MUM-3 is the largest IT-load building in this registry: 85 MW at full build, across 130,000 square metres — 1,398,000 square feet — on the Navi Mumbai campus at Rabale, inside the MIDC industrial area. Ground was broken in January 2026 with Stirling & Wilson building it. Delivery is targeted for 2027, and Q1 2028 on two of the operator’s own pages. It is 3.5% of the entire 2,420.4 MW registry in one building, and 2.9 times the floor area of everything Iron Mountain operates in India today. The 120 MVA substation is what makes the number credible: 120 divided by 85 is 1.41, exactly a facility-to-IT ratio for a design PUE around 1.4 with no spare capacity.
1At a glance
2Eighty-five megawatts, and what it is bigger than
| Comparison | MW | MUM-3 is | Note |
|---|---|---|---|
| AirTrunk HYD1, the largest site in the file | 243 | 0.35× — smaller | Facility capacity, at least |
| NTT Mumbai 9, the largest operating building | 62 | 1.37× larger | Facility power, not IT |
| NTT’s eight Mumbai records combined | 161 | 53% of them | Chandivali plus Andheri plus Vikhroli plus Fort |
| Iron Mountain’s six operating buildings | 20.3 | 4.2× larger | One building, four times the company |
| Maharashtra’s counted capacity | 786.4 | 10.8% of the state | Would take the state to 871.4 MW |
| The whole 2,420.4 MW registry | 2,420.4 | 3.5% | One building, one thirty-fifth of India |
Four point two times its own company is the number that matters commercially. Iron Mountain operates 20.3 MW in India after five years and one acquisition. MUM-3 alone is 85 MW. The platform is not growing in India; it is being replaced in India.
Every other figure on this chart is a capacity statement with no power infrastructure attached to it. MUM-3 has a 120 MVA dedicated substation, which is a distribution utility commitment with a physical footprint and an energisation date.
Divide 120 MVA by 85 MW and you get 1.41. That is not a coincidence and it is not a marketing ratio — it is what an engineer sizes a connection to when the building will draw 85 MW of IT load plus cooling and losses.
That single division is why this registry treats 85 MW as a probable number and treats 3 GW in Raigad as a press release.
31.4 million square feet on five acres
MUM-3 is the clearest vertical build in the Indian market, and the arithmetic of floor area against site area proves it without needing a storey count.
| Measure | Value | Reading |
|---|---|---|
| Floor area | 130,000 m² / 1,398,000 sq ft | Published · |
| Site, if the five-acre MUM-3 parcel | 20,234 m² | Acquired for the third building in 2023 · |
| Floor area ratio | 6.4 | Six storeys of full-site coverage · |
| Site, if the whole 5.5-acre campus | 22,257 m² | 5.8 · Five to six storeys with setbacks |
| Existing MUM-1 and MUM-2 combined | 14,923 m² / 160,631 sq ft | What 8.8 MW occupies · |
| MUM-3 relative to that | 8.7× | The same campus, eight times the floorspace · |
| MUM-3 relative to the 44,700 m² platform | 2.9× | One building, three platforms · |
A floor area ratio of 5.8 to 6.4 on a parcel in the MIDC industrial belt means the building is five or six storeys of near-full-site coverage. There is no greenfield alternative on a five-acre site: 85 MW at ground level would need roughly 18 acres at typical low-rise data hall coverage.
The registry does not claim a storey count. It claims the ratio, which is arithmetic from two published numbers, and the ratio only admits one kind of building.
Multi-storey data halls mean vertical chilled-water distribution, heavy floor loadings, and chilled water plant either on the roof or in a basement. They also mean a single point of failure stack — one riser serving five floors is a different risk profile from five independent halls.
That is why the market pays a premium for land: the alternative to five storeys in Rabale is 18 acres somewhere a tenant does not want to be.
4What 85 MW costs
Iron Mountain has not published a capital cost for the 85 MW configuration. The 2023 announcement carried US$170 million for a 32 MW building. Everything below the first row is a Gridvik estimate at a stated rate, and is labelled as one.
| Basis | Rate | 85 MW implies | Status |
|---|---|---|---|
| 2023 announcement, 32 MW | US$170m = US$5.31m per MW | US$451m / ₹3,987 crore | Rate is published; the multiplication is ours |
| AirTrunk programme rate | US$6.0m per MW | US$510m / ₹4,508 crore | Gridvik estimate |
| Per square foot, at the 2023 rate | US$121 per sq ft | US$170m at 32 MW | Coincidence of scale, not a rule |
| Disclosed capex for 85 MW | none | — | The only row that is a fact |
At ₹3,987 to ₹4,508 crore, MUM-3 is a ₹4,000 crore plus single building — which would make it one of the largest single private data centre investments recorded in this registry, alongside NTT’s NAV2 at ₹5,500 crore for 107 MW and NTT Kolkata at ₹2,000 crore for a 40 MW campus.
The reason this matters for a data buyer is different. Do not use the 2023 rate to forecast anything. A 32 MW enterprise colocation building and an 85 MW AI-ready building do not have the same cost curve: the AI building carries liquid cooling readiness, far higher power density per hall, and a much larger substation relative to load.
5What 85 MW does to the registry
| Measure | MW | Share | Note |
|---|---|---|---|
| Maharashtra counted today | 786.4 | — | 88 records |
| Maharashtra with MUM-3 at delivery | 871.4 | +10.8% | One building moves a state by an eighth |
| Registry counted today | 2,420.4 | — | 241 additive records |
| Registry with MUM-3 at delivery | 2,505.4 | 3.5% | Largest single addition possible in the file |
| Iron Mountain India counted today | 11.5 | 0.48% | Three records carry no value |
| Iron Mountain with MUM-3 | 96.5 | 4.0% | The company becomes a top-five platform in one building |
Iron Mountain goes from 0.48% to 4.0% of the national counted total on the delivery of one building. That is not a normal growth step and it is why this record is the hinge of the whole Iron Mountain family in this file.
It also means the registry has a genuine sensitivity: if MUM-3 slips from 2027 to 2030, 85 MW of nationally counted capacity moves with it, and no other single record in the 242 can absorb that swing.
6Procurement
7Limits and open questions
- Nothing is built. Ground was broken in January 2026; no capacity, no phase schedule and no tenant is published.
- Delivery is 2027 or Q1 2028 depending on which operator page is read. Both are live simultaneously.
- The site is ambiguous. The 2023 announcement describes a five-acre parcel acquired for the third building; the campus is separately described as 5.5 acres. The floor area ratio is 6.4 on the five acres and 5.8 on the campus.
- No capex is disclosed for 85 MW. Every rupee figure beyond the US$170 million for 32 MW is a Gridvik estimate at a stated rate.
- 85 MW is IT load as stated by the operator, which is unusual in this registry and rests on the 120 MVA substation inference.
- The 1.41 ratio assumes a power factor of 1.0. At 0.95 the implied ratio is 1.34, which is conservative rather than exact.
- A floor area ratio is not a storey count. Five to six storeys is the only design that fits, but the count itself is inferred.
- What would close it. A phase schedule with megawatts attached to years. Nothing else in this record is missing.
8Sources
Sources: the Iron Mountain MUM-3 and Mumbai location pages; the Iron Mountain Data Centers India platform page; the January 2026 MUM-3 groundbreaking release; Data Center Dynamics on the groundbreaking; W.media; ETTelecom; Communications Today; Structure Research; the Baxtel MUM-3 entry; and the June 2023 Web Werks and Iron Mountain announcement.