In a commercial environment it is mandator for the installer/electrician to provide available fault current calculations on equipment.
A) True
B) False
Part A of 110.24 states, "Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved." The calculation shall be documented and made available to those authorized to design, install, inspect, maintain, or operate the system.
The formula to calculate available fault current (AFC) is:
- ISC = I x M
- Where:
- ISC: is the available short-circuit current
- I: is the available fault current at the beginning of the conductor
- M: is the multiplier, calculated as 1 ÷ (1 + F)
- F: is calculated as (1.732 x L x I) ÷ (C x E_(L_L))
- ISC: is the available short-circuit current
In this formula, the variables are:
- L: is the length of the conductor
- C: is a combination of factors, including the type and size of the conductor, and the properties of the raceway
- E_(L_L): is the line-to-line voltage of the electrical circuit
To calculate fault current, you also need to know the total resistance from the power source to the fault location. This involves identifying all possible sources of current, such as electric generators, synchronous motors, induction motors, and the electric utility system.
The maximum available fault current, also known as the available short-circuit current, is the highest amount of current that can flow through an electrical system during a short circuit.
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