This post was written by Josh Weiner, Solar Expert Witness & Solar Engineering Expert. Mr. Weiner has been at the forefront of the solar energy industry for over 20 years and is an industry leader on solar-plus-storage design. Josh’s expertise spans both in-front of and behind-the-meter initiatives including residential, commercial, utility, grid-scale, and ev charging solar and storage applications. 

SepiSolar project engineers use one powerful solar design tool to perform many system design functions. Some are completed in minutes. Others take hours. This points to a perennial challenge we face as a customer service organization committed to continual improvement.

When all support requests go into a single queue, quick and easy tasks don’t always get resolved quickly enough.

The fact is, our solar design tool is necessarily complex for highly trained engineers working through a streamlined process. But if you separate the pieces, you’ll find several user-friendly tools that contractors can use themselves. A wire size calculator and a string configuration calculator, for example.

Later this year, SepiSolar will provide six web tools for contractors to perform quick and easy design tasks. Our engineers will continue to handle any project design changes, or all of them if you’d like. We are always available with a wide variety of customer support resources. This is just one opportunity, when the cost of waiting for support on a simple design change exceeds the benefit, for SepiSolar to empower our customers to use some of our solar design tools yourself.

When to use our design tools

Each time a change request comes in to SepiSolar, an assistant project manager or operations manager tags it according to priority, complexity, time needed, and root cause. Tickets go into queue for the engineering team sorted by priority.

If a contractor submits a support ticket to resize a run of wire estimated to be 100 feet but later measured at closer to 200 feet, the complexity and time required would be set to “low.” The same might be true for a contractor wanting to move an inverter from inside a building to a location outdoors, who needs to know if the wire size must change.

Using SepiSolar’s wire size calculator, you enter inputs such as current through the conductor, number of wires in the conduit, and the project site’s maximum ambient temperature. The calculator auto-populates electrical resistance and generates the wire size that you need.

When there’s no urgency, let SepiSolar run the calculation. On the other hand, if you’re in the field and a quick calculation can prevent a return trip to the project site, direct access to our wire size tool can save time and money, eliminating a costly truck roll.

Complex and time-consuming design changes will continue to go directly to the engineering team. For example, an EPC might want to replace 60-cell modules with 72-cell modules in the plan set for an 850 kW agricultural project. Our engineers would use the new module specifications to recheck wire sizes and overcurrent protection and redraw the module array as needed.

If an EPC wants to replace central inverters with string inverters, in order to optimize the project for cost, then instead of using these calculator tools, our engineers would begin a consultation to help with inverter selection, AC wiring design, and DC wiring design, which is what creative, solutions-focused designers do best.

DIY solar design tools

The solar design tools that SepiSolar will make accessible to our customers on the web are simple and user friendly. These tools are not revolutionary. They just make a contractor’s job a little easier, one day at a time. Here’s how they work.

Wire sizing calculator

Oversizing electrical wire means overspending on materials, given the current that will flow through the system. To undersize means the system is carrying so much current that you risk melting conductor wire or insulation. Correct wire sizing avoids both extremes based on conductor material (copper or aluminum), current through the conductor, environment (in conduit, direct buried, or open air), how many conductors in conduit, insulation type (THHN, THWN-2, USE-2, XHHW), and ambient temperature plus adders for the environment.

String sizing calculator

Module string configuration introduces another set of tradeoffs. If you connect too many modules in series, the system can exceed an inverter’s maximum input voltage, causing equipment damage. Connect too few modules and you might fall short of the minimum input voltage required to start up the inverter. To determine string configuration, look at your solar module datasheet and input the following data points into the SepiSolar calculator: solar module manufacturer, model name or number, and quantity; inverter manufacturer and model name or number; racking type (e.g., flush or tilted roof mount, ground mount); maximum, minimum, and average high and average low temperature at the project location.

Conduit sizing calculator

The National Electrical Code limits how much wiring can go inside a conduit. It does so to control heat gain, manage risk of wire damage, and preserve space to eventually add more wires. In the NEC, you can find fill tables for frequently used wire and conduit types and equations for any application. Or input wire and conduit specifications into our conduit sizing calculator to get a fast and dependable conduit size.

120% rule

The 120 percent rule refers to a simple calculation used to confirm that the size of an electrical distribution panel in a home or business facility is large enough to handle the capacity of the circuit breakers feeding it. Most of these residential electrical panels have a 100 amp or 200 amp main breaker. For solar projects interconnected on the customer side of the meter, the National Electrical Code allows total ampacity from all sources up to 120 percent of the busbar or conductor rating. Why? In brief, it’s because when you connect a supply source to the service panel, it has the opposite effect of connecting a load source to the service panel. Instead of reducing capacity on the busbar, the solar generator actually increases capacity.

Voltage drop calculator

Voltage drop is a measure of efficiency in an electrical circuit. A 1 percent drop in voltage equals a 1 percent power reduction at the end of the line. Electrical calculator programs are generally available, but they don’t always consider all solar project inputs, as noted in Solar Pro. (See Issue 3.2, ‘Voltage drop in PV systems’) To calculate voltage drop, input circuit type: AC (1-phase or 3-phase) or DC, nominal voltage, current, wire gauge and material (aluminum vs copper), and the length of your conductor run.

Load calculator (structural and electrical)

Before issuing project permits, local authorities will compare system specifications to the electrical load and structural load requirements on site. Using SepiSolar’s load calculator, customers can size a building’s electrical service capacity based on service voltage (1-phase or 3-phase) and the sum of all motor loads, continuous loads, and non-continuous loads. To generate structural load limits, customers can also enter module weight and quantity, the number of modules racked in portrait and landscape, racking rail weight, rack type (tilt up or flush mount), weight adders (microinverters, optimizers, ballast blocks), and a measurement of wind exposure, such as wind speed, exposure zone, or the mean height of the roof.

Unlike other design tools

Contractors looking for solar design tools will find a variety of options on the market. Some products generate project proposals for use in the sales process along with plan sets for permitting and interconnection. Others require experience with sophisticated CAD software.

SepiSolar’s user-friendly tools are relatively simple in comparison. These are calculators used by licensed engineers, governed by codes and standards that protect public health and safety. They do not take the place of a licensed engineer. They complement the engineer’s work, empowering contractors to make code-compliant calculations yourself and increasing the value of the SepiSolar service.