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Insight · grid stability

The 900 MW limit: why Ireland's grid can't lose more data-centre demand at once

EirGrid caps the demand Ireland's grid can lose in one fault at 900 MW. It plans for 90% of data-centre demand dropping off at once, so about 1,000 MW of data centres fills the limit, and EirGrid told the CRU that was reached in June 2026 (CRU202704, Jul 2026). The CRU's fix, Grid Code MPID 345, applies from 27 October 2026.

By StackPath · published 3 October 2026 · updated 10 October 2026 · how we know · corrections

What is the 900 MW load-rejection limit?

Every week EirGrid and SONI publish the operating limits they run the all-island system to. Since the update of 9 January 2026 one of them reads "Maximum total system load rejection": 900 MW. It is not in the update of 2 January 2026 (2026 week 2); the weeks in between were not found online. The latest update, week 42 of 2026 (published 9 October 2026), still lists it.

"Load rejection" means demand vanishing from the grid. Lose too much at once and generators speed up, frequency rises and protection can trip plant. The limit covers data centres and other large users together with power being exported on the interconnectors, because losing an export looks the same to the system as losing demand.

900 MW
Most demand the grid can lose at once
EirGrid/SONI, week 42 2026
~1,000 MW
Data-centre peak, Jun 2026
EirGrid via CRU202704
90%
Share EirGrid assumes could drop in one fault
CRU202704
480 MW
Largest recorded drop, Jun 2026
CRU202704 Table 1

Why data centres fill it

When a fault dips the voltage, a data centre protects its servers by switching to its own batteries and generators. To the grid, its demand disappears in milliseconds and comes back later. EirGrid plans for a worst case in which 90% of all data-centre demand goes at once. At a data-centre peak of about 1,000 MW, 90% is 900 MW: the whole limit.

"the 900 MW load rejection limit was reached" (CRU202704)

Real faults have not yet reached that worst case, but each one has taken more demand off the grid:

DateFaultData-centre demand lost, MWShare of data-centre demand
7 January 2022Limerick: Killonan-Kilpaddoge 220 kV Fault7416%
13 December 2022Dublin: Kellystown-Woodland 220 kV Fault20434%
26 January 2025Dublin: Poolbeg 220 kV Reactor Fault32144%
8 May 2025Dublin: North Wall – Poolbeg 220 kV Fault38752%
6 June 2026Dublin: Poolbeg 220 kV Reactor Fault48052%

EirGrid figures as tabled in CRU202704 (Table 1), 8 July 2026.

Figure 1

Why the grid can only lose 900 MW at once

In a voltage dip data centres can drop off the grid in milliseconds. EirGrid plans for a worst case in which 90% of their demand goes at once (recorded faults dropped 16–52%), so about 1,000 MW of data centres fills the 900 MW limit, and that happened in Jun 2026.

