Poweredge calculate power for Redundant policy

Hi,

iDRAC treats Grid A/B as purely a PSU‑capacity/topic‑of‑failure concept, not as an automatic detector of whether your PSUs are fed from independent UPS/PDUs/ phases.[1][2][3]

## 1. Grid A/B power source requirements

- iDRAC does **not** detect whether both PSUs are on the same UPS, PDU, or electrical phase; it only knows if each PSU has valid input, output, and status.[1][3]

- A/B Grid Redundant means: PSUs are split into grid A and grid B (in a 2‑PSU R760 that is effectively N+N), and the platform power budget must be such that one whole grid can carry the entire calculated worst‑case load.[1][2][3]

- The requirement for “two electrically independent feeds” is a **design best practice**, not something iDRAC validates; if you plug both PSUs into the same UPS, Grid A/B can still be reported as healthy if the capacity/budget rules are satisfied.[1][2][4][3]

- iDRAC will only show “PSU Redundancy Lost” when, according to its internal power model and PSU ratings/conditions, one grid by itself cannot support the full calculated requirement, or when one PSU/grid is faulted or offline.[1][5][3]

So: having both PSUs on the same UPS does **not** by itself force “Redundancy Lost”; the alert is driven by calculated capacity or PSU health, not feed independence.[1][3]

## 2. How PSU sizing is actually evaluated

We don't use your observed peak (e.g. 465 W) to decide redundancy state; Grid A/B / PSU Redundant are based on an internal power budget for the *configured* system, plus PSU capabilities and derating, not on telemetry history.[2][6][3]

Key points:

- The platform uses a configuration‑based maximum power requirement that takes into account CPUs, memory population, NVMe/SAS drives, PCIe cards, GPUs/accelerators, fans, etc.[2][6][3]

- This “worst‑case configuration power” is compared against the *available power per grid* (one PSU or set of PSUs) after considering:  

  - PSU nameplate rating and input voltage (de‑rated at low line),  

  - thermal/efficiency derating region,  

  - redundancy policy (A/B grid vs PSU redundant vs not redundant).[2][6][3]

- If $$ \text{available power of one grid} < \text{calculated maximum configuration power} $$, redundancy is flagged as lost, even if your real‑world load is far lower (e.g. 465 W).[2][6][3]

For a 2 × 800 W R760 with Grid A/B:

- On high‑line input (e.g. 200–240 V), each 800 W PSU can typically deliver the full 800 W; on low line (100–120 V), its maximum usable output may be lower, which is taken into account for redundancy evaluation.[2][3]

- If the internal model for your particular CPU/MEM/PCIe/GPU configuration says “this system may require >800 W worst‑case,” iDRAC will show “PSU Redundancy Lost” under Grid A/B, even though iDRAC telemetry shows ~465 W peak.[2][6][3]

Deterministic sizing for 16G:

- The recommended deterministic method is to use the Enterprise Infrastructure Planning Tool (EIPT) / Online Power Calculator and the **PSU Guide** for your platform, and then choose the smallest PSU option marked as “Full Redundancy” (FR) for that configuration.[6][3]

- In EIPT, PSU capacities are color‑coded:  

  - Gray – cannot support configuration;  

  - White – Fault Tolerant Redundancy (supported but may throttle/degrade after redundancy loss);  

  - Green – Full Redundancy (minimum PSU capacity that supports full performance before and after redundancy loss).[6]

So to “guarantee” Grid A/B is considered valid:

1. Model your exact R760 configuration in EIPT.  

2. Select PSU wattage where the tool marks the configuration as FR/green with N+N (A/B) redundancy.  

3. Ensure input voltage matches that assumption (e.g. high‑line vs low‑line).  

With that, the same sizing logic used by iDRAC for redundancy validation is satisfied.[2][6][3]

## 3. Where to see the internal power basis / reasons

You cannot see the exact full formula, but you can get several key pieces:

- iDRAC exposes:  

  - Current and historical power consumption and its own “Power Cap” limits,  

  - PSU ratings, input type (high‑line/low‑line), and status,  

  - Redundancy mode (Not Redundant / A/B Grid / PSU Redundant).[3]

- The **calculated maximum configuration power** and redundancy validation are **not** shown as a direct number in iDRAC, but are embedded in:  

