How to Size Main Panel, Load Center, and Consumer Unit?

How to Size Main Panel, Load Center, and Consumer Unit? . Proper sizing of a load center, panelboards, and distribution board is essential to ensure the safe and efficient distribution of electrical power in residential, commercial, and industrial settings. Oversizing or under sizing these components can lead to inefficiency, safety hazards, and regulatory non-compliance.

In this guide, we’ll walk you through the fundamentals, considerations, and calculation methods needed for correctly sizing these critical electrical components.

Sizing Main Panel, Load Center, Panelboards, Distribution Board and Consumer Unit According to NEC and IEC?

Planning is the first and most critical priority for all electrical wiring projects. Proper estimation and analysis, based on accurate calculations, are essential when designing and installing a power distribution system in both residential and commercial applications. This is because accurately determining the size of main panels and load center ensures they can safely and efficiently handle the current load, as well as any potential future loads.

Load centers and distribution boards should be sized in compliance with NEC, IEC, or other relevant regional codes. This process also involves selecting appropriately sized wires and cables, choosing the correct size of MCBs (Miniature Circuit Breakers), and calculating the ratings for plugs and outlets. In today’s step-by-step guide, we will demonstrate how to select the right size panelboard (whether it’s a load center, distribution board, or circuit breaker panel) according to NEC and IEC standards, with worked examples.

Panelboard, Load Center and Distribution Board or Consumer Unit

These terms—load center, panelboard, distribution board, or consumer unit—are often used interchangeably. In short, a panelboard or distribution board is a collection of protective devices such as circuit breakers, designed to safely control and distribute electrical power to various load points, including branch and final circuits.

Terms used in the US: Panelboard, load center, breaker box, service panel, or main electric panel.
Terms used in the UK & EU: Distribution board (for commercial use), consumer unit (for residential use), circuit breaker box, or main panel.

We covered these topics in detail in our previous post. Now, let’s proceed to the step-by-step tutorial. The following example can also be used to determine the capacity of the main circuit breaker, as well as calculate the overall electrical load of a home.

Importance of Proper Sizing

Safety: Prevents overheating, fires, and electrical faults.
Efficiency: Ensures optimal power distribution and prevents energy waste.
Regulatory Compliance: Meets electrical codes like the NEC (National Electrical Code).
Future Expansion: Accommodates additional loads without the need for costly upgrades.

How-to-Size-a-Load-Center-Panelboard-Distribution-Board-Consumer-Unit-NEC-IEC

How to Size a Main Panel and Load Center for 120/240V – NEC?

The common voltage levels for residential applications in the USA are 120V and 240V single-phase. Three wires (identified as Hot 1 with black color, Hot 2 with red color, and Neutral with white color) from the secondary side of the split-phase transformer enter the meter box and the main service panel (main switch breaker).

In this case, the available voltage in a single phase distribution system is as follow:

Voltage between any Hot (Hot 1 or Hot 2) and Neutral = 120V
Voltage between Hot 1 and Hot 2 = 240V

The following figure shows the overall overview of a panel box, level of voltage between different conductors and number of circuit breakers.

Main-Panel-Wiring-for-120V-and-240V-According-to-NEC

Tips : The following example is based on VA (Volt × Amperes) which is known as apparent power. You may use the active or real power (in Watts) which is equal to Apparent power × power factor or VA × PF as power factor in residential buildings are almost unity (1). In this case, the apparent power in VA is equal to real power in W “Watts”.

Now lets see the following example to determine the suitable size of load center (or main and subpanel for electricity distribution).

Tips 2 : This example to size a load center is based on NEC which is applicable in North America, especially in US and Canada which follows NEC and CEC. Check the other examples for IEC and UK/EU right after this example.

Following is a general overview of a main service panel and its different parts including the space for future load points and 120V & 240V circuits.

Inside-Main-Breaker-Box-Panel-120V-240V-NEC

Example 1:

Calculate the right size of load center or main electric panel for an 1500 ft² (square foot) or 139.35 m² (square meters) home floor plan having the following load points:

Air conditioner: 240V × 25A = 6000 VA = 6 kVA
Electric range: 240V × 35A = 9600 VA = 8.4 kVA
Electric heater: 240V × 30A = 7200 VA = 7.2 kVA
Clothes dryer: 240V × 15A = 3600 VA = 3.6 kVA
Dishwasher: 120V × 10A = 1200 VA = 1.2 kVA
Garbage disposal = 120V × 8A = 960VA = 0.96 kVA
Two Small appliances circuits in the kitchen for refrigerator, blinder, etc.
General lighting, fans, bathroom appliances and future load etc.

Sizing-Load-Center-Panelboard-Distribution-Board-Consumer-Unit-According-to-NEC-and-IEC

Solution:

Let us find and calculate the power rating for different home appliances based on floor plan without basement and garage using NEC codes and related tables.

General Lighting Load:

The minimum general lighting load including non-appliance receptacles for home appliances e.g. TV, table light etc.) for a dwelling is 3 VA per ft² (NEC Table 220.12).

