Engineering guide
N+1 vs 2N: what you can maintain without shutting the load down
Redundancy decides what can be serviced with the load still protected. Internal N+1 lets one module be swapped on line, but shared frame parts still need a bypass window. Parallel N+1 lets one whole UPS be isolated. 2N lets a complete system, batteries and switchgear included, come out of service while the other carries the load.
Key facts
- XPC M90U-80 N+1 capacity: 60 kW with 20 kW modules
- Galaxy VS in maintenance bypass: batteries not available as backup
- Toshiba G9000 parallel: up to 4 or 8 units depending on rating
- Mitsubishi 9900B parallel: up to 8 units
- Delta Modulon DPH parallel: up to 8 units
Internal N+1: one module more than the load needs
In a modular frame, N+1 means one extra power module. XPC puts numbers on it for the M90U-80: with 20 kW modules, the N+1 capacity is 60 kW, one module below the full frame, and the modules are hot-swappable without risk to the critical load. Schneider Electric's Galaxy VS allows power modules to be inserted or removed in any operation mode on units with the Live Swap label, and ABB describes its DPA modules as operating completely independently of each other.
The limit is the shared part of the frame: static switch, controls, bypass. Work there needs a bypass window, and on a Galaxy VS the batteries are not available as backup while the UPS is in maintenance bypass.
Parallel N+1: one unit more than the load needs
Several complete UPS units share the load, with one more than needed. Manufacturers publish the limits: up to 4 or 8 Toshiba G9000 units depending on the rating, up to 8 Mitsubishi 9900B units, up to six ABB SG Series units, and Delta's Modulon DPH up to 8 units. One unit can be isolated for maintenance while the others carry the load. The parallel system still has shared behaviour: Vertiv's NX manual lists a System Transfer message, when the whole paralleled system transfers to bypass at the same time.
2N: two complete systems
Two independent systems each carry the full load, feeding dual-corded equipment or a static transfer switch. One complete system, its batteries and its switchgear included, can be taken out of service while the other carries the load.
What can be serviced with the load still protected
| What needs work | Internal N+1 (modular frame) | Parallel N+1 (separate units) | 2N (two systems) |
|---|---|---|---|
| One power module | On line, where the frame supports hot swap | On line, inside the affected unit | On line |
| One complete UPS | Bypass window | That unit is isolated, the others carry the load | On line |
| Shared static switch, controls or bypass | Bypass window | Bypass window for the shared parts | On line |
| Output switchgear | Shutdown or external bypass | Shutdown or external bypass | On line, one side at a time |
Planning maintenance around redundancy
Our data centre UPS service plans each visit around the redundancy the site actually has, and strings for redundant battery banks are matched through the data centre battery program. Definitions of N, N+1 and 2N: UPS redundancy explained.
Related pages
Common questions
Can an N+1 UPS be maintained without a shutdown?
Partly. A redundant module or unit can be serviced on line, but parts shared by the whole system, such as the static switch, controls or output switchgear, still need a bypass window or a shutdown.
Is the load protected during maintenance bypass?
It is powered by the utility without battery backup. Schneider Electric's Galaxy VS manual states the batteries are not available as an alternate power source in maintenance bypass.
What does 2N add over N+1?
A complete second system. Each one carries the full load, so a whole system, batteries and switchgear included, can be maintained while the other runs.
How many UPS units can run in parallel?
It depends on the model: up to 4 or 8 Toshiba G9000 units depending on the rating, up to 8 Mitsubishi 9900B units, up to six ABB SG Series units.
Does modular N+1 need any shutdown at all?
For module work, often not. For the shared frame parts, yes: plan a bypass window and keep it short.
Sources, checked October 4, 2026:
- Xtreme Power Conversion, M90U-80 product page
- Schneider Electric, Galaxy VS operation manual 990-5910G, pp. 16, 60
- ABB, DPA 250 S4 brochure 4NWP104514R0001, p. 7
- Delta, Modulon DPH 50 to 600 kVA features page
- Toshiba International Corporation, G9000 Series product page
- Mitsubishi Electric, 9900B UPS product page
- ABB, SG Series IEC product page
- Vertiv, Liebert NX 40 to 200 kVA user manual SL-25217, p. 123
Send us the details
Fill this in and your own mail client opens with everything already written out. Nothing is sent from this site.
Brand names are trademarks of their owners and are used only to identify the equipment we service. We are not the manufacturer and are not affiliated with it.