supply side connections

SUPPLY SIDE CONNECTIONS

Design

SUPPLY SIDE CONNECTIONS AND BACKFEED

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

OCPD RATINGS

What’s overcurrent protection?

An overcurrent protection device (OCPD) is a device that is meant to protect against overcurrents such as:

• Overload

• Short circuit

An OCPD rated for the backfeed current is always required for any interconnection type. NEC 705.11(C) Overcurrent Protection

The BASELINE DT. will do this math for you when you input the circuit sizes:

• Max of 13 modules per circuit

• The OCPD rating in the BASELINE DT. tells you how much solar we are backfeeding into the existing residential system.

So how much PV can you land on the supply side?

• You can land up to the rating of the Service for a supply side connection. 

Example:

A 200A Service can handle up to 200A of PV on the supply side.

NEC 705.11(A) Output Rating: The sum of the power source continuous current output ratings on a service, other than those controlled in accordance with 705.13, shall not exceed the ampacity of the service conductors.

SUPPLY SIDE TAP - BREAKER TYPE INDOOR DISCONNECT

• Indoor OCPD Load Center AC Disconnect for a supply side tap uses a different piece of equipment:

  • Instead of a fused disconnect, a breaker type disconnect is used 

• This is another example of a meter/main using manufactured conductors that cannot be tapped. 

• Notice that the feeders attached to the load side of the topmost service disconnect are not manufactured type feeders and could be tapped if needed.

• This is another example of generator equipment you might see associated with an ATS type generator system.

• The ATS can be a separate enclosure, or it might be included in the body of the generator itself.

• If you see this, you will need to figure out where this equipment appears in the residential system wiring and ensure that we interconnect on the supply side of this equipment.

• You should always look at photos of any load center with the deadfront on as this type of equipment can very easily be missed otherwise which can result in a design change at install and you will be liable as the designer.

2020 NEC & SUPPLY SIDE BREAKER

As summarized above, the 2020 edition NEC prevents the use of supply side breaker connections in most situations. Let's dive into the changes to service requirements that were instated with the 2020 edition of the NEC:

230.85 Emergency Disconnects:

For one- and two-family dwelling units, all service conductors shall terminate in disconnecting means having a short-circuit current rating equal to or greater than the available fault current, installed in a readily accessible outdoor location. If more than one disconnect is provided, they shall be grouped.

2017 NEC and prior: there was previously no requirement for there to be an accessible emergency disconnecting means outdoors. In many older markets such as Virginia, Oregon, South Carolina, etc., you will often see "stand alone" meters, meaning that the main disconnect/MSP is indoors somewhere which was allowed in older versions of the code and may be grandfathered in.

The principle difference is that the 2020 NEC requires an outdoor accessible emergecny service disconnect.

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.

230.67 Surge Protection:

(A) Surge-Protective Device. All services supplying dwelling units shall be provided with a surge-protective device (SPD).

(B) Location. The SPD shall be an integral part of the service equipment or shall be located immediately adjacent thereto.

Exception: The SPD shall not be required to be located in the service equipment as required in (B) if located at each next level distribution equipment downstream toward the load.

(C) Type. The SPD shall be a Type 1 or Type 2 SPD.

(D) Replacement. Where service equipment is replaced, all of the requirements of this section shall apply.

2017 NEC and prior: This is another service requirement that did not exist prior to the 2020 NEC.

The principle difference is that the 2020 NEC requires each service to be equipped with an SPD.

SUPPLY SIDE BREAKERS - SOLAR READY PANEL

So, what exactly is a Solar Ready Panel?

Some Meter/Main combos have a solar ready position at the top of the panel. Usually marked by:

• Bus diagram showing a dedicated slot for secondary customer generation.

• A slot in the MSP deadfront labeled as a second service disconnect spot

• A spot at the top of the bus that is separate from the rest of the MSP busing.

A solar ready position is a designated space made for secondary power sources and can always be used regardless of code year. This will make your job a lot easier if you see this.

SUPPLY SIDE BREAKERS - MAIN LUGS PANEL

In this configuration, double check how your bus is being fed. 

Notice a 2P 125A breaker installed in this panel. Is this the main breaker?

No. If you turned this breaker to the off position, the bus is still energized by the main lugs coming from the meter.

• It’s only the main breaker if it is attached to the main lugs.

• In this configuration, both the 125A breaker and the 20A breaker are considered service disconnects, therefore, the 20A PV breaker beside it is a supply side connection.

In addition, it appears that there are feed through lugs at the bottom of this panel, meaning that we would need to ensure that the feeders connected to them are properly protected if we wanted to consider a load side connection. 

• completing a supply side connection would mean that we are not even connected to the bus supplying the feedthrough conductors, so we would not need to worry about protecting those feeders in anyway.

EXAMPLE 2:

In this example, we have a split bus panel where the top 12 spots are considered service disconnects. The breaker on the lower left of the upper bus is supplying the lower bus.

• Notice that is appears that this bus would be over the 6 handle rule, however, we have breakers on this bus that are not in use.

• We can disregard these breakers that are not in use which would make the total 4 handles on this bus.

Now we need to start screening and qualifying the equipment.

For this MSP, we need to create a homeowner obligation to remove this shelving because it violates the required working space clearance for the panel.

We'll add this to our data collection.

When you see a split bus panel, you should make sure that 6 handles are allowed in the area you are working in. This can be confirmed in the AHJ/Utility requirements in the BASELINE DT by plugging in the correct AHJ/Utility and checking for this. 

If you are familiar with the general rules of the market, such as this project which is in Oregon, you may already have an idea of if it is allowed or not.

• All of Oregon is on a custom statewide code that will allow for 6 handles.

Now we need to confirm the bus layout and that we're not over 6 handles.

On a first glance, it appears that the top six breakers are on the upper supply side bus with everything bellow these breakers being on a separate sub load bus.

• We can also confirm that there are no conductors connected to the "spare," so supply side breaker should be on our radar as the preferred interconnection for the time being.

We also need to confirm that the tandem breakers at the bottom of the bus are allowed.

The labeling confirms both the bus configuration and that tandems are allowed in the bottom most 2 positions only.

While we're looking at the label, we can also verify the bus rating. This will be added to our data collection.

We also need to look for any other code violations or damage on this panel.

• The only other issue of note here is that this is an I-T-E panel, so only I-T-E original breakers or Siemens brand breakers should be installed in this panel.

  • The two breakers in the red boxes are Bryant and Eaton type breakers that will need to be replaced. 

We will add this to our data collection.

The 200A bus should be plenty to accomedate the system backfeed, but we still need to do our due diligence.

The financials of this project specify an 18 panel system. This will produce 30A of backfeed when the backfeed calculation is completed. We should be in the clear to complete a supply side breaker as planned.

Here's what our electrical data collection should look like on paper:

1. Everything we need to complete the electrical part of the design is here

2. Having this written out in short hand ensures that none of this will be missed on the backend of the design process.

   • This includes not just the interconnection type, bus rating and make/model callout but also homeowner obligations and replacement breakers for the noncompatible breakers that are currently installed.