What a voltage dip does to data-centre demand, and the 900 MW limitFour steps: a grid fault dips the voltage; data centres switch to back-up power and about 90% of their demand drops off; frequency rises; they reconnect. Below, a bar shows peak data-centre demand of about 1,000 MW, of which 900 MW could drop, filling the 900 MW load-rejection limit. Then four measures EirGrid uses to stay inside it.01 · 0 MSvoltageA fault on the gridA short circuit on ahigh-voltage line pulls thevoltage down for a fractionof a second. 4 of the 5faults EirGrid has reportedwere in Dublin.02 · MILLISECONDSdata-centre grid demandData centres let goTo protect their servers,sites switch to batteries(UPS) and back-up generators.In some faults, EirGridestimates, up to 90% could goat once.03 · NEXT SECONDSfrequency (50 Hz dashed)Frequency jumpsSupply now exceeds demand, sofrequency rises above 50 Hz.Plant must ride throughchanges up to ±1 Hz/s.04 · THENdemand returns in a rushThey come backSites reconnectautomatically: a second jolt.The CRU's December 2025decision records EirGrid'sconcern that this makesdisturbances worse.The budget: how much demand the grid can lose in one goData-centre demand peaked at about 1,000 MW in late June 2026 (EirGrid to the CRU, CRU202704).90% of 1,000 MW = 900 MW that a single fault could take off the grid.900 MW could drop (90%)limit 900 MWpeak data-centre demand ~1,000 MW (Jun 2026)Reached in Jun 2026. Interconnector exports count against the same 900 MW, so EirGrid cuts exports when data-centredemand is high. A 100 MW campus without ride-through would add about 90 MW to the budget.How EirGrid stays inside it (week 42, 2026)28,000 MWsInertia floorwas 23,000; morespinning plant kept on75%SNSP capinstant share of wind,solar and HVDC imports;80% trial paused200 MWBattery charging capkept as a shockabsorberExports cutInterconnector exportscount against the samelimit
What a voltage dip does to data-centre demand, and the 900 MW limitFour steps: a grid fault dips the voltage; data centres switch to back-up power and about 90% of their demand drops off; frequency rises; they reconnect. Below, a bar shows peak data-centre demand of about 1,000 MW, of which 900 MW could drop, filling the 900 MW load-rejection limit. Then four measures EirGrid uses to stay inside it.01 · 0 MSvoltageA fault on the gridA short circuit on ahigh-voltage line pulls thevoltage down for a fractionof a second. 4 of the 5faults EirGrid has reportedwere in Dublin.02 · MILLISECONDSdata-centre grid demandData centres let goTo protect their servers,sites switch to batteries(UPS) and back-upgenerators. In some faults,EirGrid estimates, up to90% could go at once.03 · NEXT SECONDSfrequency (50 Hz dashed)Frequency jumpsSupply now exceeds demand,so frequency rises above 50Hz. Plant must ride throughchanges up to ±1 Hz/s.04 · THENdemand returns in a rushThey come backSites reconnectautomatically: a secondjolt. The CRU's December2025 decision recordsEirGrid's concern that thismakes disturbances worse.The budget: what the grid can lose at onceData-centre demand peaked at about 1,000 MW in late June 2026 (EirGrid to the CRU, CRU202704).90% of 1,000 MW = 900 MW that a single fault could take off the grid.900 MW could drop (90%)limit 900 MWpeak data-centre demand ~1,000 MW (Jun 2026)Reached in Jun 2026. Interconnector exports count againstthe same 900 MW, so EirGrid cuts exports when data-centredemand is high. A 100 MW campus without ride-through wouldadd about 90 MW to the budget.How EirGrid stays inside it (week 42, 2026)28,000 MWsInertia floorwas 23,000; morespinning plant kept on75%SNSP capinstant share of wind,solar and HVDC imports;80% trial paused200 MWBattery charging capkept as a shock absorberExports cutInterconnector exportscount against the samelimit

When a fault dips the voltage, data centres protect their servers by switching to on-site batteries and generators, so their demand can vanish from the grid in milliseconds and frequency jumps. EirGrid's weekly operating rules cap the demand and exports the system can lose in one event at 900 MW; planning for 90% of data-centre demand at risk, roughly 1,000 MW of data centres uses it all, and EirGrid told the CRU that peak was reached in late June 2026. To stay inside the limit EirGrid keeps more spinning plant on (inertia floor 28,000 MWs), caps the instantaneous share of wind, solar, batteries and HVDC imports (SNSP) at 75%, limits battery charging and cuts interconnector exports. The step sketches are schematic, not measured traces.

Estimate"A 100 MW campus adds about 90 MW" is our arithmetic with EirGrid's 90% assumption.