This way, the general lighting load for 1800 ft² (given in example):

3 VA × 1500 ft² = 4500 VA = 4.5 kVA

Small Appliances Load

There should be at least two 120V, 20A small appliance circuits i.e. in the kitchen for small appliances such as coffee maker & toasters etc. (NEC Article 210.11(C)(1). These circuits should be rated at 1.5 kVA (NEC Article 220.52(A). This way, the small appliances loads in the given example house:

2 × 1500 VA = 3000 VA = 3 kVA

Laundry Circuit

There should be at least one 120V, 20A circuit for the laundry area (NEC Article 210.11(C)(2). The minimum VA rating of the laundry circuit should be 1.5 kVA (NEC Article 220.52(B). This way, the load rating in the laundry area:

1500 VA = 1.5 kVA

This way, the total general lighting and small appliances including laundry circuit rating:

General lighting = 4.5 kVA
Small appliance load = 3 kVA
Laundry circuit = 1.5 kVA

Total of general lighting and small appliances:

= 4.5 kVA + 3 kVA + 1.5 kVA = 9 kVA

Demand Factor

As we know that all electrical appliances are not operational at once i.e. (only one can be used as either electric heater or refrigerator depending on the temperature). Similarly, all equipment are not always ON continuously such as electric iron, water heater, lighting, fans etc. For this reason, the first 3 kVA is rated at 100% while the remaining load can be rated at a demand factor of 35% (NEC Table 220.42). This way

The first 3 kVA at 100% = 3 kVA
Remaining 6 kVA (9 kVA – 3 kVA) at 35% = 2.1 kVA

Net total of general lighting and small appliances;

= 3 kVA + 2.1 kVA = 5.1 kVA

Tips : According to NEC 210.20(A), Continuous load (which lasts for 3 hours or more simultaneously) should be rated at 125% when selecting a wire size for it. The non-continues load circuit (i.e. general lighting) may be rated at 100%.

Read More : Calculate the Number of Panels for a Load without Battery Backup

Large Appliance Loads

High power rating e.g. large appliances with continuous and non-continues operation should be handled differently. We have the following high power rated appliances in the above example:

Air conditioner: 240V × 25A = 6 kVA
Electric range: 240V × 35A = 8.4 kVA
Electric heater: 240V × 30A = 7.2 kVA
Clothes dryer: 240V × 15A = 3.6 kVA

As we have already mentioned above, an air conditioner or electric heater can be used at the same time i.e. only one appliance is needed to operate based on the temperature. In this case, appliances with larger ratings should be taken into account (NEC® Article 220.82(C)). In our example, the rating of electric heater (7.2 kVA) is greater than the air-conditioner (6 kVA), so we will consider the heater then, i.e. 7.2 kVA

The rest of the appliances should be rated at 100% except electric range as it is used for a short time i.e. it is non-continuous as compared to other appliances. The allowable demand factor for 7.2 kW electric heater is 5.76 kW (NEC Table 220.55). We assumed the power factor is unity i.e. “1” where apparent power = real power e.g. kVA = kW. This way;

Electric range: = 8.4 kVA
Electric heater = 5.76 kVA
Clothes dryer: = 3.6 kVA

Net total of large rated appliances:

= 8.4 kVA + 5.76 kVA + 3.6 kVA = 17.76 kVA

Miscellaneous Loads

The miscellaneous loads given in the example are:

Dishwasher = 1.2 kVA
Garbage disposal = 0.96 kVA

Net miscellaneous load rating:

= 1.2kVA + 0.96 kVA = 2.16 kVA

Total Load

Load Points	kW or kVA Rating
General lighting, laundry & small load	5.1 kVA
Net large appliance load	17.76 kVA
Miscellaneous appliance load	2.16 kVA
Total Load	25.02 kVA

Required Service

The common supply voltage levels in the US supplied to residential homes are 120V/240V. Thus, we may use the highest level of voltage to determine the required service (for amps) by using the following formula.

I = P / V

Where:

I = Current in Amperes
P = Power in Watts (or kVA where PF = 1)
V = Voltage in Volts

Putting the values;

I = 25.02 kVA / 240V

I = 104.25A

It means, the required service is 105A. But we have to add the future expansion and safety factor as well.

Future Load:

It is important to add a space of at least two branch circuits for the future expansion. The minimum of 2 breakers each of min 10A space should be added i.e. 2 × 10A = 20A

The total Amps = 20A + 104.25A = 124.25A

Safety Factor

It is recommended to add a safety factor of 20% to the total amperage as circuit breakers and their operations in the load center are affected by the rise in the temperature. This way, the total current in the amperes:

Net total Amps = 20% + 124.25A = 149A

The suitable size of load center or main panel = 150 Amperes

Based on the above calculations, the right size of load center or panelboard is 150A which is nearest available to the calculated value.

How-to-Determine-the-Number-of-Circuit-Breakers-in-a-Panelboard-Load-Center-Distribution-Board

For this 150A main panel, the right wire size for service is #1 AWG Copper or #2/0 AWG Aluminum based on 83% rule in NEC – Table 310.12(A). The minimum size for Ground wire is #8 AWG Copper or #6AWG Aluminum.

