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Why do I need to worry so much about ampacity?

on Thu Sep 07, 2017 9:20 pm
Whats with Rule 4-004? what are they talking about? Is it really that important?
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Re: Why do I need to worry so much about ampacity?

on Thu Sep 07, 2017 9:23 pm
well, I guess it's important if:
1) you want to get the answer correct and pass
2) you don't want installations to go up in flames

Rule 4-004 Ampacity of wires and cables

When a conductor carries current, especially at ampacity levels of 80% or more of its ampacity rating, the
temperature of the insulated conductor begins to exceed the ambient temperature and continues to increase as
the conductor’s current increases. The temperature will rise even further if heat dissipation is restricted by
• proximity to other current-carrying conductors;
• the sheath of the cable or the material of the raceway; or
• the environment in which the cable or raceway is installed, where the ambient temperature exceeds 30 °C.
Heat can cause damage to the conductor’s insulation and nuisance tripping of temperature-sensitive
overcurrent devices (e.g., low-melting-point fuses and time-limiting-type circuit breakers); it can also lead to the
breakdown of terminations or equipment. The conductor’s temperature must be kept at an acceptable level.
This can be achieved by
• reducing the number of heat-producing conductors in a raceway or cable;
• increasing the movement of air around conductors or cable;
• increasing the separation distance between conductors; or
• applying correction factors to reduce the required current flow that causes the heating.

see also table 5A and 5C for derating factors
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