How to Size Wire for a 200 Amp Service (NEC Guide)

The “83 Percent Rule”: Sizing Service Conductors with NEC 310.12

The primary reference for sizing residential service conductors is found in NEC 310.12 of the 2023 National Electrical Code. This section specifically addresses single-phase dwelling services and feeders rated from 100 to 400 amps. It permits the service conductors to have an ampacity of not less than 83% of the service rating. This provision exists because the diversified loads in a dwelling unit rarely, if ever, draw the full 200 amps for a sustained period.

The calculation is straightforward:

• Service Rating: 200 Amps
• Required Ampacity: 200 Amps × 83% = 166 Amps

With this minimum ampacity of 166A, we turn to the ampacity values in NEC Table 310.16 (often referenced in conjunction with 310.12). It’s standard practice to use the 75°C column for service conductor calculations, as most equipment lugs are rated for 75°C.

• A 2/0 AWG copper conductor (THHN/THWN-2) has an ampacity of 175 amps at 75°C.
• A 4/0 AWG aluminum conductor (THHN/THWN-2) has an ampacity of 180 amps at 75°C.

Both of these options exceed the required 166A minimum, making them the standard correct sizes for a typical 200 amp service conductor installation. It is critical to note that this 83 percent rule only applies when the conductors supply the entire load of a dwelling unit. It cannot be used for sub-panels that only feed a portion of the home’s load, such as a detached garage.

Copper vs. Aluminum: Choosing Your 200 Amp Service Conductor

Both 2/0 copper and 4/0 aluminum are code-compliant choices, but they have distinct differences in cost, size, and installation requirements. As an electrician, advising a client or making a decision for a project requires weighing these factors.

2/0 Copper Wire for 200 Amp Service

Copper is an excellent conductor with high tensile strength and lower resistance compared to aluminum. A 2/0 copper wire is smaller in diameter than its aluminum counterpart, which can make it easier to pull through conduit and terminate in tight spaces. Its primary drawback is cost, as copper prices are significantly higher than aluminum.

4/0 Aluminum Wire for 200 Amp Service

4/0 aluminum wire is the most common choice for 200 amp service entrance conductors, primarily due to its significant cost savings. While it is larger and requires more care during installation—such as using anti-oxidant compound and ensuring proper torque on lugs—it provides a safe and reliable installation when done correctly. The increased size is necessary to achieve an equivalent ampacity rating to copper.

Key Takeaways for Conductor Choice:

• Cost: Aluminum offers substantial savings, making it the preferred material for most residential service laterals and entrance conductors.
• Size & Flexibility: Copper is smaller and more flexible, which can be an advantage in challenging conduit fill scenarios.
• Installation: Aluminum requires specific installation techniques, including the use of listed dual-rated lugs and anti-oxidant paste to prevent oxidation and ensure a solid connection.
• Authority Having Jurisdiction (AHJ): Always verify if your local jurisdiction has any amendments or preferences regarding conductor material for service entrances.

Step-by-Step Guide to Sizing a 200 Amp Service

Correctly sizing all components of a 200 amp service is crucial for a safe installation and for passing a journeyman exam prep question. Follow these steps, grounded in the NEC handbook.

1. Perform a Residential Load Calculation: Before anything, complete a thorough load calculation per NEC Article 220. This determines the actual expected load and confirms that a 200 amp service is appropriate for the dwelling. This calculation must account for general lighting, appliances, HVAC (a continuous load), and potential future additions like EV chargers. For a deeper dive into modern requirements, explore how the 2023 NEC changes dwelling unit load calculations.
2. Select the Service Entrance Conductors: Based on NEC 310.12, select either 2/0 AWG copper or 4/0 AWG aluminum for the ungrounded conductors, assuming no significant ambient temperature correction is needed.
3. Size the Grounded (Neutral) Conductor: Per NEC 220.61, the neutral conductor can often be sized smaller than the ungrounded conductors, as it only carries the unbalanced load. The calculation involves the loads from lighting, cooking equipment, and the non-continuous portion of other loads. However, it cannot be smaller than what’s required by NEC 250.24(C).
4. Size the Grounding Electrode Conductor (GEC): The GEC connects the service to the grounding electrode system (e.g., ground rods or a water pipe). Its size is determined by NEC Table 250.66, based on the size of your service entrance conductors.
5. Size the Bonding Jumper (Main): The main bonding jumper connects the grounded conductor to the equipment grounding conductor at the service disconnect. It is also sized according to the ungrounded service conductors, using NEC 250.102(C), which often directs back to the values in Table 250.66.
6. Verify Overcurrent Protection: The main disconnect must be rated for the service, so a 200 amp breaker is required. This device provides overcurrent protection for the service conductors. Understanding the rules for disconnects, including how many service disconnecting means are allowed, is a key part of service design.