Sources: EirGrid/SONI Weekly Operational Constraints Update, week 42 2026, 2026-10-09 · CRU202704 (EirGrid figures: ~1,000 MW peak, 90% drop, limit reached), 2026-07-08 · CRU202705, EirGrid FRT update, 2026-06-26 · CRU/2025/236 s.5.1.7, 2025-12-12

What EirGrid does to stay inside it

The limit is full, so EirGrid manages the system more tightly. In its own operating updates and its June 2026 report to the CRU (CRU202705):

  • More spinning plant. The inertia floor rose from 23,000 to 28,000 MWs in the update of 8 May 2026, using two new synchronous condensers.
  • Less wind at the margin. The planned trial of raising the instantaneous share of wind, solar, batteries and imports (SNSP) from 75% to 80% was paused; the cap stays at 75%.
  • Batteries held back. System-wide battery charging is limited to 200 MW so storage can absorb power when data centres drop.
  • Exports cut. Interconnector exports count toward the same limit, so EirGrid counter-trades to reduce them, including exports of surplus wind.

The CRU also notes that EirGrid issued a fault ride-through demand-curtailment procedure to affected users on 30 June 2026. No data on its use has been published.

What MPID 345 changes

On 22 September 2026 the CRU approved Grid Code modification MPID 345 (CRU2026145). The Grid Code is amended five weeks later, on 27 October 2026. It applies to every current and future demand facility connected to the transmission system, data centre or not.

"must remain connected and operate stably during and after any fault disturbance" (CRU2026145, p.17 (printed 7))
  • Ride through faults, withstand frequency changes of ±1 Hz/s, and recover 90% of pre-fault demand within 500 milliseconds.
  • Report and apply. Non-compliant sites must report and seek a derogation before 27 October 2026.
  • Compliance plans from existing data centres within six months (by about Apr 2027, our arithmetic). Their group derogation runs 24 months, to about Oct 2028.
  • A cap per site. Each transmission-connected data centre gets a Demand Utilisation Threshold (DUT): a monthly-average cap on its non-compliant demand, set at its highest 2025 monthly average plus a share of any headroom. Peaks up to 10% above it are allowed.

The decision does not raise the 900 MW limit. It shares it out. The total of all caps is the limit divided by a "demand loss factor", which EirGrid has not published, less allowances for distribution-connected sites, peaks and 2026 connections.

Figure 2

The fix: ride through faults, or accept a cap, until about October 2028

From 27 Oct 2026 every transmission-connected data centre must stay on through grid faults. Most can't yet, so each gets a capped allowance of non-compliant demand until about 27 Oct 2028, and the 900 MW limit itself stays.

