Connect For...?
What interconnection rule would allow a solar breaker to land anywhere on an MSP?
The "Sum of All Handles" Rule
According to 690.15(A), the placement of equipment disconnects must follow what two rules?
The disconnect must be within sight and within 10' of the equipment
What are the two main types of OCPD used in electrical systems, and how are they different?
Fuses = One-time use. If too much current passes through, the fuse "blows" and needs to be replaced
Breakers = Multiple uses. Too much current will "trip" the breaker, which can be switched back to complete the circuit again
Refer to NEC Table 310.15(B)(16). What size copper conductor would be appropriate given 45A of current? (Assume 90°C temp. rating)
#8 CU conductors (rated up to 55A)
Define Ohm's Law
Ohm's Law describes the relationship between voltage, current, and resistance; it is typically expressed as V = IR but can be rearranged algebraically
According to NEC 705.11, a supply-side tap must be located between which two pieces of equipment?
The net meter and the main service disconnect
A customer is interested in rooftop solar. What site conditions must be considered to select the appropriate equipment?
1. Roof plane size, orientation, and pitch
2. Framing and roof surface material
3. Shading throughout the day
4. Snow and wind loading (roof zone specific)
5. Access - fire safety and routine maintenance
1. What formula is used to calculate the minimum OCPD for most systems?
2. What should you do if this calculation results in a "non-standard" OCPD size?
1. Maximum Circuit Current x 1.25
2. Round up to the nearest standard size
Explain the difference between Equipment Grounding Conductors (EGCs) and Grounding Electrode Conductors (GECs)
EGCs are used to connect (bond) all metal equipment in the system together. The GEC connects the system to the grounding electrode (typically a rod)
[In the tree analogy, the EGCs are the branches, and the GEC is the trunk]
What is the temperature coefficient (VoC)?
This number is used in calculations to determine how the voltage of a module / system will be affected by changes in temperature
What is the "120% Rule," and how is it used?
This rule is used to determine how many amps of PV power can land on a breaker. It is found with the equation (Busbar Amps x 1.2) - Main Breaker Amps = Allowed PV breaker size
What factors must be considered when locating a battery on a job site?
1. Wall support - attach to wood studs if possible
2. Working / ventilation clearances
3. Proximity to other system equipment
4. Temperature disclosure form (if located outside)
A job site has multiple grounding electrodes. What action should be taken to ensure system safety?
Bond all electrodes together
What is voltage-drop, why does it matter, and how can it be addressed?
V-drop occurs when resistance in long wire runs causes the output voltage to reduce. Many jurisdictions only allow a certain amount of V-drop in PV systems; e.g., Oregon limits V-drop to 2%. It can be addressed by upsizing wires or repositioning equipment to be co-located as close as possible
When would the current produced by a PV module be higher than the nameplate rating?
If irradiance exceeds the standard test condition of 1000 W/m2, then current will exceed the nameplate rating
An excited salesperson informs you they are about to sign a contract on their largest system yet. They show you eval pics of a new build... with a meter main. Explain to the salesperson why they'll need a design review.
The physical design of a meter main often makes tapping difficult--feeders may be short, or feed-through lugs may be used instead. For a load-side tap, the MSP may not have a main breaker. Finally, inspectors may count the lugs / MSP main towards the "sum of all handles."
What are some jurisdiction-specific "red flags" to note when designing solar in Washington and Oregon?
1. Washington Cities Electrical Code jurisdictions - additional disconnect required within 10' of tap
2. City of Portland - Note rafter size and span, additional engineering and paperwork needed
3. Proximity to the ocean and/or Columbia River
4. Allowable vent coverage
When can we exclude fire safety setbacks for systems over 33% / 25%?
If the installation is occurring on a detached, non-inhabitable structure or if other fire mitigation strategies (e.g. sprinklers) are in place and the fire marshall signs off
What internal and external factors can affect the ampacity of conductors?
Internal = Increased current raises the temperature and thus lowers the ampacity
External = Ambient air / weather conditions, the number of conductors in conduit, sunlight exposure, and proximity to roof can all raise the temperature of conductors, thus lowering ampacity
How can you calculate the output current of a PV array using microinverters?
(Output Current of Microinverter x # of Microinverters in Parallel)
What are some workarounds if the electrical equipment does not have space for a supply-side interconnection?
1. Use a "tap box" to interconnect
2. Downsize the main breaker to allow the solar to land on the MSP
3. Perform a load-side tap
A customer is interested in a ground-mount system. What factors must be considered when determining the size and location of the system?
1. Total Array Footprint, i.e. individual module Footprint (F = Cos of Tilt Angle x Module Length) plus Distance between rows (D = Cos of [180° - Azimuth°] x Length of Shadow)
2. Setbacks from property lines and access pathways
3. Slope of land
4. Orientation of array (S-Facing preferred, E- or W-Facing will see longer shadows in the morning or evening respectively
5. Vegetation / other obstructions around array
1. What is the difference between a ground fault and an arc fault?
2. How can you protect a PV system against faults?
1. Ground fault = A +/- conductor makes contact with anything bonded to ground. Arc fault = Current "arcs" or "jumps" across a high-resistance failure point in the circuit
2. Ensure conductors are sound (i.e. no nicking, chafing, pinching), keep conductors away from the elements, account for thermal expansion/contraction when positioning equipment
Define the following conductors used in a PV System: PV Source Circuits, PV Output Circuits, and Inverter Output Circuits
1. PV Source Circuits: "Strings" and "Homeruns," i.e. any conductors running between modules and connecting the modules to a combiner box. (DC Power)
2. PV Output Circuits: Conductors between the combiners and the inverter. (DC Power)
3. Inverter Output Circuits: Conductors running from the inverter to the point-of-interconnection. (AC Power)
1. Explain how wiring PV modules in series and parallel can affect the output voltage and current of the system
2. What are some advantages / disadvantages for each type of wiring?
1. With series wiring, voltage is additive but current remains the same. With parallel wiring, current is additive but voltage remains the same
2. Series = Best for unshaded conditions, lower amperage allows for smaller wire sizes, additive voltage may be needed to reach nominal-voltage-to-charge of battery systems
Parallel = Panels operate "independently," so this is best for mixed-light conditions