Critical Considerations Beyond the Main Conductor

Grounding Electrode Conductor Sizing (NEC Table 250.66)

Proper grounding electrode conductor sizing is non-negotiable for safety. Using NEC code Table 250.66, you select the GEC size based on your ungrounded service conductors.

• For 2/0 AWG copper service conductors, the GEC must be at least #4 AWG copper.
• For 4/0 AWG aluminum service conductors, the GEC must be at least #2 AWG copper or #1/0 AWG aluminum.

Remember the exceptions in NEC 250.66(A), (B), and (C). For instance, if the GEC connects only to a ground rod, it is not required to be larger than #6 AWG copper.

Voltage Drop Calculation for Long Service Runs

While the NEC doesn’t mandate a specific voltage drop percentage for services, it provides strong recommendations in its informational notes (FPNs) to limit voltage drop to 3% for feeders and 5% total for the system to ensure efficiency. For a long service lateral, a voltage drop calculation is essential. A significant drop can cause equipment to run inefficiently or fail. If a long run (e.g., over 150-200 feet) results in a drop over 3%, you must increase the conductor size beyond the 2/0 or 4/0 minimum. For example, you might need to upsize to 3/0 copper or 250 kcmil aluminum to mitigate the voltage loss.

Load Calculations: The Foundation of Correct Service Sizing

All wire sizing begins with accurate load calculations as prescribed in NEC Article 220. With homes now including high-draw appliances, heat pumps, and EV chargers, relying on assumptions is a critical error. An accurate load calculation ensures that the 200 amp service is sufficient and prevents oversizing or dangerous undersizing. Master the essential skills needed for these calculations and more. Master residential service calculations with our expert-led courses.

Don’t forget modern requirements like outdoor emergency disconnects, which are now mandatory for one- and two-family dwellings. Understanding how outdoor emergency disconnects are required is just as important as conductor sizing for a compliant installation.

Primary Sources

• NFPA 70, National Electrical Code (NEC), 2023 Edition

Frequently Asked Questions (FAQ)

What size wire for 200 amp service is required by the NEC?

The NEC generally permits 2/0 AWG copper or 4/0 AWG aluminum conductors for a 200 amp residential service. This is based on NEC 310.12, which allows the conductor’s ampacity rating to be 83% of the service rating (166A minimum).

Can I use 4/0 aluminum wire for a 200 amp service?

Yes, 4/0 aluminum wire is a very common and code-compliant choice for a 200 amp service entrance. It has an ampacity of 180 amps at 75°C, which exceeds the 166A minimum required by the 83 percent rule. Proper installation techniques, including using an anti-oxidant compound, are critical.

How do I size the grounding electrode conductor for a 200 amp service?

You must use NEC Table 250.66. The size is based on your service entrance conductors. If you used 2/0 copper, you need a #4 AWG copper GEC. If you used 4/0 aluminum, you need a #2 AWG copper or #1/0 AWG aluminum GEC.

Does voltage drop affect the size wire for a 200 amp service?

Yes, significantly. While 2/0 copper or 4/0 aluminum is the minimum size, you must perform a voltage drop calculation for long service runs. If the calculated drop exceeds the recommended 3-5%, you must upsize the conductors to compensate and ensure proper voltage at the service panel.