The MPID 345 fault ride-through rule: timeline to October 2028 and how the cap is sized12 Dec 2025: CRU flags the problem. 9 Jan 2026: 900 MW limit appears. 1 Apr 2026: EirGrid proposes MPID 345. 8 May 2026: Inertia floor raised. Jun 2026: Limit reached. 8 Jul 2026: CRU asks for more views. 22 Sep 2026: CRU approves MPID 345. 27 Oct 2026: Rule in force. 27 Apr 2027: Compliance plans due. 27 Oct 2028: Group derogation ends. The aggregate cap: 900 MW limit divided by the demand loss factor gives about 1,000 MW of non-compliant data-centre demand, less allowances for distribution sites, a peak margin and 2026 connections, whose sizes are not published.NOT TO SCALE · EVENTS IN ORDER12 Dec 2025CRU flags the problemThe large-energy-user decision recordsEirGrid's concern that data centres droppingoff and reconnecting worsens faults.9 Jan 2026900 MW limit appearsFirst listed in EirGrid's weekly operatingconstraints (absent in the December 2025edition).1 Apr 2026EirGrid proposes MPID 345A Grid Code rule for alltransmission-connected demand: ride throughfaults, withstand ±1 Hz/s, recover 90% ofdemand within 500 ms.8 May 2026Inertia floor raisedTo 28,000 MWs, using two new synchronouscondensers.Jun 2026Limit reachedData-centre demand peaks at about 1,000 MW;exports already cut.8 Jul 2026CRU asks for more viewsThree options for setting each site's cap;closed 29 July 2026.22 Sep 2026CRU approves MPID 345With a derogation framework that caps eachexisting site's non-compliant demand.27 Oct 2026Rule in forceSites must report non-compliance and apply fora derogation before this date.~27 Apr 2027Compliance plans due (our arithmetic)Data centres file a plan within 6 months.~27 Oct 2028Group derogation ends (our arithmetic)The 24-month derogation for data centresconnected before end-2026 runs out.as of 9 Oct 2026How the cap on existing sites is sizedCRU2026145b, the derogation framework900 MWload-rejection limit (unchanged)÷ demand loss factor (not published)~1,000 MWmost non-compliant demand allowedminus three allowances, sizes not published?distribution−?peak margin−?2026 sites= shared out as one cap (DUT) per siteOne DUT per sitevalues not publishedEach transmission-connected data centre's DUT= its highest 2025 monthly-average demand + apro-rata share of any headroom. Peaks up toDUT +10% are allowed; it is measured on30-minute metering. Compliant demand is notcapped.
The MPID 345 fault ride-through rule: timeline to October 2028 and how the cap is sized12 Dec 2025: CRU flags the problem. 9 Jan 2026: 900 MW limit appears. 1 Apr 2026: EirGrid proposes MPID 345. 8 May 2026: Inertia floor raised. Jun 2026: Limit reached. 8 Jul 2026: CRU asks for more views. 22 Sep 2026: CRU approves MPID 345. 27 Oct 2026: Rule in force. 27 Apr 2027: Compliance plans due. 27 Oct 2028: Group derogation ends. The aggregate cap: 900 MW limit divided by the demand loss factor gives about 1,000 MW of non-compliant data-centre demand, less allowances for distribution sites, a peak margin and 2026 connections, whose sizes are not published.NOT TO SCALE · EVENTS IN ORDER12 Dec 2025CRU flags the problemThe large-energy-user decision recordsEirGrid's concern that data centresdropping off and reconnecting worsensfaults.9 Jan 2026900 MW limit appearsFirst listed in EirGrid's weeklyoperating constraints (absent in theDecember 2025 edition).1 Apr 2026EirGrid proposes MPID 345A Grid Code rule for alltransmission-connected demand: ridethrough faults, withstand ±1 Hz/s,recover 90% of demand within 500 ms.8 May 2026Inertia floor raisedTo 28,000 MWs, using two new synchronouscondensers.Jun 2026Limit reachedData-centre demand peaks at about 1,000MW; exports already cut.8 Jul 2026CRU asks for more viewsThree options for setting each site'scap; closed 29 July 2026.22 Sep 2026CRU approves MPID 345With a derogation framework that capseach existing site's non-compliantdemand.27 Oct 2026Rule in forceSites must report non-compliance andapply for a derogation before this date.~27 Apr 2027Compliance plans due (our arithmetic)Data centres file a plan within 6months.~27 Oct 2028Group derogation ends (our arithmetic)The 24-month derogation for data centresconnected before end-2026 runs out.as of 9 Oct 2026How the cap on existing sites is sizedCRU2026145b, the derogation framework900 MWload-rejection limit (unchanged)÷ demand loss factor (not published)~1,000 MWmost non-compliant demand allowedminus three allowances, sizes not published?distribution−?peak margin−?2026 sites= shared out as one cap (DUT) per siteOne DUT per sitevalues not publishedEach transmission-connected data centre's DUT = itshighest 2025 monthly-average demand + a pro-rata shareof any headroom. Peaks up to DUT +10% are allowed; it ismeasured on 30-minute metering. Compliant demand is notcapped.

The CRU approved Grid Code modification MPID 345 on 22 Sep 2026; it takes effect on 27 Oct 2026 and applies to all current and future transmission-connected demand, data centre or not. A compliant site must stay connected through a fault, withstand frequency changes of ±1 Hz/s and recover 90% of its demand within 500 milliseconds. Existing data centres get a 24-month group derogation, with a compliance plan due within six months, and meanwhile each has a Demand Utilisation Threshold (DUT) capping its non-compliant demand. The caps are sized from the load-rejection limit, but EirGrid has not published the demand loss factor, the allowances or any site's DUT; distribution-connected sites wait for an ESB Networks code change that is not yet public.

