Take 83 amps and utilize the 75degC column in Table 310.15 (B) to establish that the service requires overhead service entry conductors of 4 AWG copper, type THWN. A detailed examination of the old, deleted table yields the same findings. A sneak peek at the NEC is provided here. For a complete examiniation of all wiring requirements for this service, see page 23 of our free book.
So, for a 200-amp service, you may still use a 4/0 AWG aluminum or a 2/0 AWG copper, but only from the 75 degree C column in Table 310.15 (B). For 200 amps, you'll need a minimum of #6 ground or #4 aluminum. If you're not sure what wire size to use, call your local electrical supply house.
The table below shows the amp capacity of various metals at different temperatures:
Temperature (degrees C) | Metals | Ampacity ---|20 | Aluminium | 0.08" diameter | 200 | Copper | 0.02" diameter | 40 | Iron | 0.058" diameter | 100 | Steel | 0.045" diameter | 80 | Bronze | 0.025" diameter | 160
As an example, if you want to provide a 200-amp load and it's 20 degrees C outside, then you should use #6 aluminum cable because that's the smallest wire size that will meet the requirements. If it's 90 degrees F outside instead, then you should use #4 copper cable instead because smaller cables are more suitable for higher temperatures.
The important thing is that you use the right cable size regardless of the temperature.
1 3/0 AWG THHN insulated Copper is rated at 225 amps and falls under the 90 degree C or 194 degree F temperature tables. The safe working limit for temperatures internal to a metal-sheathed cable is 125 degrees C (257 degrees F). At 225 amps, the cable will get very hot and should not be placed in direct contact with any other material that would cause it to break down first.
The ampacity of wires within a cable is based on the number of conductors involved. In this case, there are two conductors inside the cable so the 300-amp rating applies to each individual conductor. The cable itself is also referred to as "thickness" material because it provides protection and support for the wiring within it. Conduit is thinner than the cable itself; therefore, it carries less current than the cable.
Conduit is used to channel electric power along a route from one point to another. It comes in various sizes and shapes but usually consists of a tube or duct containing several strands of wire wrapped around a cylindrical core. The term "conduit" also refers to the material itself. Electricity can only flow through a conductor so to allow more current to pass, many conductors are grouped together.
Table 310.15(B), originally Table 310.16, is one of the National Electrical Code's most-referenced tables (NEC). It includes conductors with permitted ampacities of insulated copper and aluminum (or copper-clad aluminum) rated up to and including 2,000 volts (V). The table provides information on the maximum allowable voltage based on the overall size of the conductor involved. For example, if you were to use a 16-gauge wire to supply an area of 120 square feet at 12 V, the total current would be 1.2 A or 180 mA, well below the capacity of the wire.
The table was developed by the Underwriter's Laboratory, which tests products designed to protect against electrical shock. The table has been revised over time to take into account new technology and improved ways of measuring voltage with respect to ground. For example, prior to 1973, the voltage with respect to ground was not considered when calculating the maximum permissible voltage for cables. That is no longer true today because modern equipment uses differential detection techniques that can identify problems with low voltage near the end of a cable run before such problems develop into serious hazards.
Also, the original table did not consider voltage between conductors. This became relevant when fiber-optic communications cables began to appear in the market place. These cables contain several strands of fiber optic material each capable of conducting light but not electricity.
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