4.0 Permits‎ > ‎

4.2 Electrical

Here we will perform the electrical validation. Basically, this entails selecting the right electrical parts, wiring, conduit and applying proper derating.


Resources

Pacific Energy Center's Guide to California Climate Zones
National Electrical Code 2008 Edition
NABCEP PV Installer Resource Guide, March 2012
A Guide to PV System Design and Installation
PV Power Systems and the 2005 NEC: Suggested Practices by John Wiles
Grounding Photovoltaic Modules - Solarabcs.org
Utility External Disconnect Switch - Practical, Legal and Technical Reasons to Eliminate the Requirement - Solarabcs.org


Electrical Parameters

The system shall be comprised of sixteen (16) solar PV panels/modules and sixteen (16) microinverters.

PV Module Ratings ValueUnits 
Module Make  Renesola 
Module ModelJC250M-24/Bb  
Max Power-Point Current (Imp)8.31 
Max Power-Point Voltage (Vmp)30.1 
Open-Circuit Voltage (Voc)37.4 
Short-Circuit Current (Isc)8.83 
Maximum Power (Pmax)250 
Voc Temp Coefficient-0.3 %/o
Table 4.2.1: PV Module Ratings

Inverter RatingsValue Units 
Module MakeEnphase  
Module ModelM215-60-2LL-S22 
Maximum DC Volt Rating45 
Maximum Power @ 40oC215 
Nominal AC Voltage240 
Max AC Current0.9 
Table 4.2.2: Inverter Ratings

Misc Parameters:
  • AC Cable from Junction Box to Service Panel = 40 feet
  • AC Conductors in Couduit = 4
  • Proposed Conduit = 3/4" EMT
  • Proposed Wire = 10 AWG THHN or THWN-2


Temperature and Conduit Fill Corrections for Conductor Ampacity

Figure 4.2.1: Zone 12 Climate Information

The Summer 1% Bulb Temperate is 100oF or 37.78oC
Publication Expedited Permit Process for PV Systems, Appendix F: Temperature Tables for Sacramento, High % Temp is 38oC. So the numbers agree very well.

UPDATE: Recalculated for ZONE 11, RED BLUFF, 104oF or 40.0oC 
Table 4.2.3: Conduit Fill Table for EMT

From the above table, 3/4" EMT can support up to 10 wires

Table 4.2.4: Conduit Fill Adjustment Factor

A single Enphase Engage cable can support up to 17 microinverters. Since I have 16 panels, it can be supported by a single cable, which will have 3 current-carrying conductors. So I do not need to derate. However, I want to plan for the possibility of adding another string of microinverters in the future. This will add 3 more 10 AWG wires in the conduit. So I will use an Adjustment Factor of 0.8.

Table 4.2.5: Ambient Temperature Adjustment for Rooftop Conduits

The EMT conduit should not be left lying on top of the roof tiles. It needs to be supported. NEC 358.30 (A) states "EMT shall be securely fastened in place at least every 3m (10 ft). In addition, each EMT run between termination points shall be securely fastened within 900 mm (3 ft) of each outlet box, junction box, device box, cabinet, conduit body, or other tubing termination."

I will be securing my EMT conduit between 0.5" to 3.5" above my roof top. So the Ambient Temperature Adjustment is 22oC

Temperature Adjustment Basis 
= Summer 1% Temp + Sunlit Conduit Temperature Adder
= 40oC + 22oC
= 62oC

Table 4.2.6: Correction Factor from NEC Table 310.16

From the above table, looking at the column (4th) for 90oC THHN/THWN wire and at an Ambient Temp of 62oC, we have a correction factor of 0.71.

Table 4.2.7: Allowable Wire Ampacity before Correction

Corrrected Wire Ampacity = Allowable Wire Ampacity * Correction Factor * Conduit Fill Adjustment Factor

 Wire Gauge Before Correction After Correction
 12 30A 13.9A
 10 40A 18.6A
 8 55A 25.5A
Table 4.2.8: Corrected Wire Ampacity


Wiring Calculations

Enphase recommends protecting each Engage cable string with a 20A breaker. NEC states that the breaker must be at least 25% more than the maximum current, but less than the corrected wire current.

