Electrical

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.

Terminology

Microinverter/Inverter - A device(s) that converts the DC power generated from the solar panels to AC power.

PV - Photovoltaic.

BOM - Bill of material

BOS - Balance of system

1-line or Single Line - A simplistic symbolic representation of an electric power system.

3-line or Circuit Diagram - A more complex diagram showing each component of the electrical diagram.

AC - Alternating Current

DC - Direct Current

Net Metering - Is a billing system that allows users with a solar array to send excess or unused solar energy back to the utility grid for nearby users to consume.

MSP - Main Service Panel

Example of a 3-line diagram depicting different components.

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).

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.

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:

Common type of conductors we use.

THHN
NM-B (ROMEX)
SER
UF (Trenching cable)
Bare copper (for grounding)

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.

250.24(A)(5)

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.

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.

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

The most common types of grounding methods that we use.

2 grounds rod spaced 6ft apart
Cold water bond (CWB)
Ufer (or concrete encased electrodes)

Type of grounding clamps

Acorn clamp
Irreversable splice

Cold water bond

INTERCONNECTIONS

*Note if the customer has an upgrade between site survey and installation please have the crew send in photos. And if we can continue with the OG interconnection then we should proceed "as planned" if it doesn't cause extra work.

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.

How does a MSP/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

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.

Types of breakers we use

Normal 2p breakers, 2 space 240v
Normal 1p breaker, 1 space 120v
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
GFCI (Top right)
AFCI (Bottom right)

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.

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”

Load side interconnections

Backfeed considerations:

On a load side connection, you are not at liberty to backfeed up to the rating of the bus bar. There are a couple different rules you can use but here at Ion we default to the code article that gives you the most liberties:

120% rule

It should also be noted that when connections are made to sub panel feeders, the sub panel/feeders should be protected with a main breaker. This is also not a rule for supply side connections.

**Basically if you have MSP has a bus rating of 200A and a main breaker rating matching at 200A. This is what the math should look like for the 120% rule:

LOAD SIDE TAP

This interconnection type is not used very often. Here's the basics of what you need to know:

Mainly used in a full main disconnect enclosure (MSP) with the sub panel being somewhere in the middle of the home.
Must be connected to sub panel feeders. Cannot tap general load branch breakers.
Sub Panel feeders must be protected with a main breaker on the other side.
Load taps connected to a branch breaker must be relocated to the furthest position from the main lugs
1. Follows the 120% rule.
2. Sub panel that is being tapped on too must have a main breaker to protect the bussing.

Supply side interconnections

Identify supply side buses by simply investigating how the main bus is being supplied:

Any MSP bus being fed directly through lugs from the meter is considered a supply side bus.
- Note that supply side buses need to comply with the 6-handle rule: no more than 6 handled disconnects on a supply side bus.
Sub panels being fed through lugs are not supply side buses. These will need to be fed from a breaker.

In the images below, supply side buses are highlighted green while load side buses are highlighted purple.

What is backfeed?

Backfeed is a term used to describe the total output from the inverter feeding into the residential loads.
How is it calculated?
NEC 690.9 (B): Backfeed device ratings shall be no less than 125% of the [inverter] maximum continuous output current.
Simple formula:
# of inverters x Max inverter output current x 1.25 = Backfeed needed
Example: 19 module system using IQ8+ inverters
(19) x 1.21 = 22.99A
22.99A x 1.25 = 28.74 A
30A Overcurrent Protection Device (OCPD) Needed

SUPPLY SIDE TAP

Here's the basic run down:

A Supply Side Tap can be used when the meter is detached from the MSP.
Supply side taps can never be used on a meter/main combo.
Must have at least 5” of Service Conductor for a typical tap using piercing connectors which is standard here at Ion.

HOW DO FUSES WORK?

The thin filament inside the fuse will melt when met with overcurrent and fulfil the same purpose as a breaker. Fuses always need to be replaced when the filament breaks

A standard fuse is pictured to the right.
Next to it, we have fuse reducers.
- Reducers are used when the fuses are smaller than the standard size for the disconnect.

Supply Side Tap for Automatic Transfer Switch Generator Systems:

When a Generator is in use with an automatic transfer switch (ATS), interconnection needs to default to a supply side tap in the ATS enclosure.
When a Generator has a manual interlock kit, we need to default to a supply side tap in the MSP. (pictured lower right)
- If the manual interlock kit is in a sub panel, a load side breaker may still be used in the MSP.

***Note the why we needed the solar to be on line side of generators. So apparently when the generator is running and the demand from the loads falls below the output of the PV system the PV system will try to backfeed the generator an at some point it will disconnect because V and Hz are out of spec but the PV will continuously try to backfeed to the generator causing the generator motor to slow down and eventually burnout.

TAP BOXES

Some utilities require that the supply side tap occur in a separate junction box.
Usually you'll see these in Texas.

Special considerations:

Requires a TDR:
- A TDR is a request for the utility to cut power to the meter temporarily to make it safe for our installers to cut and splice feeders.
Requires special materials to be added to the case

Here's the basic run down:

Best form of interconnection when available.
Can backfeed up to the bus rating of the panel for supply side connections.
Only available when:
- AHJ uses 2017 NEC or earlier and there are less than 6 service disconnects on the supply bus.
- AHJ allows for grandfathering service equipment with 6 handles on the 2020 NEC
- Load center has a designated solar ready position regardless of NEC Code year.
Not available when AHJ enforces the single main disconnect rule per 2020 NEC

230.71(B) Maximum Number of Disconnects:

Two to Six Service Disconnecting Means. Two to six service disconnects shall be permitted for each service permitted by 230.2 or for each set of service-entrance conductors permitted by 230.40, Exception No. 1, 3, 4, or 5. The two to six service disconnecting means shall be permitted to consist of a combination of any of the following:

(1) Separate enclosures with a main service disconnecting means in each enclosure

(2) Panelboards with a main service disconnecting means in each panelboard enclosure

(3) Switchboard(s) where there is only one service disconnect in each separate vertical section where there are barriers separating each vertical section

(4) Service disconnects in switchgear or metering centers where each disconnect is located in a separate compartment

2017 NEC and prior: a residence was still only allowed up to 6 service disconnects, however, it used to be allowed to have all 6 service disconnects in a single enclosure.

The principle difference is that the 2020 NEC requires each service disconnect to be in a separate enclosure with only one main service disconnect per enclosure.

Workspace clearances can be found on the Code violations page.

Designs Current Interconnection flow chart.

Interconnection Flow Chart.docx

Quiz time!!!

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