EV Load Management

Biggest EV charger your panel can handle (NEC 220.87)

The load calculator, inverted: from your service size and your measured demand, the largest EV charger that fits, and how an automatic load-management device (an EVEMS) raises that answer without a panel upgrade. The branch is always sized to the full charger output.

Three Ways a Device Raises the Answer

An EVEMS relaxes the SERVICE load calculation, which NEC 220.70 and 625.42(A) permit for a listed energy management system in accordance with 750.30. It never relaxes the branch circuit. There are three device types, and the computation is different for each.

Biggest Charger by Service Size

The biggest charger that fits for an example measured peak on each common service size. These are worked examples; your own metered demand governs, so use the calculator below for your number.

Full grid across demand levels: max charger chart. With vs without a device: with vs without load management.

Two EVs on One Panel

A second EV is a common version of this question, and it has a cleaner answer than most people expect. Two full-rate chargers rarely fit a single service on their own, because two continuous loads eat headroom fast. The move is not two dedicated circuits fighting over the panel; it is one multi-charger load-management system that shares the service's spare capacity between the cars.

The key idea: a multi-port EVEMS does not grant two full chargers, it splits oneservice's headroom across both. When both cars are plugged in, each gets roughly half the available rate; when only one is charging, it gets the full rate. The device holds the combined draw under the service (NEC 220.70 / 625.42(A) / 750.30), while each branch circuit stays wired and protected for its own charger's full output, never the shared setpoint. A circuit-share switch is the other honest path when you also have a heavy appliance branch to borrow: one load at a time, never both together. Run your service and measured demand through the calculator below to see the headroom you would be sharing.

Load-Management Devices

Compute Your Biggest Charger

Enter your service size and your measured 12-month peak demand (or a 30-day recorded peak). The calculator returns the largest standard EV charger that fits and its branch, computed by the NEC 220.87 method through the same locked engines the rest of the site uses.

What voids the method, and what has to be true before a device changes the answer

The panel-capacity method this tool uses is NEC 220.87, and a load-management device can change its answer only where the code's own conditions hold. They are facts about installed equipment, so no calculator can verify them. Each list below is published whole, exactly as the engine carries it, rather than trimmed to the convenient members.

Before an energy management system relaxes the service calculation

All 7 of these, not a selection.

• The energy management system is listed: as a complete energy management system, as a kit for field installation in switch or overcurrent device enclosures, or as listed individual components assembled as a system. (NEC 750.6)
• The system is an EMS in accordance with 750.30. Both the 625.42(A) relaxation and 220.70's single-setpoint value are written on that condition, so a device that does not meet 750.30 relaxes nothing. (NEC 625.42(A) / 220.70)
• The system does not cause a branch circuit, feeder, or service to be overloaded, and where it limits the current on a conductor it also meets the duties 750.30(C)(2) through (C)(4) impose. (NEC 750.30(C))
• The system does not override load shedding controls placed to ensure minimum electrical capacity for fire pumps, emergency systems, legally required standby systems or critical operations power systems, and does not disconnect power to elevators, escalators, moving walks or stairway lift chairs, hazardous-location ventilation, emergency lighting circuits, or essential health care systems. (NEC 750.30(A) / 750.30(B))
• Where one or more pieces of equipment are provided with an integral load management control, the system is marked to indicate that control is provided. (NEC 625.42(A))
• The EMS setpoint counts as a continuous load in the load calculation. This tool checks the service RATING, which NEC 230.79 sets from the calculated load; it does not size service-entrance conductors, and NEC 230.42(A)(1) sizes those from the noncontinuous load plus 125 percent of the continuous load. Whoever sizes or verifies the service conductors has to apply that to the setpoint. (NEC 220.70 / 230.42(A)(1))
• Where the system controls electrical power through a remote means, a directory identifying the controlled devices and circuits is posted on the enclosure of the controller, the disconnect, or the branch-circuit overcurrent device. (NEC 750.50)

The NEC does not define peak load shaving, and where it does use the term, at 700.4(C)(1), it is describing a source running in parallel rather than a device that limits load. Reading an EV energy management system as a form of peak load shaving is our interpretation, and it is the conservative one: check it with the AHJ before relying on a 30-day recorded basis on a service that has one.

A branch-level device, which never reached the service calculation

NEC 625.42(A) reaches the service and feeder load calculation; only (B) changes the EVSE's own ampere rating, which is what 625.41 sizes the branch from. A branch-level device leaves the conductor, breaker and ground sized to 125 percent of the charger's full output. Of the 7 conditions listed above, 3 still attach. That is which members of that list carry over, and not a complete account of what NEC 750.30 asks of a device sitting on a branch circuit:

• The energy management system is listed: as a complete energy management system, as a kit for field installation in switch or overcurrent device enclosures, or as listed individual components assembled as a system. (NEC 750.6)
• The system does not override load shedding controls placed to ensure minimum electrical capacity for fire pumps, emergency systems, legally required standby systems or critical operations power systems, and does not disconnect power to elevators, escalators, moving walks or stairway lift chairs, hazardous-location ventilation, emergency lighting circuits, or essential health care systems. (NEC 750.30(A) / 750.30(B))
• Where the system controls electrical power through a remote means, a directory identifying the controlled devices and circuits is posted on the enclosure of the controller, the disconnect, or the branch-circuit overcurrent device. (NEC 750.50)

Before a dialled-down setting sizes the circuit

Sizing a conductor from an adjusted current setting rather than from the unit's rating is NEC 625.42(B). The 4 below are the conditions this tool can name, and 750.30(C) attaches three of them. They are not the full set: 625.40, 625.54 and 750.30's own capacity requirement are untouched by them. Size from the full rating unless a qualified person has verified these, 625.42(B) itself and the manufacturer's instructions, on site:

• Where the adjusting means is part of an energy management system, that system uses monitoring and controls to automatically cease current flow if it malfunctions. 750.30(C) applies its subdivisions where an EMS is used to limit the current on a conductor. (NEC 750.30(C)(2))
• Access to the ampere adjusting means is restricted by at least one of the five means 750.30(C)(3) names: behind a removable and sealable cover over the adjustment, behind a cover or door that needs a tool to open, behind a locked door accessible only to qualified personnel, password protected with the password held by qualified personnel only, or software whose access to the adjusting means is password protected for qualified personnel only. (NEC 750.30(C)(3))
• The equipment supplying the branch circuit is field marked with the maximum current setting, the date of the calculation and setting, identification of the loads and sources associated with the current limiting feature, and the words "The setting for the EMS current limiting feature shall not be bypassed". Those markings meet the requirements in 110.21(B) and are located so they are clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment. (NEC 750.30(C)(4))
• The adjustment was made in accordance with the manufacturer's instructions, and the adjusted rating appears on the rating label with enough durability to withstand the environment involved. (NEC 625.42(B) / 110.3(B))

NEC 625.42(B) + 750.30(C) + 110.3(B)

The fix can void the basis of the answer that recommended it

This is the part of NEC 220.87 that catches people out, and it is worth reading before you quote the job. Where a 220.87 answer came from the 30-day recording exception rather than from a full year of demand data, the engine returns this:

Your answer above used the 30-day recorded peak, which is NEC 220.87's exception rather than its main rule. That exception is not permitted where the service has a renewable energy system (solar PV, wind) or any form of peak load shaving. If you install load management here, a later 220.87 check on this service cannot rest on a new 30-day recording; it needs 12 months of utility demand data.

Frequently Asked Questions

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