Sizing Electrical Service

One of the first steps for doing your own electrical installation is sizing the electrical service to your home.  In the US, residential buildings are normally fed with a 240V service from the utility company.  This is achieved with two 120V lines and a neutral line.  Older houses were often fed with 100A services or smaller.  You will find 200A size electrical services in many modern homes.  When talking electrical service size, don't confuse service VOLTS (120V/240V) with service AMP or WATTS.  Electrical services are sized according to AMPs.

Calculating what size electrical service you need takes a little homework but it's not too bad.  One thing to keep in mind is future growth.  The cost of upgrading your service down the road because you want to add a hot tub or air conditioner is going to be a lot more than bumping up to a larger service in the beginning.  For our cabin we ended up going with a 400A service.  A typical single family home will require a 200A or 250A service.  In our case, a 400A was required due to a number of heavy power hitters including:

5-Ton Heat Pump 7,130 VA  
20 KW Backup Electric Heat 20,000 VA
On-Demand Electric Water Heater #1 (54A) 12,960 VA
On-Demand Electric Water Heater #2 (54A) 12,960 VA
Electric Range 8,000 VA
Electric Dryer 6,000 VA
Hot Tub (future growth) 11,369 VA
  80,189 VA

 Electrical services for a house or garage are typically fed by aluminum conductors, rather than copper, due to cost.  The utility company will determine the proper size, supply the conductors, and install the conductors in the conduit all the way to the electric meter on your house.  This makes sense when you understand from their standpoint they aren't getting paid for the power until it passes through the meter.  Having homeowner install the conductors to the meter would create problems with people undersizing cables or even tampering with the conductors before they reached the meter.  Be sure to communicate the sizing of your electrical service to the power company before they show up to pull conductors...especially if you needed a larger-than-normal size service as we did. For our cabin electrical service, we were required to run 4" conduit from the meter panel to their transformer and to leave a rope inside for pulling the conductors.  The power company did the rest.  They did not charge anything but a $50 connection fee.

Residential Electric Service Sizing Calculator

NEC codebook (article 220) provides the details on properly sizing an electrical service.  After studying the codebook for some time, I discovered an online spreadsheet that simplified these calculations tremendously.  I've listed two links below to this service sizing calculator.  The first link is a new template for you to use and the second link is the spreadsheet I filled out for our cabin project.

Residential Load Calc (Excel, 1,225 KB)

Load Calc filled out for our cabin (Excel, 1,225 KB)

Once you've filled out the residential load calc spreadsheet, there is a box in the spreadsheet that indicates your minimum required service size according to NEC guidelines.

 

Electrical - The Basics

Residential Electrical Guidelines and Codes

Rough-In Electrical and Pulling Cable

Common Electrical Wiring Diagrams

Breakers and Fuses

Wire Gauge and Voltage Drop Calculator

NEC Reference Tables (2011, 2008, 2005, 2002, and 1999)

Sizing Your Electrical Service

Electrical - Main Service Line Installation

Fire/Smoke Alarm Installation

Doorbell Wiring

Phone Wiring

Low-Voltage Wiring

Cable Pinouts

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