Load Calculation: The Arithmetic Behind Service Sizing > 자유게시판

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Whether a house needs a larger electrical service is usually answered with an opinion when it can be answered with arithmetic. A load calculation is that arithmetic, and understanding roughly how it works makes it much easier to judge advice.

The method starts from the idea that not everything runs at once. If you simply added the ratings of every appliance and every circuit, you would arrive at a number far larger than any house ever draws. So the calculation applies demand factors, which are standardised assumptions about simultaneity.

The general approach works like this.

A base figure is derived from the floor area of the dwelling, covering general lighting and receptacle circuits. This is a fixed number of volt-amperes per square unit of area.

Fixed allowances are added for small appliance circuits in the kitchen and for laundry.

Then the fixed appliances are listed individually: range, water heater, dryer, dishwasher, disposal, and so on, each at its nameplate rating.

Heating and cooling are handled specially. Since they rarely run simultaneously, only the larger of the two is counted.

Demand factors are then applied to portions of the total, on the basis that a house with many appliances does not run all of them at full output at the same moment.

The result is a demand in volt-amperes, which divided by the service voltage gives amperes. That number is compared against the existing service rating.

Two practical points follow from this.

First, electric heat, electric ranges and electric water heaters dominate the result. A house with gas for all three can run comfortably on a service that would be inadequate for the same house all-electric.

Second, an electric vehicle charger is a large continuous load and is treated as such, which is why it so often turns a comfortable service into a marginal one.

Asking for the calculation before agreeing to an upgrade is a fair request. It converts a sales conversation into a technical one, and sometimes the answer is that the existing service is fine and a subpanel solves the actual problem.

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