Okay, let's break down these questions about load calculations for your residential service upgrade.

Understanding the Core Issue

Your core concern is how to size the feeder conductors running from your new meter/main combo to the subpanel within the house, given that you're upgrading the service and adding new loads during a renovation. The problem is you're not taking over the entire load, or replacing the load exactly as it was before.

Question 1: Feeder Calculation Method Yes, you'll likely need to use the General Load Calculation (Article 220 Part III) for the feeder. Here's why: Optional Calculation (220.82) Limitations

The optional calculation method (220.82) is designed to simplify load calculations for the entire dwelling unit. It offers significant demand factors that reduce the overall calculated load. However, you are specifically not calculating the load for the entire dwelling on the feeder. You're only calculating what's needed for the subpanel.

"Entire Dwelling Unit" Requirement

Optional Calculations (like 220.82) apply to calculating the

service load of a dwelling. Since you are only feeding the panel, not the whole house, you cant use the optional calculation. General Calculation Necessity

Because you can't use the optional calculation, you are left with Article 220, Part III. This is a more detailed method, requiring you to add up all the connected loads on the subpanel (lighting, receptacles, specific appliances, etc.) and apply appropriate demand factors.

Why the 83% Rule Doesn't Apply Here (310.12)

You are correct that you cannot use the '83% rule' in 310.12. 310.12 only applies to 'one-family dwelling units' and you must also meet the restrictions. A feeder to a sub-panel doesn't meet those requirements.

Question 2: Calculation Methods for a Subpanel Load (Not the Entire House) The General Load Calculation (Article 220 Part III) is Your Primary Method

This is the standard method for any feeder or subpanel load calculation that doesn't qualify for an optional calculation. You will need to identify each type of load connected to the subpanel. Apply demand factors from Article 220, as appropriate (e.g., for ranges, dryers, or other large appliances).

Question 3: 83% Rule and Subpanels Correct: You Cannot Typically Use the 83% Rule for a Subpanel Feeder. As described above, this rule only applies to 'one-family dwelling units', with specific restrictions.

How to Perform the General Load Calculation for the Subpanel Feeder


Inventory the Loads

Make a detailed list of all the loads that will be connected to the subpanel. This includes: General lighting (VA/sq ft, as per NEC). Small appliance branch circuits (1500 VA per circuit). Laundry circuit (1500 VA). Specific appliances (nameplate rating): Dishwasher Garbage disposal Microwave Electric oven/range (use the table values with demand factors) Clothes dryer (use the table values with demand factors) Water heater HVAC equipment (heating and cooling – use the larger of the two) Etc. Any other fixed loads.2.

Apply Demand Factors

Using Article 220, apply the appropriate demand factors to reduce the calculated load. For example: Table 220.55 (Ranges). Table 220.54 (Clothes Dryers). 220.53 (Household Cooking Appliances).
3.

Calculate Total Demand Load

Add up all the individual loads, after applying demand factors.4.

Size the Feeder Conductors

Use the total demand load to determine the minimum ampacity required for the feeder conductors. Refer to Table 310.16 (or the appropriate table for your conductor type) to select the conductor size. Also, ensure you have the appropriate size grounding and neutral conductors.
5.

Overcurrent Protection

The overcurrent protection (breaker or fuse) at the meter/main must be sized to protect the feeder conductors.

Example (Simplified)

Let's say your subpanel will serve: General lighting: 1000 sq ft x 3 VA/sq ft = 3000 VA Two small appliance circuits: 2 x 1500 VA = 3000 VA One laundry circuit: 1500 VA Dishwasher: 1000 VA Electric Range: 8 kW (8000 VA)1.

Initial Load

3000 + 3000 + 1500 + 1000 + 8000 = 16500 VA2.

Demand Factors

Let's apply some demand factors: Lighting and Receptacles: Apply the first 3000 VA at 100%, then 35% for the remainder. 3000 VA + (16500 - 3000) 0.35 = 3000 + 4725 = 7725 VA Use Table 220.55 for the Range: Let's assume Column C applies and the demand is 8 kW.
3.

Total Demand Load

7725 + 8000 = 15725 VA4.

Amps

15725 VA / 240V = 65.5 Amps5.

Conductor Size

You would need to select conductors rated for at least 70 amps (check the specific temperature rating and adjust as needed). You would also need an appropriately sized ground.

Important Considerations



Local Codes

Always check with your local building department and electrical inspector for any specific requirements or interpretations of the NEC in your area. They have the final say.

Future Expansion

Consider future load additions when sizing the feeder and subpanel. It's often wise to upsize slightly to provide room for growth.

Voltage Drop

Calculate voltage drop in the feeder, especially for longer runs, to ensure it remains within acceptable limits (typically 3% for feeders).

Neutral Conductor

Calculate the neutral load according to 220.61. The neutral may be smaller than the phase conductors, but it must still be sized appropriately for the expected imbalance.*

Conduit Fill: Ensure you're not exceeding conduit fill limitations when running the conductors to the subpanel.By using the general load calculation and carefully documenting your calculations, you'll be able to size the feeder correctly and ensure a safe and compliant installation. Good luck with your upgrade! Flag for review