In this guide, let us discuss the importance of wire and conductor sizes in electrical systems. After that, we will see what wire size is suitable for 100-amp service panel or sub-panel wiring. If you are just starting construction on a new home or if you are planning to extend the existing electrical system with a sub-panel, then you need to know the 100 amp wire size (or wire size for appropriate service or feeder wire). WARNING: This is not a user guide on how to wire the 100 amp service entrance or sub-panel. This is just a technical discussion on the 100 amp wire size you need for such installations. If you are planning to install or set up 100 amp service panels or sub-panel wiring, then we highly recommend you consult with an experienced and licensed electrician.
A Brief Note On The Electrical Service
When we say the term “service”, we mean the wires and cables responsible for bringing electricity from the street transformer to the main circuit breaker panel in your home. The two common methods for bringing the service cables into the house are the Overhead Conduit System and Underground System. In an overhead or aerial system, the utility company brings three cables, two hot and one neutral, from the pole-mounted street transformer to the Service Head at the roof of the house. The utility is responsible for these cables and calls it the Service Drop. The service drop terminates at the service head on the roof and from here onwards, it is the responsibility of the customer to follow NEC guidelines and properly wire the cables. Here comes the Service Entrance Conductors. You have to tie the Service Entrance conductors to the Service Drop conductors and pass them through the service head on the roof and then through a Service Mast. From here, the Service Entrance cable goes to the Meter Base. You have to connect another set of Service Entrance conductors from the meter to the Service Entrance Equipment (the main control panel or breaker panel). In an Underground Service, the three conductors are insulated and are known as Service Lateral, if the utility provides them, or Underground Service Conductors, if you provide them. The underground conductors can come from a pole-mounted street transformer or a concrete pad-mounted transformer. Regardless of the type of transformer, the underground conductors are buried 4 feet in the ground and connect to the meter base at your premises. From the meter, we have to use a Service Entrance conductor to connect to the Service Entrance Equipment (the main service panel or circuit breaker panel).
100 Amp Wire Size
Depending on your contract with the utility, the Service capacity or amperage can be a minimum of 100 amps and can go all the way up to 400 amps. The 100 amp service is the bare minimum nowadays. If you have this service, then what is the 100 amp wire size you should use for your Service Entrance Conductors? Before proceeding, you need to remember that we need two sets of Service Entrance conductors; one set between the Drip Loop (roof Service Head) and the Meter Base and the other between the Meter Base and the Main Service Panel. The three common types of conductors are Copper, Aluminum, and Copper-Clad Aluminum. Depending on the type of conductor you chose, the 100 Amp Wire Size varies. In the following table, we mentioned the NEC recommended 100 Amp Wire Size for Service Entrance Conductors. Note that this table is as per the NEC Table 310.15(B)(7). Nowadays, NEC recommends the size of the conductors must support an amperage of at least 83% of the rating or capacity of the Service. But this time, we have to follow the NEC Table 310.15(B)(6) to get the size of the conductor. If you calculate properly, then you will get similar results as the previous table. In the following table, we mentioned the conductor size in AWG for Copper, Aluminum, and Copper-Clad Aluminum at 75°C temperature for some common ampacities. This table is based on the NEC 310.15(B)(6) table. From the above table, if we apply the 83% rule, then the minimum required conductor size must be calculated for 83% of 100 Amps i.e., 83 Amps. So, the 100 Amp Wire Size with an 83% rule is AWG 4 for Copper Conductors and AWG 2 for both Aluminum and Copper-Clad Aluminum Conductors. Apart from the Service Entrance conductors, the feeder wires i.e., wires from the main service panel or the circuit breaker panel to the sub-panel or different circuits must also follow the 83% rule based on the NEC 310.15(B)(6) table. In the 2020 revision, the NEC brought table 310.15(B)(7)back and renamed it 310.12.
