Okay, this is a multi-faceted problem. Here's a breakdown of how to approach evaluating the electrical service and feeder capacity for adding electric ranges to these apartments

1. Gathering Information (Crucial First Step):
Nameplate Information



Main Service

Look at the main service disconnect. Note the amperage (400A total seems low for 10 units, so double-check that). Note the voltage (likely 120/240V single-phase). Note the available fault current (this is critical for safety).

Feeder Disconnects (Apartment Group A & B)

Carefully document the amperage of each 400A feeder breaker serving the groups of 5 apartments.

Existing Apartment Panels

Open the electrical panel in one or two representative apartments. Note the panel's amperage rating. Write down a list of everything currently connected to that panel (lighting, outlets, existing appliances – even small ones). Include the wattage or amperage of each item.

HVAC Equipment

Get nameplate information on any existing heating and cooling systems in the apartments (even if gas-fired, there's likely an electric blower).

Water Heaters

Are these gas or electric? Get nameplate information if electric.

Building Permits

Find out if there were building permits filed and approved. Those plans will have the electrical design

Future Plans

Ask the client if they plan to install any additional electric load (EV chargers in the future? Mini Splits? etc.)

2. Load Calculation - The Core of the Problem

You'll need to perform a load calculation according to the National Electrical Code (NEC). Here's the basic process (consult the current NEC for specific articles): NEC Article 220

This is the bible for load calculations.

Dwelling Unit Loads (NEC 220.11 - 220.19)

This is where you start.

General Lighting & Receptacle Loads (220.12)

Use 3 VA per square foot of living space for each apartment. This covers basic lighting and general-purpose outlets.

Small Appliance Branch Circuits (220.14(H)(1))

NEC requires 2 small appliance circuits at 1500 VA each for the kitchen. (3000 VA total minimum)

Laundry Circuit (220.14(H)(2))

1500 VA (required for each dwelling)

Electric Range/Oven (220.55)

This is the big one.



Column C of Table 220.55 is most likely the value you will use.

Demand Factors

The NEC allows for demand factors. As you add more ranges, the incremental load increase is less than the full nameplate ratings.

Example

If you have 5 ranges, and each range is rated 12kW, you

do not add 5 x 12kW = 60kW to the load. Use Table 220.55, Column C.

Other Fixed Appliances (220.53)

Dishwashers, garbage disposals, electric dryers, water heaters, etc. For 4 or more fixed appliances, you can apply a 75% demand factor.

Heating & Cooling Load (220.82)

Take the larger of the heating or cooling load. Don't add them together.

Feeder/Service Load Calculation



Apply Demand Factors (220.42)

Once you have the total load for all the apartments, you might be able to apply demand factors from NEC Article 220.

Existing Loads

You need to factor in the existing loads on the service.

Calculations

Calculate the total load for a single apartment with an electric range (worst-case scenario). Calculate the total load for all five apartments on each 400A feeder (using appropriate demand factors). Calculate the total load on the 800A (2x400A) service.

3. Comparing Load Calculations to Service and Feeder Capacity



Service Capacity

Compare the calculated total service load (including existing loads) to the actual 800A service rating. If the calculated load exceeds the service capacity, the service is undersized. You'll need to consider a service upgrade.

Feeder Capacity

Compare the calculated feeder load (for each group of 5 apartments) to the 400A feeder rating. If the calculated load exceeds the feeder capacity, the feeder is undersized.

4. Determining Appropriate Range Size

Based on your load calculations, you can determine the maximum size (wattage/amperage) of electric range that can be safely installed without overloading the service or feeders.

5. Voltage Drop Considerations

NEC Article 210.19(A)(1) and 215.2(A)(1) outline voltage drop requirements. Excessive voltage drop can damage appliances and cause them to operate inefficiently. Calculate voltage drop from the main service to the furthest apartment and from the apartment panel to the range outlet. Use voltage drop calculators (many are available online) or the formulas in the NEC Annex. If voltage drop is excessive, you may need to increase the conductor size of the feeders or branch circuits.

6. Conductor Sizing and Overcurrent Protection



NEC Articles 210.19 & 215.2

Size conductors appropriately for the calculated load and voltage drop requirements.

NEC Article 240

Protect conductors with the correct size overcurrent protection (circuit breakers or fuses).

Range Circuit

An electric range typically requires a dedicated 40A or 50A, 240V circuit. (Consult the range manufacturer's instructions for the exact requirements.)

7. Grounding and Bonding

Ensure proper grounding and bonding of the electrical system per NEC Article 250. This is critical for safety.

8. Safety Considerations



Available Fault Current

Calculate the available fault current at the main service and at the apartment panels. Ensure that the overcurrent protection devices (breakers and fuses) have a suitable interrupting rating to safely clear a fault. This is absolutely crucial. If the interrupting rating is too low, a fault could cause the breaker to explode, posing a serious hazard.

Arc Flash

Be aware of arc flash hazards when working on energized equipment.

Lockout/Tagout

Follow proper lockout/tagout procedures when working on electrical systems.

9. Documentation and Reporting

Keep thorough records of all calculations, measurements, and observations. Prepare a report for the client outlining your findings, including: Service and feeder capacity assessment. Recommended range sizes (if any). Any potential problems or limitations. Recommendations for upgrades (if necessary). A clear disclaimer stating that this is a preliminary assessment and that a more detailed engineering study may be required.

Tools You'll Need



National Electrical Code (NEC) – current edition

Clamp Meter (Amp Meter)

To measure actual current draw on existing circuits.

Voltage Meter

To measure voltage.

Calculator

For performing load calculations.

Tape Measure

To measure square footage of apartments.

Flashlight

For inspecting panels and equipment.

Laptop or Tablet (optional)

For using online calculators and accessing the NEC.

Camera

For documenting nameplate information and existing conditions.

Important Notes



This is a simplified overview. Electrical load calculations can be complex. The NEC is a complex document. Consult with a licensed electrical engineer or a qualified electrician with experience in load calculations and service upgrades.

Permits

You will almost certainly need permits for any electrical work. Check with your local building department.

Local Codes

Local codes may have additional requirements beyond the NEC.

Liability

Be aware of your liability. If you make a mistake in your calculations, you could be held liable for any resulting damages.

Safety First: Electrical work is dangerous. Take all necessary precautions to ensure your safety and the safety of others. If you are not comfortable working with electrical systems, hire a qualified electrician.*By following these steps, you can provide your client with a well-informed assessment of their electrical service and feeder capacity and help them make the right decisions about installing electric ranges. Good luck! Flag for review