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How to Find the Right Wire Size for 100 Amp in AWG?

What Size Wire is Needed for a 100-Amp Subpanel Running 80 to 100 Feet to a 100-Amp Breaker?

The proper size of the wire for a 100-amp load circuit (e.g., subpanel) depends on factors such as the distance between the main panel and the load circuit (i.e., subpanel) as the voltage drop over the length of the run is serious consideration. Other factor affecting the wire size are the voltage (120V or 240V), ambient temperature, the number of wires in a bundle and the type of wiring used. Let’s perform electrical calculations to determine the perfect wire size for a 100A circuit in both ideal and real scenarios.

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In the US, a 25kVA, 7200V/240V distribution transformer provides electric power at 100 amps as follows:

  • Power = Voltage x Amps
  • Power = 240V x 100A
  • Power = 24,000 Watts = 24kW.

Based on this calculation, the ideal wire size for a 100 Amp breaker is #1 AWG for copper and #1/0 AWG for aluminum, according to the AWG Size chart and NEC Table 310-15B (formerly 310-16) Article 310.60 (given below). However, this may not be a suitable approach when considering distance, as it can lead to high voltage drops. To address this, we will perform a more detailed calculation based on NEC and related tables to determine the correct wire size for a 100 Amp circuit over a distance of 100 feet.

How to Find the Right Wire Size for 100 Amp Subpanel in AWG

According to NEC (National Electric Code);

  • A 100 Amp circuit should be utilized for 80% of its rated load, i.e., a minimum of a 125A power circuit can be used for a 100A load circuit (220-2 Code – NEC).
  • Add 20% of additional ampacity for every 100 feet of distance to counter the voltage drop (310-16 Code – NEC).
  • In the case of a load center and main panel, a diversity (demand factor) of 35% should be considered for general lighting (NEC – 220.45).

Good to Know: All calculations are based on a median temperature of 167°F (75°C). Wire sizes may vary with changes in temperature. Refer to the wire chart and table for different ambient temperatures and their related ampacities

Wire Size for 100A Breaker

Now, let’s find the right size wire for a 100-amp circuit with distances ranging from 80 to 100 feet.

First, we will apply (220-2 NEC), i.e., the minimum wire ampacity of the power circuit should be 1.25 times that of the rated load circuit, for example.

Wire Ampacity = 100 Amps x 1.25 = 125 Amps.

Required Wire Size: #3AWG Cupper (#1 AWG for Aluminum)

Wire Size for 100A Subpanel at 0ft

If the subpanel is installed right beside the main panel, the same rule of thumb applies to the wire size of the subpanel as calculated for the circuit breaker above. However, this is a rare case where the subpanel is almost attached to the main panel. Let’s continue to find the wire size in AWG for a 100A panel at a distance of 100ft.

Wire Size for 100A Subpanel at 100ft

Since there is a distance of 80-100 feet between the main panel and subpanel, the length of the run becomes a critical factor. So, we will apply 310-16 Code – NEC.

Wire Ampacity for 100 ft away = 125A + 25A = 150 Amps.

Required Wire Size: #0AWG (1/0 AWG) Cupper (#3/0 for Aluminum)

based on the above calculation and AWG chart and tables based on NEC, the right size wire for 100 amp circuit at a distance of 100 feet from the main panel to subpanel is 1/0 AWG (AKA #0 AWG). It means, an the wire having ampacity of 150A is sufficient to deliver 100A to the the load circuit located 100 ft away.

For a 100-amp subpanel that is 80-100ft away from the main panel, you would need to consider both the current capacity and the voltage drop due to the distance. Here’s what you need to know:

  • The wire size should be selected such that it can carry 80% of the rated current of the breaker, which in this case is 100 amps.
  • For copper wires, a #0 AWG wire with a 150 amp median capacity is most appropriate. This wire size can handle up to 120 amps (80% of 130A), which is more than the 100A breaker rating.
  • For aluminum wires, a #3/0 AWG wire is typically used for a 100-amp subpanel having a distance of 100ft.

As the finalized size of the wire is based on calculated values and general rules of thumb, it’s always recommended to consult with a licensed electrician for electrical installations.

Wire Size for 100A Subpanel with Demand Factor

We all know that almost all loads can’t be operated at once; for example, only the air conditioner operates in the summer (while the heater is OFF), and vice versa in the winter. Similarly, electric iron, stove, hair dryer, light bulbs, washing machine, and TV, etc., are not running at the same time.

NEC 220.14 permits 220.55 and Table 220.55 to be used for calculating branch circuit loads for ranges. See note 3 of Table 220.55 too. If the load is non-continuous (e.g. water heater, ranges etc.), the demand for 1 unit in Column B of Table 220.55 is 80% of nameplate. In that case, if the rating of required load is 10kW, we will use 80% of the load i.e. 10kW x 80% = 8kW and do the calculation based on it.

Suppose the total required load for a load center, main panel (or sub-panel), is 24kVA designed for general lighting, and we need to identify the wire size for that service while considering the demand factor (diversity factor).

According to NEC Table 220.42 and (220.45), the first 3 kVA is rated at 100%, while the remaining load can be rated at a demand factor of 35%, in the case of general lighting load. This way:

  • The first 3 kVA at 100% = 3 kVA
  • Remaining 24 kVA (24 kVA – 3 kVA) at 35% = 7.35 kVA

Net total of general lighting and small appliances =  3 kVA + 7.35 kVA = 10.35 kVA

Now, let’s find the current in amperes

  • I = P ÷ V
  • I = 10.35 kVA ÷ 240V
  • I = 43.125 Amps

To add the safety factor of 80% load on the rated current, we multiply 1.25 to the calculated value as:

43.125 x 1.25 = 53.90A

To be on the safe side, add an additional 20% of ampacity for future loads. For this purpose, allocate space for two additional branch circuits to accommodate future expansions.

53.90A x 1.25 = 67.375 A

Now, add additional 20% to counter the voltage drop for service having distance of 100 feet.

67.375 A x 1.25 = 84.21 A.

According to NEC table 310.10, choose wire size of #4 AWG for copper and #2 AWG for aluminum. To be on more safe side (if load included other than lighting), AWG #3 or 2 for copper and AWG #1 or 1/0 for aluminum is suggested and recommended wire size for 100A of load canter, main panel of subpanel having a distance of 100 feet.

Good to Know:

  • The calculation is based on a 240V main panel or subpanel installation at a mean temperature of 75°C (167°F). In the case of a basic 120V panel and load circuits, change the value from 240V to 120V and find the wire size in AWG.
  • The wire size is typically determined by the ampacity (current-carrying capacity) and the voltage drop. The National Electrical Code (NEC) provides guidelines for wire sizes and ampacities for copper and aluminum conductors.
  • It is recommended to use copper wire over aluminum wire. While copper wire is more expensive, it is also more durable and has a higher ampacity than aluminum wire.
  • Additionally, use THHN or XHHW insulation. These types of insulation are designed for use in high-temperature and wet environments. Ensure the correct conduit size is used; it should be large enough to accommodate the wires without overfilling it.
  • Consider the number of wires in the bundle. If you are running multiple wires in the same conduit, you will need to use a larger wire size to compensate for the heat generated by the wires.
  • For smooth operations and safety, consult with a licensed electrician for proper wiring installation to comply with local area codes.

AWG Table and Chart:

Wire Gauge Size & Wire Ampacity Table

Click image or open in a new tab to enlarge

NEC Table 310.15(B)(16) (formerly Table 310.16) - 310.60 - ARTICLE 310 - CONDUCTORS FOR GENERAL WIRING & Allowable Ampacities of Conductors & Wire Sizes

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