverter converts array DC to AC at grid voltage and frequency, dissipates heat, and is a point of isolation. Handbook 4.3.2 lists eight consideration groups. Examiners expect you to treat them as a checklist, not as optional extras you apply only when the homeowner complains about noise.

Indoor or outdoor location and IP ratings

Ingress protection (IP) is classified under IEC 60529. An indoor utility-room chassis might be IP20 or IP21: protection against fingers and vertically dripping water only. An outdoor wall-mounted string inverter is commonly IP65: dust-tight and protected against water jets. Matching the rating to the location is not a brand preference. An IP21 inverter on an unsheltered north wall will corrode, trip on moisture, and fail isolation confidence. An IP65 inverter in a dry plant room is electrically acceptable, but you still apply the manufacturer's ambient, clearance, and mounting rules — outdoor rating does not waive fireproof backing or ventilation.

Garages, outbuildings, and coastal sites add dust, salt, and vehicle spray. Condensation forms when a cold chassis meets moist air. Outdoor enclosures often include drain paths that must not be blocked with silicone 'to make it neater'. If the manufacturer publishes separate indoor and outdoor variants, you do not make do with the cheaper indoor model behind a plywood hood and call it weatherproof.

Ventilation and thermal derating

Inverters are heat engines. Manufacturers publish an operating ambient range and a derating curve: above a stated temperature, often around the mid-40s °C, AC output is reduced to protect semiconductors. Airing cupboards, unventilated lofts in high summer, and cupboards packed with towels destroy that curve. Clearance distances on the datasheet — a stated gap above the heat sink and at the sides — are part of the specification. Covering the unit with loft insulation, or mounting it in a sealed riser, is a location fail even if the IP rating looks adequate on paper.

Dust on a heat sink is a slow thermal fault. A garage inverter above a workbench that throws plaster dust needs a cleaning access path, which is why access for servicing sits next to ventilation in the handbook rather than in a separate 'nice to have' list.

Access for servicing

You must be able to read the display, operate AC and DC isolators, apply a lock-off, replace the unit, and inspect labels. A loft hatch with no boarded standing area, a chassis mounted at 3.5 m with no means of access, or an inverter jammed behind a stacked freezer does not meet 4.3.2. Service access also drives the AC isolation outcome in 4.4.1: if the inverter is in a different room from the consumer unit, you will need a local AC isolator (section 13.2). Do not 'solve' access by leaving isolators unlabelled in a dark eaves void and hoping the next person has a head torch.

Proximity to DC components

Keep the inverter close to the array isolator, strings, and any DC combiner so that DC cable length is minimised. Long DC runs increase voltage drop, I²R loss, electromagnetic disturbance, and the length of conductor that remains live in daylight after AC isolation. The 2922 DC-cable content already asked you to consider circuit length; 4.3.2 repeats the point from the inverter end. A plant-room inverter under the array roof slope is usually better than a hallway cupboard that needs 25 m of DC in the fabric, even if the hallway looks tidier.

Weight, fixing, flammability and fireproof mounting

String inverters are heavy relative to a lighting accessory. Fixings must match the substrate: sound masonry or a manufacturer-specified mounting plate, not hollow plasterboard without noggins. Pull-out and shear loads include the cable bundle and any snow or wind load on an outdoor hood.

Manufacturer flammability requirements are explicit in 4.3.2(vi), including fireproof mounting. If the manufacturer requires a non-combustible or fire-rated backing, you do not mount the chassis on combustible lining — timber matchboarding, foam-backed plasterboard, or decorative cladding. Use the specified fire-rated board or masonry. This is a specification item, not optional styling. Do not mount on combustible lining where the manufacturer requires fire-rated backing.

Labelling and other manufacturer requirements

Even at siting stage, the location must allow the 4.5 labels (section 13.4) to remain readable: the inverter isolation warning, ON/OFF indication, and manufacturer plates. Other manufacturer requirements include altitude limits, exclusion of corrosive atmospheres, residual-current type, whether the unit may sit in a habitable room, and gland entry direction. Follow the installation manual. 2922 will not reward 'we always put them in the airing cupboard'.

MCS 5.6.7 — arc-fault detection

MCS 5.6.7 requires that where an arc-fault detection function is present in the inverter, it shall be enabled. Do not disable arc-fault detection to silence nuisance trips. Investigate loose DC connectors, damaged insulation, and poor crimps. The 2922 handbook points you at manufacturer location rules; MCS adds this inverter-function rule on top. They work together: a well-sited inverter with arc-fault detection left off is still an MCS non-conformance.

Location-check table

Scenario: airing cupboard versus garage masonry versus outdoor north wall

A 3.6 kW G98 string inverter needs a home. The airing cupboard is warm, close to the cylinder, often timber-lined, poorly ventilated, and used for storage. Ambient temperature will sit on the derating curve. Combustible lining fights the fireproof-mounting rule. Service access disappears behind towels. Treat it as unsuitable unless the manufacturer explicitly permits that environment and you can prove fire backing, clearance, and access — most will not.

A garage masonry wall is usually the stronger indoor option: cooler, solid fixings, space for isolators, and a shorter route from a gable array. Still check IP for dust and damp, vehicle impact, frost, and whether the garage is treated as a habitable space if the manufacturer forbids that. Keep DC entry short through the garage roof space rather than looping DC through the house to 'hide' the inverter.

An outdoor north wall avoids solar gain, which helps thermal derating, but it demands a true outdoor IP rating, UV-stable glands, vermin protection, and a fire-rated backing if specified. Isolation handles must remain operable in wet weather and lockable in the off position. Do not site the inverter where a ladder cannot be used safely, or where labels face a fence 100 mm away.

Choose the location that satisfies the whole 4.3.2 list, not the one that hides the inverter from the homeowner.