LOAD SIDE CONNECTIONS

LOAD SIDE CONNECTIONS

Design

Here at ION we will default to installing a main breaker to protect sub panels in most cases.

This is what the diagram should look like:

For this system, we are backfeeding 35A of solar, so this bus should be more than capable of supporting this.

Remember, the main disconnect outside will be considered the main breaker for the sub panel enslosure in this type of scenario. The disconnect outside is rated for 200A. 

The math should look like this:

(200 * 1.2) - 200 = 40A

This is what the diagram should look like:

• The drawing always needs to show all enclosures between the point of interconnection (PV breaker) and the utility meter.

• Even though the main breaker is in a separate enclosure, this system behaves the same and has the same rules as if the main breaker was in the same enclosure as all of the loads (green boxes for visual)

• 40A of available backfeed is plenty of backfeed available for the 30A of backfeed this system will produce. 

• The red box indicates the point of interconnection: Load side breaker - sub panel - main with no loads.

This is a setup that is more common in areas like New Mexico. This meter/main combo has two service disconnects in a single enclosure. The preferred method of interconnection would be to interconnect in this enclosure if it is possible.

The first thing you'll notice about the MSP in this enclosure is that it appears to be full. Here's the problem: the labeling appears to be faded to a point that it won't be legible and the surveyor didn't get photos of it so it is unknown if any of the spaces will allow tandems. In addition to this, a supply side tap is not an available option for a meter/main combo.

Now we need to look for alternative options. We can start by looking at the second service disconnect in this enclosure.

This setup can look a little confusing but focusing on just the second service disconnect, you'll see that the setup is actually identical to the previous example. It is a main disconnect feeding a distribution panel elsewhere in the home. 

This is what the connected sub panel looks like. The feeders appear to come from the top wall of the enclosure. This doesn't necessarily mean that this panel is not back-to-back with the main disconnect, but it is an indicator that we need to look for other indicators of its location. 

In addition, we have many slim breakers in this panel. We need to ensure that this load center allows thin breakers and if it does, we may be able to thin further to accommodate solar.

The labeling indicates "A/B" for each breaker space. This means that you may use thin breakers on all breaker spaces. This is great news for us as we now know we can thin breakers to make space for a 2P PV breaker.

This label also has a detail for alternate main breakers. The listed breakers go up to 200A. The surveyor didn't get a photo of the proper bus rating, but we can use the 200A figure since we know that is the max main breaker size.

We will plan to consolidate 4 of the 1P breakers onto slims to make space for a 2P breaker. This will be added to our data collection during this step.

Wait, there's no main breaker?

Remember, this set up is like the last one. The main disconnect feeding this panel acts the same as if it was installed where the green box is.

The main breaker is rated for 200A. Remember to account for backfeed:

(200 * 1.2) - 200 = 40A

This diagram looks extremely similar to the last one.  This system backfeed for this system is 30A, so 40A will be plenty to handle this. Notice that we don't need to do anything special on the diagram when thinning breakers, they're still existing loads.

In this example, the main panel is full of tandem breakers and GFCI breakers that cannot be thinned. Remember, an outdoor interconnection is always preferred, so we need to look for more options.

The breaker in the red box is feeding a sub panel somewhere on the interior of the home. Note that if the sub panel is not back-to-back then it should not be considered for interconnection.

Assuming the sub panel is not back-to-back, we should consider a load side tap, though we should still do our due diligence and make sure to get a proper location on the sub panel to rule it out in the next few steps.

When load side tap connections are made to a feeder connected to a branch breaker, then the branch breaker that the feeders are connected to essentially becomes the PV breaker and should follow the 120% rule. First, we need to collect the bus rating and main breaker rating. We saw the main breaker is 200A in the previous photos and this photo shows that the bus is also rated for 200A.

(200 * 1.2) - 200 = 40A

The sub panel appears to be in the basement and away from the MSP. Since it is away, we should rule out trying to interconnect on it alltogether. We will want to complete a load side tap, but there's one more factor we need to consider.

this sub panel needs a main breaker protecting it. There are a couple of options here. We could do a proper main breaker kit if the panel supports it, if not, we could consider a backfed main breaker using a typical 2P branch breaker.

This is what the diagram should look like. Notice the tap highlighted. The new main breaker in the sub panel is indicated as New (N) and we have a fusible AC disconnect ahead of the point of interconnection rated for PV backfeed.

The knock out on the deadfront confirms that the panel should be able to accept a proper main breaker.

But, the listed model number for the proper main breaker is under these wires so it is not visible in the photos. The labeling does call out alternatives we could fall back on if needed, but indoor HOM type load centers usually use a common main breaker that is also used for QO models and we can use other context clues to confirm this.

QOM breakers are the easiest to call out by inspecting the main lugs. Note the lug spacing and the anchor space for the hold down kit. These lugs are detachable to allow for QOM type breaker tabs to land on them.

This is the QOM main breaker that should be called out:

• The spacing is identical to the existing lugs

• The hold down coordinates with the anchoring on the panel as seen in the photo.

• The breaker profile matches the knockout on the deadfront.

Note that QOM is only compatible with Square D HOM and QO type load centers and you need to thoroughly investigate every time you call one out. This breaker is not compatible with every HOM or QO model; always use the labeling when it is available and legible and use context clues as we did in this example.

This is what the diagram looks like. Notice the callout for a new main breaker in the sub panel.

