electrical load centers

ELECTRICAL LOAD CENTERS

Sector 2

1 | RESIDENTIAL ELECTRICAL SYSTEMS

HOW PV INTERCONNECTION WORKS

Most residential PV installations are grid tied. Grid tied solar is a PV supplemented electrical system that still relies on the utility grid for power at some point during the day. Grid tied solar will not power a residential home in the event of an outage; only battery backup systems can provide power in the event of an outage.

What does interconnection mean in terms of PV?

Interconnection is how:

Customer generated electricity can be used by the existing electrical system.
Excess generated electricity is exported to the grid
Essentially, it’s how the system is physically connected to the utility and home electrical system.

Below is an example of a 3-line diagram depicting PV interconnection. The red box is the point at which the PV system joins the existing electrical system.

The existing residential electrical service will have the most bearing on how interconnection can be designed. Before we talk about the specifics of interconnection, it is important to understand how Residential Electrical Systems Work.

HOW RESIDENTIAL ELECTRICAL SYSTEMS WORK

Power from the utility is supplied either overhead or underground.
The utility owns the power transformer, service lines, and the meter.
The customer owns the meter base, riser, and all electrical panels/load centers in the home supplied by this meter.
The meter records customer consumption.

In short, it is important to understand that transformers, service lines and meter bulbs are the responsibility of the utility to maintain. We are responsible to ensure that all customer owned equipment will be able to accommodate our PV system.

SERVICE ORIENTATION

Service orientation refers to how the utility meter receives power from the utility. The two standard methods are: Overhead (pictured left) or Underground (pictured right).

MAIN SERVICE PANELS

When new associate surveyors and designers first begin looking at electrical panels, there tends to be some slight confusion regarding what would be considered a Main Service Panel, and what would be considered a Sub panel. This distinction is nonetheless important for PV design purposes.

So, what is a Main Service Panel (MSP)?

Freeway Analogy:

A freeway, or interstate highway, is a main distribution rout that travelers take to get to backroads and residential/business streets.
They usually have the most lanes and the most traffic but not always.
Some interstate sections are 2 lanes and sparse traffic.
- What makes it the interstate is that it is still the main road travelers use to get from point A to point B.

An MSP behaves the same way for residential electricity:

It is the main point where electricity is distributed to loads (circuits in the home.)
It is the main load center that all other load centers in the home are connected to.
It doesn’t always house the most breakers, but it is still connected to all electrical loads in the home.
- Some MSPs are a single main disconnect beside the meter.

IDENTIFYING THE MAIN SERVICE PANEL

An MSP will always be supplied directly from the meter. \

One of the easiest ways to confirm this is if the meter and panel are connected via conduit.

The photo to the right shows an MSP directly next to the meter connected via conduit highlighted in red.

Sometimes, the MSP might be indoors or not directly adjacent to the meter.

Another great way to identify the MSP will be to look for Service Entrance Conductors consisting of 3 wires landing in the main lugs of the panel.

The photo to the right demonstrates what this will look like on a typical panel.

SUB PANELS

Now that we know what an MSP is and what it should look like, we need to talk about sub panels.

What is a Sub Panel?

A sub panel is any panel supplied by the MSP or another sub panel. A sub panel can be indoors or outdoors. Sub panels can sometimes be housing more breakers than the MSP.

Identify sub panels using the following methods:

- 4 wires entering the enclosure and landing in the main lugs or terminal bars.

There is another key difference between MSP and Sub panels that explains why a sub panel has a 4 wire set up vs why an MSP has a 3 wire set up. Before we get into that, we need to define what each of these wires, or conductors, are.

CONDUCTORS

"Conductor" is the proper terminology used to describe electrical wires. Typically, you will hear conductors described as "live," "neutral," or "ground," each serving its own specific purpose in residential circuitry. The code language differs slightly from the terms used previously to refer to these wires:

What is an Ungrounded Conductor?

Ungrounded Conductors are black or red
Ungrounded Conductors are often called live or hot wires colloquially.

- Ungrounded Conductors are the main wires carrying current in residential circuitry.

What is a Grounded Conductor?

Grounded Conductors are white or gray.
Grounded Conductors are often called neutral wires colloquially.
Grounded Conductors direct circuit current back to the main service panel and ultimately to where grounds and neutrals are bonded in the main service disconnect enclosure.

What is an Equipment Grounding Conductor (EGC)?

EGCs are green or bare
EGCs are often called ground wires colloquially.
EGCs are used to reduce the risk of electrical overload, fires, and electrocution by redirecting excess electrical charge back to the earth rather than this charge jumping through you or an appliance in the home.

Often, people misunderstand that ground and neutral serve different purposes and can cause some to use these terms interchangeably.

So, what’s the difference between ground and neutral wires?