Wires and Wire Sizes
Wires are conductors than can carry electric current through them. The opposite of conductors is insulators. Copper is one of the most common and popular types of conductors. The next best thing is Aluminum. We usually call Wires with some sort of plastic enclosures Cables. In a cable, we can have more than one wire with respective plastic insulators surrounding them. In an earlier guide, we made the American Wire Gauge or AWG Wire Size Chart. If you want more in-depth knowledge on that topic, we highly recommend you check that guide out. For the purpose of this discussion, we will only cover the basics. We represent the size of the wire in the form of numbers known as AWG Numbers. When we say the size of the wire, it usually means the diameter of the wire. An interesting thing about the AWG system is that the bigger the AWG number, the small the wire size. For instance, if you remember in the 15 amp vs 20 Amp Outlet comparison guide, we mentioned that the wire size of the 15 Amp circuit uses a 14 AWG wire while the 20 Amp circuit uses 12 AWG wire. Here, the 14 AWG wire is smaller (or rather thinner with less diameter) than the 12 AWG wire. This goes on for all the AWG Numbers. Number 1 AWG is the thickest of all wires and as the AWG Number increases, the thickness decreases. We have even thicker wires than Number 1 AWG. The naming scheme for these wires is slightly different. Some common AWG wire sizes that are thicker than AWG 1 are AWG 4/0 (or AWG 0000), AWG 3/0 (or AWG 000), AWG 2/0 (or AWG 00), and AWG 1/0 (or AWG 0). Of these, the AWG 4/0 is the thickest with a diameter of almost half an inch.
Amperage or Ampacity
The thickness or the AWG of the wire determines the maximum current it can carry safely. We call this the Ampacity or the Amperage of the Wire. If you are familiar with electrical terms, then you might know that we measure electric current in Amperes. The following table gives the Ampacity of different AWG Wire sizes. In the table, we mentioned the amperage for copper, aluminum, and copper-clad aluminum wires with different temperature ratings.
Voltage Drop in Wires
There are two things that happen when you force too much current through a wire. The first thing is the power loss in the form of I2R losses. The other thing is a drop in the voltage. While power losses can be minimal and don’t impact the performance of the equipment on the other end of the wire, you need to be concerned about the voltage drop. Assume you are running a 1-hp motor from a 120V outlet. Depending on the length of the cable, the voltage at the motor can be 119V (at approximately 10 feet) or drop to 112V (at approximately 100 feet). This difference in voltage at the outlet and the device or equipment is known as Voltage Drop. The Voltage Drop in the wire can lead to inefficient operation of the device or equipment. If there is a 10% voltage drop in the wire i.e., only 90% of the rated voltage is present at the load, in this case, a 1-hp motor, the motor operates only at 80% of its rated power. It is practically impossible to maintain zero voltage drop. But by picking the proper wire Size, we can minimize the voltage drop in the wires. The calculations of Voltage drop become very important if you are planning to wire a sub-panel at 100 feet, 150 feet, or 200 feet away from the main panel. As a rule of thumb, in addition to the 83% rule that NEC mentions, you need to consider an additional 20% per 100 feet length of the wire before calculating the amperage and the wire size.
Conclusion
Selecting the correct wire size, be it for a simple lighting circuit or the main service entrance conductor is very important. All the NEC guidelines help us in a safer utilization of electricity without any mishaps. Speaking of Service Entrance conductors, the minimum service capacity nowadays is 100 amps for small homes (but it is usually 200 amps for medium to large homes). If you have a 100 amp service or planning to wire a 100 amp sub-panel, then you need to know the 100 Amp Wire Size that you have to use for the Service Entrance conductors. In this guide, we saw the basics of a typical electrical service layout with both overhead and underground systems. After that, we took NEC tables as a reference and laid out the possible 100 amp wire sizes for the three common conductors i.e., copper, aluminum, and copper-clad aluminum. Then, we made a new table with the new NEC-recommended 83% rule. As a bonus, we saw the basics of wire and wires sizes, ampacity or amperage of different AWG Wire Sizes, and even an important topic of voltage drop. We hope that this guide on 100 Amp Service Entrance or Sub-Panel Wire Size could help you understand how to calculate the wire sizes properly for a given service capacity. If you feel we missed something or want us to add anything, do let us know in the comments section. It will not only help us but other readers as well. Comment * Name * Email * Website
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