A note regarding backfeed:

When a connection is made to a feeder coming from a main disconnect only type panel (in other words, not from a branch breaker) the 120% rule does not apply. The rules will be similar to a supply side tap wherein we are at liberty to backfeed up to the rating of the feeders.

HOW TO COMPLETE A LOAD CALCULATION

How do you complete a load calculation?

The first thing to understand: what is a Volt-Ampere (VA)?

• VA is a figure used for sizing breakers and conductors.

• VA = VxA in a circuit.

Example: 

A 20A 2P breaker has a Voltage of 240V:

20A x 240V = 4800VA

What about a 20A 1P breaker? A 1P breaker is rated for 120V:

20A x 120V = 2400VA

This is the basic principle. There are a few code specifics that change this.

But wait, why aren’t all of our values here the unadulterated version of the VA calculation?

The NEC has specific minimum guidelines for load calculations which are built into our design tool.

NEC 220.82(B)(1) General Lighting and Receptacles:

3 VA/sq. Ft  for general lighting and general use receptacles. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit. The calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

Looking at the photo on the right, which loads would be accounted for in this portion of the calculation?

Any light or receptacle breaker. 

That means that these breakers don't need to be accounted for anywhere else, we can use the square footage figure only (this figure goes in the very top section).

Example:

See the example on the right: Section (1) General Lighting and Receptacles.

• In this example, the home has a square footage of 3500 sq. ft. 

• The DT will use the square footage figure you plugged in and multiply by 3 VA.

Next, we factor in small appliances. Small appliances are 1P/120V appliances. Typically: washer, microwave, fridge, etc.

NEC 220.82(B)(2) Small Appliances and Laundry:

1500 VA for each 2 wire (1P) 20A small appliance branch circuit and each laundry branch circuit.

Looking at the photo on the right, which loads would be included in this portion of the calculation?

Any small appliance (fridge, microwave, etc) attached to a single pole breaker.

For these fields, we only need the quantity, as each will count as 1500 VA for the calculation.

Example:

See the example on the right: Section (2) Small Appliances and Laundry:

• In this example, we counted 5 small appliances (Microwave, Furnace, water softener, "tankless", Disposal/Dish)

• "Laundry" will always have a minimum value of 2. This increases if, for whatever reason, the customer has 3 laundry units or more.

• The DT will then multiply the quantity selected by 1500VA for each of these figures.

Next, we factor in larger appliances. Specific appliances are any 2P/240V appliance. Typically: Dryer, Electric Range, Oven, etc.

NEC 220.82(B)(3)&(4) Specific Appliances, Ranges, Dryers, Water Heaters: Nameplate rating of all major appliances.

What’s a nameplate rating?

Nameplate rating is the maximum power rating of the appliance per the manufacturer specifications. 

Here at ION, we just use the breaker rating since the appliance nameplate rating will be rated at or below the breaker rating. If the surveyor got photos of nameplates, use this rating.

** note that some AHJs/Utilities may require that we use the specific nameplate rating in some cases **

Looking at the photo on the right, which loads would be included in this portion of the calculation?

Example:

See the example on the right: Section (3) Specific Appliances, Ranges, Dryers, Water Heaters:

• In this example, we have a "well", dryer, and "garage" all on 2P breakers.

  • Any that are not on the directory drop down can just be added as "Other 240V Appliance)

• Input the breaker ampacity next to each appliance.

• The DT will complete the VA calculation for you.

  • This calculation assumes that breakers are sized at 125% of nameplate rating for these appliances.

Finally, we will factor in Heat and Air Conditioning Loads.

NEC 220.82(C) Heating and Air Conditioning Loads: The largest of the following six selections shall be used:

1. 100% nameplate rating of the air conditioning

2. 100% nameplate rating of the heat pump when it is used without any supplemental electric heating. 

3. 100% nameplate rating of the heat pump compressor and 65% of the supplemental electric heating for central electric space heating systems

4. 65% of the nameplate ratings of electric space heating if less than four separately controlled units

5. 40% of the nameplate rating of electric space heating if four or more separately controlled units

6. 100% nameplate rating of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value.

For most markets, surveyors will not get name plate ratings for these units as they aren't specifically required by the AHJ. W

• We will assume the worst case scenario and use the circuit breaker size for this calculation instead as it will likely be rated higher than the actual nameplate rating of the unit.

** note that some AHJs/Utilities may require that we use the specific nameplate rating in some cases **

Example:

See the example on the right: Section (4) Heating and Air Conditioning Loads:

• In this example, we have a single AC unit on a 20A breaker.

• Input the breaker ampacity in the associated box. In this example, we only need to use AC unit 1.

• The DT will complete the VA calculation for you.

  • This calculation assumes that breakers are sized at 125% of nameplate rating for these units.

Now that we have all of this added, how is it applied to the design?

In the example to the right, the total VA load on this residential service came out to 30968 VA.

Now we divide this number by the continuous voltage of the bus bar: 240V

30968 VA / 240V = approximately 129A total load as seen at the bottom of the load calc page. Now we know that the minimum size main breaker for this bus rounds up to 150A, so the 175A main breaker this designer has selected is sized appropriately according to this load calc.

Essentially, this calculation is completed to justify downsizing the main breaker by establishing how much load is on this bus.

• If the total load is over the size of the new proposed breaker (example: you want to derate from 200A down to 175A but the load calc says the total load is 180A) then you will need to explore other avenues.

• If the total load is at or below the threshold for the new main breaker size, you are good to continue and submit for review.

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:

1. Labeling on the load center indicates "CH" as part of the model number

2. The breakers have tan handles.

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