Neutral wires are a designated path for active current to return to the main service entrance only.
Ground wires are an intentional low impedance path for stray current (fault current) to safely dispel into the earth.
- Electricity follows the path of least resistance, so grounding systems are meant to be very direct and are present throughout the circuitry of the home.

Circling back now, there is also a distinction between MSP wiring and Sub Panel wiring that needs to be noted.

Ground and neutral should never be bonded (connected) in any sub panel enclosure.
This means that sub panels will require both a neutral and a ground wire connecting to the MSP.
It will also mean that grounds and neutrals for each branching circuit need to be on separate terminal bars.
- You shouldn't see green/bare wires and white/gray wires on the same terminal bar in ANY sub panel.

The example on the right shows a typical ground bar on a sub panel. Notice, there are only wires that are either bare or with green sheathing on this terminal bar. The visible white wires are terminating on a separate insulated neutral bar not visible in this photo, but present just under the breakers.

When should ground and neutral be bonded?

Only at the main service disconnect.
Ground and neutral are bonded either by landing on the same terminal bar or by forming a connection between ground and neutral bar through other means such as a green screw bonding the insulated neutral bar to the enclosure.
- This means that in the MSP You CAN have green/bare wires and white/gray wires on the same terminal bar.

The example on the right shows a fairly common ground/neutral terminal bar as seen in an MSP enclosure. Notice that both bare AND white wires terminate on this bar.

GROUNDING ELECTRODE SYSTEM

The MSP is typically the termination point for the Grounding Electrode Conductor (GEC) which secures the home structure to the Grounding Electrode system.

This is a system devised to disperse excess current from the electrical system.
This can also be required on panels that are on a completely separate structure such as a separate panel that appears on a garage separate from the main home.

Further clarification is provided in the National Electrtical Code (NEC). The NEC is the international code governing electrical installation standards and safety under the Nation Fire Protection Association (NFPA).

There are 8 items that are listed in NEC 250.52 as allowable grounding electrodes, here is the list:

Metal Underground Water Pipe
Concrete-encased Electrode
Metal In-ground Support Structure
Ground Ring
Rod and Pipe Electrodes
Plate Electrodes
Other Listed Electrodes
Other local underground metal systems or structures

Ground rods are one of the most common grounding systems you will see on a residential electrical system.

Site Surveyors should get a photo of the existing grounding system.

WHAT IS SUPPLY SIDE?

Put simply this would be:

Equipment upstream from (above) the main disconnect breaker.
This equipment will always be energized, whether the main breaker is on or off.

WHAT IS LOAD SIDE?

Equipment downstream (below) from the main disconnect breaker
This equipment can be de-energized by switching off the main breaker.

2 | LOAD CENTERS & BREAKERS

HOW LOAD CENTERS WORK

First of all, what is a load center?

Load Center is a generic term used to refer to any breaker type enclosure. This encompasses both MSPs and Sub Panels.

How does a load center work?

The load center is fed by conductors attached to the main lugs. This supplies everything “downstream” from the main lugs.
The busing is composed of two bus bars.
- Each bus bar is supplying electricity at 120V.
- With both bus bars, the busing is capable of supply electricity at 240V.
The bus will also have a current rating which typically ranges from 100A up to 225A.

COMMON BUS TYPES

Click the hyperlinked text below to read more about bus configurations:

Meter/Main Combo

Top/Bottom Fed Main Breaker Panel

Main Lugs Only Panel

Split Bus Panel

MORE ABOUT LOAD CENTER BUS TYPES >

ELECTRICAL BUS BARS

How is electrical current voltage for each circuit controlled?

The voltage of each attached circuit will be determined by breakers contacting one bus bar (120V or 1P breaker) or both bus bars (240V or 2P breaker).

The image to the right shows what load center busing looks like under the breakers. Breakers contact the bus bar by contacting these tabs, also called "stabs."

Note that the number of spaces occupied by a breaker indicate if it is contacting only one stab or two.

The images on the right show the terminals on these breakers that directly contact tabs on the bus bar.

The Ampacity (Current) of each attached circuit will be determined by the breaker ampacity rating:

- Breakers are sized not to exceed the ampacity of the circuit conductors.
- Commonly, the current rating is located on the breaker handle

The breaker in the red box is a 240V breaker rated for 30A

BREAKERS

How does a breaker work?

Breakers plug onto the bus bar connector stabs.
Breaker ampacity is rated to match the max current of that circuit.
Breakers trip when the bimetal element inside the breaker reaches a high temperature due to overcurrent.

What is overcurrent?

When a circuit is overloaded
When there is a short in the circuit

Okay, but WHY breakers?

The Chocolate Bar analogy:

Why do Hershey's bars have perforations?

To create intentional weak points causing the bar to break only at those perforations when pressure is applied.

The same principle applies to breakers:

Breakers are an intentional weak point in the electrical design so that electrical failures don’t happen somewhere random in the home and minimize the hazard to you and your home.

