Electrical Design - Walk Through

ELECTRICAL DESIGN - WALKTHROUGH

Design

This is an example walk through of completing the electrical data collection and design.

1 | DATA COLLECTION

HOW AND WHY

OPEN THE ELECTRICAL PHOTOS

METER

MSP

BACKFEED CALCULATIONS

NARROW DOWN THE INTERCONNECTION

CHECK YOURSELF

FINALIZE INTERCONNECTION

1 | DATA COLLECTION

HOW AND WHY

It is important complete your data collection and plan the design on paper either via notes on your PC or by hand on a note pad. It is important to remember that BASELINE is a set of design tools. You need to collect data and plan your design before you fill out this tool. If you do not do this, then instead of you commanding the tool, the tool will command you; only, the tool doesn't know all the variables to look out for.

OPEN THE ELECTRICAL PHOTOS

When scrolling to the electrical phots, generally, you will see the photos in sequence with photos of the meter first. With the meter, there's a few things to pay attention to:

Is there sufficient clearance near the meter where we can mount the equipment?
Is the MSP adjacent to the Meter or away?
Note the meter height in case there are utility specific requirements regarding meter height.

METER

In this example, we have the MSP directly next to the meter. Additionally, it appears that we can most likely mount our equipment to the right of the meter.

We should still look for a photo a little farther away to verify that there's sufficient space on the right to ensure that there are no unexpected obstructions such as a gas meter or fence that might cause issues. The photo on the lower left confirms this.

We also know the meter height is about 4.5'. This could be important later. Also note that this is a ringless 200A meter can indicated by the model number ending in "200"

Lets go ahead an notate this info in our data collection now:

MSP

The first thing to look for before anything else is any potential show stoppers. For electrical, that would be damage or code violations. Make sure to scan through all the photos looking for anything that might be a red flag.

There are no indications of damage or code violations. Given the lug barriers and installed surge protective device and the overall condition of the equipment, it appears to have been installed in the last few years. Generally speaking, you don't need to scrutinize the condition of the electrical equipment as much on newer equipment. This is not a hard and fast rule as code violations are still possible even on new construction.

Notice how much space there appears to be on this bus. This tells us, given that we have sufficient available backfeed, a load side breaker will most likely be the best option for interconnection.

Next, we need to calculate the total allowable backfeed on the load side. We'll need to find:

The bus rating
The main breaker rating

We have a Bus rating of 225A and a main breaker rating of 200A. Let's notate that and then we'll calculate the backfeed:

BACKFEED CALCULATIONS

We need to comply with NEC code article 705.12(B)(3)(2), otherwise known as the 120% rule:

To do this we will multiply the bus rating (225) by 1.2 and then subtract the main breaker rating (200). This will give us the maximum current we can backfeed on the load side.

Notate the total allowable backfeed.

Now we know we can land up to 70A of solar on the load side. Now we need to know how much total backfeed will produced by all microinverters combined. As of this publishing we are using Enphase IQ8+ Micro inverters. We can use the guidelines in NEC article 690.9(B) to do this calculation: Backfeed device ratings shall be no less than 125% of the [inverter] maximum continuous output current.

The maximum continuous output current for one IQ8+ Microinverter is 1.21A according to the spec sheet. We'll multiply that continuous output by the total number of microinverters that will be used and then multiply that by the 125% (or 1.25) safety figure factored into the code.

We will round up to the nearest size OCPD (breaker or fuse). In this case, it will be 20A.

NARROW DOWN THE INTERCONNECTION

It is important to consider every interconnection type available to you. Use your best judgement and of course, make sure to use the resources available to you to check yourself.

It appears that we have supply side service conductors as a viable option.
We also have a load side breaker as a viable option

What are the factors that are going to make this decision for us?

Cost
Labor

We want to design an installable product while wasting as little effort as possible on the part of our installers. Thinking about the equipment needs.

A supply side tap would require:

- Piercing taps
- Fusible AC disconnect enclosure
- Fuses and fuse reducers

A Load side breaker would require:

- Only a breaker

Again, we're being lead to a load side breaker being the best method of interconnection.

CHECK YOURSELF

Let's use the Interconnection Flow Chart to check our judgement:

The top preferred interconnection type for this category is a supply side breaker, but this is not available to us on this MSP. Load Side Breaker is the very next option. Also note that it is listed above Supply Side Tap in the order of preference.

Now we can fully conclude that we will use a load side breaker.

FINALIZE INTERCONNECTION

The final thing we will need to complete the interconnection call out is the make of the MSP to ensure we source a compatible breaker.

Be careful with this. Notice it says EATON on each breaker. This is where it gets a little confusing at first. EATON purchased Cutler Hammer a number of years ago but continues to manufacture CH type breakers. CH type panels are easily distinguished from BR type panels because of the tan handles on their breakers.

You will call it out as Culter Hammer if:

Labeling on the load center indicates "CH" as part of the model number
The breakers have tan handles.

Now we can notate this and start filling out the electrical design in the BASELINE tool.

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