Okay, let's break down how the type of cable impacts conductor sizing in a townhome service/feeder situation under the 83% rule.

Understanding the 83% Rule (Article 310.15(C)) The 83% rule allows you to reduce the ampacity of service conductors or feeder conductors supplying an individual dwelling unit (like your townhome) if the load calculation doesn't exceed certain conditions outlined in the NEC. Essentially, it acknowledges that dwelling unit loads aren't always running at their full capacity simultaneously. It's usually applied to the service entrance conductors (SE conductors) or the feeder conductors that supply the entire dwelling unit.

Impact of Cable Type (Romex (NM-B) vs. SE Cable) on Conductor Sizing

Here's how the cable type affects your conductor size when the 83% rule applies:1. Romex (NM-B)



Temperature Limitation

NM-B cable has a 60°C ampacity rating when it's in direct contact with thermal insulation (like within an insulated wall). Even if the terminals are rated for 75 degrees C, you still have to use the 60 degrees C rating for ampacity. You use the 90 degrees C rating for correcting and adjusting only. If not installed in direct contact with thermal insulation you can use the 90 degrees C rating for correction and adjustment.

Ampacity Table

You must use the 60°C column of Table 310.16 (in the NEC) to determine the ampacity of NM-B cable when it's installed in thermal insulation.

Application

You can use romex for service entrance conductors but you will have to protect it if it is subject to physical damage.

Sizing

Using the 60 degree C column and the 83% rule can potentially reduce the size of the conductors if the load calculation allows.2.

SE (Service Entrance) Cable



Temperature Rating

SE cable is typically rated for 75°C (and sometimes 90°C, check the cable's marking).

Ampacity Table

You would use the 75°C (or 90°C, if permitted by the cable marking and terminal ratings) column of Table 310.16 to determine the ampacity.

Application

SE cable is designed explicitly for service entrance applications and can be used from the service drop/lateral to the service equipment.

Sizing

Because you're using a higher temperature rating (75°C or 90°C), you'll generally get a higher ampacity for a given conductor size compared to NM-B. This means you might be able to use a smaller conductor size with SE cable than with NM-B if the 83% rule applies and the higher temperature rating is permitted by the terminals.

In Summary



Yes, the size of the conductors (Phase/Neutral) can change depending on the cable type (Romex vs. SE) if you are applying the 83% rule. Romex (NM-B)

The lower 60°C ampacity rating could force you to use a larger conductor size compared to SE cable for the same calculated load.

SE Cable

The higher 75°C (or 90°C) rating can potentially allow you to use a smaller conductor size compared to Romex for the same calculated load, provided the 83% rule conditions are met and the equipment terminals are rated for the cable's temperature.

Important Considerations and Steps


Accurate Load Calculation

This is critical. You must perform a complete load calculation for the entire townhome according to Article 220 of the NEC. This will determine the minimum required ampacity of your service or feeder conductors.2.

Check Terminal Ratings

Always verify the temperature ratings of the terminals in your service equipment (main breaker, panelboard, etc.). You cannot exceed the lowest temperature rating. For example, if your breaker terminals are only rated for 75°C, you cannot use the 90°C ampacity of the SE cable, even if the cable itself is rated for 90°C.3.

Apply 83% Rule (If Qualified)

After you have your calculated load, determine if your situation meets the conditions for using the 83% rule as outlined in the NEC. The specific requirements are in Article 310.15(C).
4.

Ampacity Tables

Use the correct ampacity table (Table 310.16) and the correct temperature column (60°C for NM-B in thermal insulation, 75°C or 90°C for SE, depending on terminal ratings and cable marking) to determine the allowable ampacity of different conductor sizes.5.

Select Conductor Size

Choose the conductor size that has an ampacity equal to or greater than the load calculated after applying the 83% rule (if applicable).6.

Overcurrent Protection

The overcurrent protection (circuit breaker or fuse) must be sized to protect the conductor according to its ampacity rating. The overcurrent protection device cannot exceed the ampacity of the conductor.7.

Local Codes

Always check your local electrical codes and regulations. They may have additional requirements or restrictions beyond the NEC.

Example Scenario

Let's say after performing a load calculation and applying the 83% rule, you determine you need a minimum ampacity of 120 amps for your service conductors.

Using NM-B (Romex)

You would look at the 60°C column of Table 310.16. You would likely need a #1 AWG copper conductor (rated at 110 amps - which is close but not over so you would need to round up to the next size) or #1/0 AWG copper (rated for 125 amps).*

Using SE Cable (75°C terminals)

You would look at the 75°C column. You might be able to use #2 AWG copper (rated for 115 amps - which is close but not over so you would need to round up to the next size) or #1 AWG copper (rated for 130 amps).

Disclaimer:

I am an AI chatbot and cannot provide electrical design advice. This information is for educational purposes only. Always consult with a qualified and licensed electrician for any electrical work. They can properly assess your specific situation, perform accurate load calculations, ensure compliance with all applicable codes, and ensure the safe and proper installation of your electrical system. Failure to do so can result in serious hazards, including fire and electrocution. Flag for review