Maximum current 
= M215 Max AC Current * Number of Microinverters
= 0.9 * 16
= 14.4A

Minimum conductor ampacity = 14.4A * 1.25 = 18A

So 12 AWG wire after correction cannot support this current. Besides, a 20A breaker cannot support/protect the corrected 12 AWG ampacity. This design proposal is to use 10 AWG wire, which will accommodate the expected max current.

Table 4.2.8: Conductor Resistance

Next, we will calculate the voltage drop. The recommended voltage drop is between 2% to 5%. NEC Chapter 9, Table 8 provides resistance in terms of ohms/kilo-foot. 10 AWG copper for example, is 1.29 ohm/kilofoot.

Voltage drop = Max current * Resistance/foot * 2 way Wire Length

For 10 AWG, Voltage Drop
= 14.4A * 0.00129 ohm/foot * (40 ft * 2)
= 1.49V or 1.49V/240V = 0.62%

For the various common wire sizes, we can calculate the following:

 Wire GaugeLength (1-way) for 1% VdropVdrop at 40 feet (1-way) 
 12 40.65 ft 0.98%
 10 64.60 ft 0.62%
 8 103.01 ft 0.39%
Table 4.2.9: Voltage Drop for wire size and distance

Enphase Specific Voltage Calculations

Although NABCEP recommends keeping voltage drop to between 2% to 5%, Enphase recommends keeping the total drop to less than 2%. The Engage cable itself will introduce a voltage drop (more so since it's made of 12 AWG wires). Keeping within this figure will reduce chances of nuisance voltage trips. The M215 microinverter will cut power and enter AC Voltage Out of Range condition if it thinks that the line voltage is out of spec.

Enphase Tech Brief - Voltage Drop for M215 documents this recommendation.

Table 4.2.10: Enphase M215 Voltage drop/rise

Enphase recommends using a Center-Fed configuration to reduce this internal Vrise (Vrise or Vdrop, it implies the same thing). For my 16-panel array, this means cutting my Engage cable into two 8-drop lengths and combining them at the Junction Box. Splicing wires use space. While I can use a bigger box, I was thinking of mounting the box right under the edge of the panel array for a neat looking setup. I want to leave room in my Junction Box for the possibility of adding another Engage branch circuit in the future. So while going to Center-Fed yields a superb 0.19% Vrise, I should be OK with End-Fed, suffering a Vrise of 0.72%.

Total Voltage Drop
= Enphase Engage Cable Drop + Conduit Cable Drop
= 0.72% + 0.62%
= 1.34%

The last calculation to do is to size the 240V breaker at the Service Panel. My Service Panel is rated at 200A. Limit is no more than 20% of the panel max or 40A in this case. I will go with a 20A breaker. My load center will service as a branch combiner and AC disconnect. I already plan to have a 20A breaker protecting each branch circuit (only 1 branch circuit as proposed). In the future, if I add another branch circuit, it will be protected by another 20A breaker. I will have to upgrade the wiring from Load Center to Service Panel to 8 AWG and upgrade the Solar Feed breaker at the Service Panel to 30A. Maybe I'll just do this during my install.


Electrical Summary

Number of Panels = 16
Number of Branch Circuits = 1
Max Branch Current = 14.4A
Wire in Raceway = 10 AWG
Wire Length = 40 feet
Correct Wire Ampacity = 22.7A
Branch Breaker Capacity = 20A
Service Panel Capacity = 200A
Service Panel Breaker = 20A
Total Voltage Drop = 1.34%


PS: Although California is not a lighting prone area, since Enphase recommends it, I may install a Delta LA-302 or Midnight Solar MNPD-300 lightning arrestor. These 2 parts are tested by Enphase to not interrupt Envoy signals. I will probably install them in the Load Center/AC Disconnect box.

PSS: At first, I wasn't sure if I can put up these diagrams/tables from the NEC (since you have to pay to get a copy). However, I see similar tables at Home Depot in their Electrical section. Anyone can pop over and have a look. So I guess one cannot be overly copyright zealous of a spec that everyone is supposed to adhere to. Still, I will take down these NEC tables if anybody with the right authority makes such a request.
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