Tips :

It is always a good practice to follow the NEC guidelines for main panel, main lug or load center requirement and installation.
The main breaker feeding the busbar can not exceed busbar rating in the main panel or load center. (NEC – 705.12(B)(2)(3).
Sum of the ampacity of all the overcurrent protection devices i.e. breakers can not exceed the busbar ampacity rating. (NEC 705.12(B)(3)).
The NEC allows exceeding the busbar’s rating by up to 120% except the rating of OCPD protecting the busbar and 125% of the power source output circuit amperes.
This above calculation are based on the NEC guidelines. For more details, refer to NEC 210.21, 210.24, 220.110, 220.14, 220.42, 220.45, 220.53, 220.55, 240.4, 310.12, 310.12, 310-14 and 517.22

Considerations for Specific Applications

Residential Load Centers

Standard sizes: 100A, 200A, 400A.
Commonly used in homes to control lighting, outlets, and appliances.

Commercial Panelboards

Require higher amperage ratings, often 400A or more.
Must accommodate three-phase systems.

Industrial Distribution Boards

Handle complex loads such as motors and large machinery.
Require precise calculation of load balancing.

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How to Size a Consumer Unit? Single-Phase, 230V – IEC

The following example will show you how to find the right size of single phase 230V AC consumer unit or garage unit and associated MCB/MCCB to handle the residential load.

Tips :

It is a good practice to follow the IEC and IEC guidelines and requirements for consumer unit, garage unit and distribution board installation

Example 2:

Find the proper size of a single phase consumer unit if the estimated total load in a home is 12 kVA.

Solution:

Generally, the power factor of residential homes having normal loads is considered as unity “1”. This way, the total load in kVA = kW i.e. the apparent power is equal to the real power in watts due to the absence of power factor.

Now, we will have to first calculate the load current using the general formula of current in amperes for single phase circuits.

First of all, we will find the required amperes by using the three phase current formula.

P = V × I × Cos Ф

I = P ÷ (V × Cos Ф)

Putting the values:

I = 12 kW ÷ 230V

I = 52.17A

Diversity Factor:

The general diversity factor is 80% of the connected load for residential applications. (You may select the proper % according to the load type in the IEC 60439). In this case,

80% × 52.17A = 41.74 A

Future Expansion:

The general rule of thumb for safety factor is 20%. So you may add it as well if needed.

20% × 41.74A = 50 A

Safety Factor

The minimum safe range of the safety factor is 20-25%. So we will it add it to the calculated value of load current as follows:

25% × 50 A = 62.5 A

This way, we may select the nearest standard available rating of MCCB which is 63A for the single phase 230V consumer unit. Based on the calculation, this 63A MCB or MCCB is the right size to handle a 12kW load in residential homes.

How to Size a Distribution Board? 3-Phase, 400V – IEC

In the following example, we will show you how to calculate the right size of three phase 400V distribution board which is mostly applicable in countries following the IEC rules e.g. UK, EU and former British colonies.

Read More : How to Install a CCTV Camera?

Example 3:

What is the right size of a three phase distribution board if the estimated total load in a home is 50 kVA. The load is a combination of single phase and three phase systems (400V & 230V AC) including air-conditioning, refrigerator, electric range, water pumps, washing machines and general lighting points etc. Consider the power factor = 0.9.

Solution:

First of all, we will find the required amperes by using the three phase current formula.

P = √3 × V × I × Cos Ф

I = P ÷ (√3 × V × Cos Ф)

Substituting the values:

I = 50kW ÷ (√3 × 400V × 0.9)

I = 80.18A

Diversity Factor:

The general diversity factor based on IEC is 80% for mixed load in residential applications. (see IEC 60439 for more details). In this case,

80% × 80.18A = 64.15 A.

Future Expansion:

The general rule of thumb for safety factor is 20%. You may add it accordingly when needed.

20% × 64.15A = 76.98 A

Safety Factor

The minimum safe range of the safety factor is 20-25%. It may also be used as future expansion if needed. Just add it to the calculated value of the load current as follows:

25% × 76.98 A = 96.22 A

Now, the nearest standard available rating of MCCB shall be 100A for the three phase 400V distribution board which is the suitable size to handle a 50 kW load.

Common Sizing Mistakes to Avoid

Underestimating Peak Demand: Leads to frequent tripping and overloads.
Ignoring Expansion Needs: Future growth can necessitate costly upgrades.
Skipping Code Requirements: Non-compliance can result in penalties and safety hazards.
Overloading a Circuit: Causes overheating and potential fires.

FAQs

1. What is the difference between a panelboard and a load center?

A load center is typically for residential use and houses fewer breakers, while a panelboard is designed for commercial or industrial applications with higher capacity and advanced features.

2. How do I calculate the size of my load center?

Sum the total electrical load in watts, add 125% of continuous loads, convert to amperage, and choose a load center with a higher rating.

3. What is the NEC 125% rule?

The NEC requires sizing electrical systems to handle 125% of the continuous load to ensure safety and prevent overloads.

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How to Size Main Panel, Load Center, and Consumer Unit?