Configuring MRP Types, Procedures, and Lot Sizing for the Planning Run

Explanation

Once the basic purpose of MRP is understood, the next layer is configuring how each material is planned, which is controlled primarily through MRP type, procurement type, and lot-sizing procedure on the material master, combined with planning run scope settings executed by MRP controllers. MRP type determines the planning procedure applied to a material. Consumption-based types (e.g., reorder point planning, forecast-based planning) trigger procurement when stock falls below a threshold or is driven by statistical forecast, and are typically used for low-value or high-volume materials where precise deterministic planning is not cost-justified. Deterministic MRP types (standard MRP) calculate requirements directly from actual demand records (sales orders, planned independent requirements, dependent requirements) and are used for materials where exact demand visibility exists, typically finished goods and key components. Choosing the wrong MRP type is a common source of planning noise - applying reorder point logic to a material with lumpy, large-batch demand often produces frequent small procurement proposals that do not match real consumption patterns, while applying deterministic MRP to true consumption-driven commodity items can create unnecessary planning overhead without meaningfully better outcomes. Procurement type governs whether a material is planned as in-house produced (creates planned orders that convert to production orders), externally procured (creates purchase requisitions that convert to purchase orders or are consumed by scheduling agreements), or both (with a special procurement key directing specific scenarios like subcontracting or stock transfer). Lot-sizing procedure determines how MRP groups net requirements into a single procurement proposal quantity. Static lot-sizing (fixed lot size, exact lot-for-lot) creates proposals matching demand precisely or in fixed batch sizes. Periodic lot-sizing (e.g., weekly, monthly lot sizing) bundles multiple days of net requirements into single proposals to reduce transaction volume and take advantage of setup or ordering cost efficiencies. Optimizing lot-sizing procedures use cost-based algorithms weighing ordering cost against holding cost, which are less commonly used in practice because they require accurate cost parameters that many organizations do not maintain rigorously. The planning run itself is executed with parameters that control scope and processing mode: total planning run (processes the whole plant) versus single-item planning (processes one material, useful for troubleshooting or urgent replanning), and net change planning (only replans materials with changes since the last run) versus regenerative planning (replans everything regardless of change, typically used for periodic full recalculation or initial setup). Planning run mode selection has significant performance implications - regenerative planning across a large plant can run for hours, while net change planning in the background is designed for frequent, incremental updates and is the standard operational mode in production environments. Scheduling parameters (lead time scheduling, basic dates versus detailed capacity scheduling) also factor into MRP results because they determine order start and finish dates, which affects whether the system schedules dependent requirements and receipts within or outside the planning horizon, directly influencing whether shortages appear as exceptions.

Code example

Example MRP type and lot size configuration decision matrix (conceptual, not a system screenshot): Material: Finished good FG-1000 (make-to-stock, high demand visibility)- MRP type: PD (standard/deterministic MRP)- Procurement type: E (in-house production)- Lot-sizing procedure: EX (lot-for-lot / exact order quantity)- Planning time fence: 5 days (protects near-term orders from automatic rescheduling) Material: Fastener/bolt RM-5000 (low value, high volume, consumption-driven)- MRP type: VB (reorder point planning)- Procurement type: F (external procurement)- Lot-sizing procedure: periodic lot size (weekly)- Reorder point and safety stock maintained based on historical consumption This contrast illustrates why one uniform MRP configuration is rarely appropriate across an entire material portfolio - planners typically segment materials (e.g., using ABC/XYZ classification) and apply differentiated MRP type and lot-size strategies per segment.

Real project scenario

During a rollout, a plant applied standard MRP (PD) uniformly to all materials including low-value hardware components, resulting in thousands of small daily planned orders for items that could have been managed with simple reorder point logic. The planning team consumed excessive review time on low-value exceptions. A subsequent material segmentation project reclassified low-value, high-volume items to consumption-based MRP with periodic lot sizing, which reduced daily exception volume significantly and let planners focus attention on critical, deterministically planned materials.

Common mistakes

• Applying a single MRP type across all materials without segmentation by value, volume, or demand pattern. • Setting lot-for-lot (exact) lot sizing on materials with high setup costs, generating excessive small production orders and setup time loss. • Misunderstanding net change versus regenerative planning and running regenerative planning routinely, causing unnecessary system load and long run times. • Not setting a planning time fence, allowing MRP to reschedule or cancel orders that are already in near-term execution, disrupting shop floor plans. • Forgetting that changing MRP type or lot-size procedure on an existing material does not retroactively fix already-created open orders; only future planning runs are affected.

Best practices

• Segment materials (ABC/XYZ or similar classification) and assign differentiated MRP type and lot-size strategies rather than a single default. • Use net change planning as the standard operational mode; reserve regenerative planning for periodic full recalculations or significant master data changes. • Always configure a planning time fence for materials with active shop floor execution to prevent unwanted rescheduling of orders in progress. • Periodically review lot-sizing outcomes (order frequency, quantity variability) against actual demand patterns and adjust configuration as business conditions change. • Coordinate MRP type/lot-size changes with planners before go-live since the effect only applies prospectively, not to existing open orders.

Interview angle

Interviewers assess whether a candidate can justify MRP type and lot-size selection based on material characteristics rather than reciting definitions. Be prepared to explain the trade-off between deterministic and consumption-based planning, when periodic lot sizing is preferable to lot-for-lot, and why planning time fences matter for protecting shop floor stability. Explaining net change versus regenerative planning and its performance implications is also frequently probed.