Generator and Emergency Power Coordination: Where Drawings Stop and Field Problems Start

NEC, NFPA 110, and the Three Branches of Emergency Power

The drawings have to clearly show three categories of load: emergency systems (NEC Article 700, life safety), legally required standby (Article 701, code-required but not life safety), and optional standby (Article 702, owner preference). Each has different separation requirements. Emergency feeders cannot share raceways with normal power. Legally required standby has slightly looser rules. Optional standby is whatever the engineer decides.

The most common drawing error: a single ATS feeds a panel that mixes emergency and optional loads. This is a code violation that's not always obvious from the one-line because the panel schedule looks normal. The reviewer has to trace each circuit back to its branch designation. NFPA 110 then layers on top: classification (Type, Class, Level), starting time, fuel supply duration, and ventilation requirements.

Generator Sizing Is a Coordination Problem, Not a Math Problem

The generator sizing calculation depends on which loads are connected, the starting current of each motor load, and the load step assumptions. The calculation is engineering work, but the drawings have to support it. Specifically: the connected load shown on the panel schedules has to match the load assumed in the generator sizing.

When those don't reconcile — and they often don't, especially on projects where the panel schedule is updated late in design while the generator sizing is locked in early — the generator either undersized or oversized. Undersized means it won't start under load. Oversized means it sits at low load and accumulates carbon, called wet-stacking, which damages the engine.

The drawing review should reconcile: (1) the sum of generator-fed panel schedule loads, (2) the largest motor on the system and its starting current, and (3) the load step calculation behind the generator size. If those numbers don't match across drawings, the generator is wrong even if each individual drawing is right.

Fuel, Ventilation, and Combustion Air

A diesel generator needs fuel storage sized for the required duration (NFPA 110 Class). Most healthcare and data center applications require 96-hour Class. Indoor generators need combustion air, exhaust routing, and radiator ventilation that pulls hundreds of CFM through the room.

The architectural and mechanical drawings have to show: (1) generator room volume sized for the heat rejection, (2) combustion air louvers sized to the engine's consumption, (3) radiator discharge sized to the radiator airflow, (4) exhaust pipe routing with thermal expansion and proper insulation, (5) exhaust termination clear of intakes, and (6) sound attenuation that meets the noise ordinance at the property line.

We've seen generator rooms designed for the wrong engine's heat rejection because the spec was updated but the room sizing wasn't. The result was a generator that overheats and shuts down within 20 minutes — the opposite of what an emergency power system is supposed to do.

ATS Coordination Across Drawings

Automatic transfer switches show up on the electrical one-line, but the room they live in shows up on the architectural plan, the conduit pathway shows up on the electrical plan, and the load they serve shows up on the panel schedule. All four drawings have to agree.

Common errors: ATS shown in one location on the one-line and a different location on the architectural plan. ATS rated for less amperage than the panel it feeds. ATS missing the bypass-isolation switch required for testing without dropping power. ATS room without the working clearance required by NEC 110.26 — see our NEC panel clearance reference.

Selective Coordination and Arc Flash

NEC Article 700.32 requires selective coordination of overcurrent protective devices in emergency systems. The breakers have to coordinate so that a fault on a downstream branch trips only the local breaker, not the upstream main. This requires a coordination study that's usually a separate deliverable from the drawings.

The drawings should reference the coordination study and show the breaker types and trip settings that the study assumes. If the panel schedule shows generic breakers and the coordination study assumes specific electronic-trip models, the building gets built with the wrong gear. This is a quiet code violation that surfaces only during commissioning when the breakers don't coordinate as designed.

Same applies to arc flash labeling. The arc flash study assumes specific equipment and clearing times. The labels installed on the gear have to match the study. If the gear changes between design and construction, the labels are wrong and the people opening the panels are at risk.

When the System Has to Actually Work

Generators are the system everyone hopes never runs. When they do run, the building's most critical loads depend on the drawings being right two years earlier. The teams that consistently commission generator systems on schedule build a standard generator-coordination review into preconstruction: every panel reconciled, every drawing cross-referenced, every assumption documented. The cost of doing this is a few engineering hours. The cost of skipping it is an emergency that doesn't get powered.