Automatic transfer switch (ATS) load shedding is an automated power management strategy that disconnects high-amperage appliances when total electrical demand approaches the standby generator’s maximum output capacity. Without load shedding, large inductive loads—such as central air conditioning compressors, heat pumps, electric water heaters, and EV chargers—can stall the generator engine or trip its main circuit breaker during a power outage. Integrated load management lets a moderately sized generator safely back up an entire electrical service panel while maintaining strict compliance with electrical code standards.
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| LOAD SHEDDING QUICK SUMMARY |
+----------------------+--------------------------------------------------+
| Primary Purpose | Prevents generator engine stalls & breaker trips |
| Code Requirement | NEC 702.4(B): Generator capacity must equal peak |
| | demand OR use automatic load management |
| Control Methods | Low-voltage 24VAC HVAC interruption or inline |
| | 240V high-voltage contactor disconnect |
| Priority Order | Non-essential high loads shed first, restored |
| | last after system stabilization |
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1. Why Load Shedding Is Required: The NEC 702.4(B) Mandate
When connecting an automatic standby generator to an entire residential service panel, you cannot simply assume the homeowner will avoid turning on heavy appliances during an outage.
Under NEC Article 702.4(B)(2), an automatic transfer system must meet one of two strict sizing criteria:
- Full Capacity Sizing: The generator must be physically sized to carry the full calculated load of the home according to NEC Article 220 calculations. On a 200-amp service, this often requires a 30 kW to 48 kW liquid-cooled commercial generator.
- Automatic Management Sizing: If a smaller, air-cooled generator (typically 14 kW to 26 kW) is installed on a 200-amp service, the system must include an automatic load management system. The system must automatically shed loads to prevent the generator from being overloaded.
For sizing details, see Sizing a Transfer Switch for Whole-House Loads and System Management: What Is Electrical Load Shedding.
2. How ATS Load Management Works
Load management systems continuously monitor power conditions and open control circuits when capacity thresholds are crossed.
HOW LOAD MANAGEMENT MONITORS POWER
[Utility Fails -> Generator Starts -> ATS Transfers House to Backup]
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v
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| ATS / Generator Monitors Line Current & Frequency |
+-------------------------------------------------------------+
| |
(Frequency Drops Below 58 Hz (Current Exceeds 85%
or Engine Bogs Down) of Rated Capacity)
| |
+--------------------+--------------------+
|
v
+-------------------------------------------------------------+
| Load Shed Controller Signals Priority Relays to OPEN |
+-------------------------------------------------------------+
|
+-----------------------+-----------------------+
| |
v v
[Priority 1: Lock Out EVSE] [Priority 2: Drop AC Stage 2]
Frequency-Drop Sensing (Digital Power Management)
Some smart management modules monitor the AC frequency of the generator feed. When a heavy motor starts, the sudden mechanical drag pulls engine speed down, causing electrical frequency to drop from 60 Hz down toward 57 or 58 Hz. The load shed module detects this frequency sag and immediately drops its managed circuit to protect the engine.
Current Transformer (CT) Measurement
Advanced smart ATS units use integrated current transformers (CTs) clamped around the generator’s main feeder conductors. The ATS controller calculates real-time kilowatt and amperage draw. If continuous current passes a pre-programmed threshold (such as 85% of rated capacity), the controller initiates a staged shed sequence.
To review system operation, see How an Automatic Transfer Switch (ATS) Works and Automated Control: How Automatic Load Shedding Systems Work.
3. High-Voltage Contactors vs. Low-Voltage HVAC Interruption
Depending on the appliance, load shedding is executed using either high-voltage power interruption or low-voltage control-loop interruption.
LOW-VOLTAGE HVAC CONTROL (24VAC) HIGH-VOLTAGE RESISTIVE CONTROL (240V)
[Thermostat Y-Wire] [240V Branch Circuit Breaker]
| |
v v
+---------------+ +-------------------------------+
| NC Relay on | | Heavy-Duty Contactor (50A/60A)|
| ATS Logic Bd | | (Normally Closed) |
+---------------+ +-------------------------------+
| |
v v
[AC Compressor Contactor Outside] [Water Heater / EV Charger / Range]
(Interrupts small 24V signal; (Physically cuts full 240V line
safe, low heat, no heavy wiring) conductors via external enclosure)
Method 1: Low-Voltage HVAC Lockout (Thermostat Interruption)
Central air conditioners and heat pumps consume massive starting currents (locked-rotor amps). However, you do not need to cut the 240-volt line power to shed an air conditioner:
- The Wiring: The 24VAC cooling control wire (traditionally the yellow “Y” wire) running from the indoor thermostat to the outdoor condenser is routed through a normally closed (NC) relay inside the ATS.
- The Action: When shedding occurs, the relay opens, breaking the 24V thermostat call. The outdoor compressor and condenser fan shut down gently through their normal control circuit, while the indoor furnace fan continues to circulate air.
- Details on compressor inrush: High-Inrush Sizing for HVAC & Heat Pumps.
Method 2: High-Voltage Inline Contactors
Appliances like electric water heaters, hot tub heaters, and electric clothes dryers lack external low-voltage control loops.
- The Wiring: These appliances require an external load-shed contactor box. The full 240-volt branch circuit passes through heavy-duty mechanical contacts held closed by a spring.
- The Action: When the ATS commands a shed, it energizes or de-energizes the contactor coil, physically breaking both ungrounded (hot) line conductors.
4. Priority Staging and Reconnection Sequences
Load management relies on priority staging to ensure that critical home systems take precedence over discretionary electrical loads.
PRIORITY LEVEL APPLIANCE ASSIGNMENT RECOVERY DELAY AFTER RESTORATION
Priority 1 Primary HVAC / Heat Pump 5 Minutes after transfer
Priority 2 Secondary / Upstairs AC 8 Minutes (staggered delay)
Priority 3 Electric Water Heater 12 Minutes (only if KW capacity exists)
Priority 4 EV Charger / Hot Tub Locked out until utility grid returns
When the transfer switch first transfers to generator power, it holds all managed loads in an open (disconnected) state. Once the engine stabilizes at 60 Hz and baseline home circuits are powered, the controller initiates a timed ladder logic sequence:
- Staggered Re-engagement: Priority 1 engages. The controller monitors generator output for several minutes. If voltage and frequency remain stable, Priority 2 engages.
- Dynamic Shedding: If Priority 1 and Priority 2 are running, and an unmanaged load (such as a 1,500-watt microwave or well pump) suddenly kicks on, the controller immediately sheds Priority 2 to protect engine stability.
- Lockout Logic: High-demand appliances assigned to Priority 4 remain locked out completely until utility power returns.
For advice on setting priority tiers, review Logic Hierarchy: How to Set Load Shedding Priority Levels.
Diagnostic Troubleshooting: Common Load Shedding Faults
SYMPTOM LIKELY CAUSE NEXT DIAGNOSTIC STEP
AC won't run during outage, Low-voltage relay stuck open Verify 24VAC across ATS control
even with no other loads. or wiring open-circuit. terminals; test continuity of Y-loop.
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Generator stalls immediately Contactor failed closed; Verify auxiliary contactor drops
when AC compressor starts. load management not shedding. open when generator frequency sags.
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Appliance drops out and Load module timer failure or Inspect module LED status codes;
never restores. under-frequency threshold set check generator engine governor
too aggressively. for RPM stability under load.
If a load shed module fails, do not bypass the control relay permanently unless you have manually switched off the corresponding appliance breaker. Bypassing load management on an undersized generator creates an immediate fire hazard from overheated windings and can cause severe engine bog-down during peak household demands.