  - EIPT’s “Estimated Maximum” / “Full Redundancy required PSU size” output for your entered configuration,  

  - BIOS/iDRAC power management policy behavior and alerts.[2][6][3]

What the internal model considers (per Dell power‑management whitepapers):

- Component‑level TDP and configuration rules (CPU SKUs, memory DIMM count/type/speed, NVMe/SAS drive count, PCIe / GPU type and count).[2][6]

- PSU derating based on input voltage and temperature; “maximum usable capacity” can be less than nameplate rating at low line or high ambient.[2][6][3]

- Platform overhead for fans, chipset, NICs, etc.[2][6]

How to see reasons/alerts:

- System Event Log (SEL) / iDRAC “Hardware -> Power -> Events” will show entries like `RDU0012 Power supply redundancy is lost` when redundancy evaluation fails; these do not include the numeric power comparison but are your main indicator.[5][3]

- There is no per‑component breakdown of the internal “worst‑case” model in GUI/CLI; that detail is abstracted into EIPT and design documentation.[2][6][3]

## 4. Documentation and best‑practice references

Key official materials for 16G PSU redundancy and sizing:

- “PowerEdge: Power Settings” knowledge base article – explains A/B Grid Redundant, PSU Redundant, and how redundancy depends on configuration and PSU capacity.[3]

- “Strategically Manage Power in PowerEdge Server PSUs Using …” whitepaper – describes redundancy modes (N+N A/B grid, N+1, non‑redundant), derating, and how PSUs are sized against platform config.[2]

- “Full Redundancy vs. Fault Tolerant Redundancy for PowerEdge Server PSUs” – discusses how 15G/16G tools classify PSU options (FR vs FTR) for a given configuration and shows the logic for required PSU capacity.[6]

- Power distribution / data center design whitepapers (e.g. chassis/PDUs) – describe best practices for mapping PSUs to separate PDUs/circuits to achieve *true* grid independence.[4]

For your specific R760 deployment:

- Use EIPT with your exact R760 BOM to obtain “Estimated Maximum” and the minimum PSU capacity flagged as FR in N+N mode, and choose that or higher.  

- Ensure the server’s input voltage and environment match the assumptions in the calculator.  

- Wire PSUs to independent PDUs/UPS feeds to achieve *actual* grid redundancy, even though iDRAC does not enforce or detect feed independence.[2][4][6][3]

Sources:

[1] PowerEdge PSU redundancy options - differences? https://www.dell.com/community/en/conversations/poweredge-hardware-general/poweredge-psu-redundancy-options-differences/647fa024f4ccf8a8de51abad

[2] [PDF] Strategically Manage Power in PowerEdge Server PSUs Using ... https://objects.icecat.biz/objects/mmo_135024374_1758638246_2314_703856.pdf

[3] PowerEdge: Power Settings | Dell US https://www.dell.com/support/kbdoc/en-us/000202926/poweredge-power-settings

[4] Power Distribution Systems for the ... https://i.dell.com/sites/csdocuments/Business_smb_merchandizing_Documents/pt/br/pe_m1000e_selection_whitepaper.pdf

[5] PowerEdge: PSU Redundancy Lost - part 5G4WK | Dell US https://www.dell.com/support/kbdoc/en-us/000215889/psu-redundancy-lost-dell-dpn-5g4wk

[6] Full Redundancy vs. Fault Tolerant ... https://infohub.delltechnologies.com/en-us/p/full-redundancy-vs-fault-tolerant-redundancy-for-poweredge-server-psus/

[7] Stopping iDRAC Reporting "Power supply redundancy is lost" as I ... https://www.dell.com/community/en/conversations/power-cooling/stopping-idrac-reporting-power-supply-redundancy-is-lost-as-i-have-moved-from-2-psus-to-one/685040b816c3a94aace0c4e9

[8] PSU Failover behaviour in r720 https://www.reddit.com/r/homelab/comments/u70tll/psu_failover_behaviour_in_r720/

[9] Power Supply troubleshooting for the ATP or SEDR 8880 and ... https://knowledge.broadcom.com/external/article/174386/power-supply-troubleshooting-for-th

e-atp.html

[10] Stop R710 iDrac complaining about loss of power to spare PSU https://www.reddit.com/r/homelab/comments/eaj7e4/stop_r710_idrac_complaining_about_loss_of_power/