EstimateCompliance-plan and derogation-end dates are our arithmetic from the effective date (+6 and +24 months).

Sources: CRU2026145, Grid Code Modification MPID 345 and Compliance and Derogation Framework: Decision Paper, 2026-09-22 · CRU2026145b, EirGrid MPID 345 Compliance and Derogation Framework (Sept 2026, as decided), 2026-09-22 · CRU2026145c, CRU letter of direction to the GCRP chair (also on eirgrid.ie as 'MPID345 CRU Approval, effective 27/10/2026'), 2026-09-22 · CRU202704, 2026-07-08

What about sites on ESB Networks' system?

MPID 345 covers only transmission connections. Checked on 3 October 2026, no matching Distribution Code change is public. The CRU says ESB Networks:

"intends to propose a related modification to the Distribution Code, to address the FRT risk at a distribution level and has commenced customer engagement" (CRU2026145, p.44 (printed 34))

esbnetworks.ie still lists Distribution Code v8.0 (approved February 2023), whose ride-through rules cover generation, not demand. At the Grid Code panel the distribution operator said a MW threshold may be needed. EirGrid's November 2025 information paper put about 150 MW of a roughly 800 MW data-centre peak on the distribution system. No site list is public.

What is not known

  • Which campuses define the limit: EirGrid has not published site-level data-centre demand, DUT values or which connections are curtailed. Only the aggregate (~1,000 MW peak, June 2026) is public.
  • Site-level DUTs: EirGrid has set one per transmission-connected data centre (2025 highest monthly average plus a pro-rata share of headroom) but has not published the values, the demand loss factor or the aggregate DUT.
  • How many sites have applied for derogations, and compliance rates: EirGrid reports these to the CRU quarterly (first report due end December 2026); publication of those reports is not stated.

Why it matters

Our reading. Grid capacity on paper is no longer the only limit. Until sites can ride through faults, every new megawatt of data-centre demand also uses up room under a fixed 900 MW ceiling. On EirGrid's 90% assumption a 200 MW campus would take about 180 MW, 20% of the whole limit. The pace at which existing sites become compliant will shape how quickly that room reopens, and the CRU has asked EirGrid for a roadmap for easing the operating measures.

The cost of the measures (extra spinning plant, curtailed wind, exports cut) falls on the system as a whole. EirGrid and the CRU have not published a figure for it.

Questions people ask

What is EirGrid's 900 MW limit?
The most demand plus interconnector export the all-island grid can lose in a single fault: 900 MW in EirGrid/SONI's weekly operating rules since January 2026. Data centres reached it at a peak of about 1,000 MW in June 2026 (CRU202704).
What is MPID 345?
A Grid Code modification requiring transmission-connected demand, including data centres, to ride through faults, withstand ±1 Hz/s frequency changes and recover 90% of demand within 500 ms. The CRU approved it on 22 September 2026; it applies from 27 October 2026 (CRU2026145).
Does MPID 345 apply to data centres on the distribution network?
No. It covers transmission connections only. ESB Networks intends a Distribution Code change, but none was public on 3 October 2026.
Does MPID 345 raise the 900 MW limit?
No. It keeps the limit and caps each existing data centre's non-compliant demand until it complies, for up to 24 months under the group derogation.

How we know

  • Limits read from EirGrid/SONI Weekly Operational Constraints Updates, 144 editions from 2022 to 2026. The first-appearance date is the first edition found that lists the limit.
  • Fault events, peak and 90% assumption as reported by EirGrid in CRU202704. Every quote on this page was matched against the source document text (all found).
  • Compliance-plan and derogation end dates are our arithmetic from the effective date. The 200 MW campus example uses EirGrid's 90% assumption and is ours.

Sources

Numbers on this page are read from our data build of 10 October 2026 and update when it changes. Text marked Our reading is our analysis, not a sourced fact. Found a mistake? See corrections; method notes are on methodology.

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