What is the Rated Ampacity of Copper AWG 4-3 Romex?

By Dave Rongey - Summary:

The amperage capacity will depend on the Type of Insulation, such as THW, THHN, etc. They all have different temperature ratings. This will show…

Electrical Question from Paula about Circuit Wiring

Received from Paula a Homeowner in Vicksburg, MS

Question: What is the rated ampacity of copper AWG 4 – 3 Romex wire with a #8 ground when used to route from an outside main panel to an inside sub-panel through a garage attic area? (Residential construction)

Dave’s Reply:
Thanks for your electrical question.
Hi Paula – The amperage capacity will depend on the Type of Insulation, such as THW, THHN, etc. They all have different temperature ratings.
Please follow this links where you will find more detailed information:
Allowable Ampacities of Insulated Wire Conductors

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2 Responses to “What is the Rated Ampacity of Copper AWG 4-3 Romex?”
  1. Dave Rongey says:

    Understanding Voltage and Amperage

    Hi Jody – Great Question,
    I understand how the topic of voltage and amperage can be confusing, let me explain without getting too technical.
    Using your example of 6/3 cable with a ground, the L1 and L2 conductors have the capacity of 55 amps each, and this amperage capacity is not added together for a total circuit capacity. Because the 6/3 cable has a neutral conductor which will be used in the circuit the voltage will be expressed as 120/240 volts. If this were a 6/2 cable with L1, L2 and a ground wire then the circuit voltage would be expressed as a 240 volt circuit.

    120/240 Volt Panel
    For example a sub panel which is fed by a 6/3 cable with a ground has the capability of providing other circuits of both 120 and 240 volt circuits.
    120 Volt Circuits
    The 120 volt circuits would have a hot (L1 or L2), neutral and a ground for applications such as 120 volt receptacle outlets and lighting.
    240 Volt Circuits
    Some 240 volt circuits may have two hots (L1 and L2), and a ground for equipment that does not require a neutral wire, such as a conventional air conditioner unit that is part of a HVAC system.
    120/240 Volt Circuits
    Other circuits may be 120/240 volt which would have two hots (L1 and L2), a neutral and a ground for equipment that does require a neutral wire, such as a new electric clothes dryer or electric range.

    As you see, the voltage of a circuit will depend on the application.

    Testing 120 and 240 Volt Circuits
    With this discussion, if you use a voltage tester to measure the voltage between L1 and L2 you will have a 240 volt reading, however if you measure the voltage between L1 or L2 to ground or neutral you will have a 120 volt reading.
    The reason why the voltage is different between L1 and L2 is because the generator that creates L1 and L2 are out of phase and not in sync.

    Neutral and Ground Wires
    You may ask why we need a neutral wire when a ground wire is typically a part of every cable? The answer is that the neutral wire is insulated and is considered a current carrying conductor. However a ground wire, insulated or not, is designated as a equipment safety grounding system which is bonded the earth and should never be used as a continuous current carrying conductor.

    Your not alone with this topic, I remember having several discussions with my father about voltage and amperage.

    I hope this helps you,

  2. jodi says:

    Using Romex 6/3 as a example: If this cable has 2 conductors and a neutral will the L1 and L2 conductor carry 55 amps or is 55 amps the total of both l1 and l2 combined. Also, if each conductor is carrying 120 volts, is the whole 6/3 cable said to be carrying 120 or 240 volts? These questions apply to a voltage drop calculation that I am working on. This topic is something that confuses me.


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