Why is an electrical load calculation required?

Installation

Why is an electrical load calculation required?

October 8, 2022

avatar
Nick Sherry
Electrical Lead

TL;DR

Before installing your EV charger, an electrician is required to perform a residential electrical load calculation to determine if an EV charger can be safely installed in your home. This is critically important due to the large electrical loads produced by these devices. EV chargers can generate 3-4x more load than an air conditioner, the next largest load in most homes.

The residential electrical load calculation is governed by the National Electric Code (NEC), a United States standard for the safe installation of electrical wiring and equipment. The code is updated and published every three years.

Here is the General Method for calculating electrical load in a single family home:

  1. General use load: This is based on the inside square footage of the home and estimates basic lighting load and required appliance circuits. To calculate, multiply the home square footage by 3VA. Add 4,500VA to account for 2 small appliances and 1 laundry appliance. Volt-ampere (VA) is a standard unit of electrical power used to determine electrical circuit component requirements. It is calculated by multiplying the voltage and amperage current in a circuit.
  2. Permanently fastened load: Locate the nameplate of each major appliances in the home sum up the VA listed on each nameplate. If an appliance nameplate cannot be located, find the breaker for that appliance, multiply breaker value by .8, then multiply by 120V for single pole breaker or 240V for a double pole breaker to estimate VA. This gives you your existing permanently fastened load. Next, add the VA for the EV charger you’d like to install. If you don’t have the exact figure you can estimate it using the formula above. For example, a 40A JuiceBox 40 is installed with a 50A breaker, so estimated VA would be: 50A breaker * .8 *240V = 9,600VA.
  3. HVAC load: Locate the VA listed on the nameplates of the air conditioning unit and/or electric furnace/heating appliance if either is installed. Use only the larger of these two figures. If the nameplates are unavailable, find the HVAC appliance breaker, multiply breaker value by .8, then multiply by 120V for single pole breaker or 240V for a double pole breaker to estimate VA.
  4. Sum the general use load and permanently fastened load calculated above. If the total is less than 10,000VA then use this figure. If the total exceeds 10,000VA then use 10,000VA + 40% of the remaining VA. Next, add the HVAC load calculated above. Finally, divide this total by 240V to get the total load in amps (A).

Your EV charger can be safely installed with a new breaker if the load calculation with it installed is less than the total electrical service available for your home. Most homes have an electrical service of between 100 to 200 amps You can determine your home’s electrical service by looking at the amperage rating on your main disconnect breaker. This is typically located in your electrical service panel or near your electric meter. The main breaker will usually be the largest breaker you have and should have its capacity written on it or adjacent to it.

If your home has insufficient electrical capacity to install an EV charger, we’ll help you determine if an electrical service upgrade is necessary. In many cases, an upgrade can be avoided by installing a more affordable energy management system like a Tesla Neurio Energy Meter, or by using a circuit sharing device such as a NeoCharge. You can learn more about these solutions with our guide to electrical service upgrade alternatives.

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