COMMON LOAD CENTER MAKES/MODELS

Follow the hyper links below to learn more about these makes and models.

EATON
- Cutler Hammer (Tan)
- Eaton BR
SIEMENS
Square D
- HOM
- QO
General Electric (GE)

COMMON DISCONTINUED LOAD CENTER MAKES/MODELS

Federal Pacific/Stab Lok
Pushmatic
Zinsco/Sylvania

MORE ABOUT MAKES AND COMPATIBILITY >

TANDEMS, TRIPLEXES & QUADS

Tandem breakers are simply breakers that can fit more circuits in less spaces to make more overall space on the bus bar for circuitry. Tandem is a generic term used to describe a few different types of these breakers:

Tandem/Duplex: 1 space, (2) 1-pole 120V circuits

Triplex: 2 space, (1) 2-pole 240V circuit + (2) 1-pole 120V circuits

Quad: 2 space, (2) 2-pole 240V circuit

The image to the right shows an MSP with tandems installed in the bottom 10 spaces:

Tandems allow you to fit 2 circuits in one branch breaker space.

Notice that the tandems in the bottom spaces are the same size as the single space breakers.

The purpose of all duplex/triplex/quadruplex breakers is to consolidate breaker space. There are a couple of reasons you may need to do this:

To correct code violations such as double landed breakers
When a panel is too full to accept a standard 2 pole PV breaker.

Note that not every load center will accept tandems

How do I know if a bus bar accepts tandems?

Look at the labeling:
- Model number:
  - tells you how many circuits allowed on the load center
  - Tells you how many breaker spaces (stabs) the load center has.

- Bus diagram:
  - Often indicates which stabs, if any, will accept tandems.

- Breaker compatibility table:
  - Often tells you which types of breakers can be used in the panel.

- Lastly, look for notched stabs if they are visible.
  - The diagram to the right shows what a solid stab vs a notched stab would look like.

In the example to the right:

The red highlighted stab is a solid stab that would not allow tandems.
The green highlighted stab is a notched stab that would accept tandems.

PANEL STICKERS

Panel stickers can be a big help in electrical design. This label tells us a variety of things ranging from equipment ratings, bus configuration, to breaker compatibility and weather resistance rating.

It can be daunting to look at a wall of information like this if you aren't sure what you're looking at.

In this segment of the training, we'll walk through how to read these labels and interpret in the information.

Often, the model number might tell you most of what you need to know:

bus rating is usually the last 3 numbers on the model number
if you aren't sure, the bus rating is usually confirmed just below it.

The number of breaker spaces and the max allowable number of circuits can usually be derived from the first four digits of the model number
if you aren't sure, this information should also be confirmed elsewhere on the label. Look for a "max circuit" label or it may be listed directly on the bus diagram as it is on this one.

The bus schematic gives us a guide to what the busing should look like.

It tells us the position of the main disconnect
It gives us an idea of which spaces can accept tandem breakers (in this case, every space accepts tandems.)

The tables below tell us about equipment compatibility:

This table tells us the main breaker type but identifying the appropriate prefix.
- We know that this load center uses an EQ9985 type main breaker. This could be useful if we need to add in a main breaker or replace it for any reason. (we will explore what those reasons would be later on)
We also know the branch breaker model types compatible with this load center based on the table next to it.

The tables right above the breaker compatibility tables also list compatible lugs, terminals and standard conductor types to be used with the equipment.

GFCI BREAKERS

GFCI breakers are meant to detect current differences between wires which would be caused by some type of interference which could result in electrical burns or shock to users. The additional neutral in this breaker is used to detect these subtle current differences and cut electricity within milliseconds when it is over a predetermined precedent.

Installed for circuits servicing areas such as:

Kitchen
Bathroom
Garage
Outdoor Areas

These breakers may not be tandemed.

AFCI BREAKERS

AFCI has a similar function but is mainly used to prevent fires. AFCI also measures for current differences between wires but it is meant to distinguish between normal arching, such as when appliances are plugged in or pulled out of outlets, and dangerous uncontrolled arching that would cause a fire. Likewise, when these breakers detect a current leak that could lead to a fire, they cut power.

Installed for circuits servicing areas such as:

Kitchen
Laundry Areas
Common Areas such as living rooms and hallways

These breakers may not be tandemed.

SURGE PROTECTIVE DEVICES

The 2020 Edition of the NEC requires that each service supplying a dwelling be equipped with a surge protection device.

What is it?

SPDs dissipate and divert surges such as lightning strikes and directs the overvoltage back to the grounding system.

This is commonly seen in AHJs enforcing 2020 NEC
Some AHJs may require us to install an SPD. This will be stated in the general AHJ requirements under “Electrical Design Requirements”

3 | QUIZ

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