TECHNICAL BRIEF
Circuit Calculations for the S280 Microinverter
Overview
Use this document as a reference guide to properly size conductors for Enphase S280 Microinverter™
branch circuit home runs. This supplements the more comprehensive technical brief, Calculating AC Line
Voltage Drop for S280 Microinverters with Engage Cables. Enphase Energy recommends maintaining the
total voltage drop on all wiring sections to 2% or less. This includes the Enphase Engage Cable™, the home
run wiring from the junction box to the microinverter subpanel, and the section from the microinverter
subpanel to the main service panel or Point of Common Coupling (PCC).
This document lists the maximum conductor lengths from the array-located junction box back to the main
service panel, assuming that a 1% voltage drop is maintained.
240 VAC Single-Phase
External Branch (Home Run) Wiring Maximum Distance to Maintain 1% VDrop
for 240 VAC Single-Phase
Microinverters per Branch for 240 VAC Single-Phase
1
2
AWG
3
4
5
6
7
8
9
10
11
12
13
14
Maximum One-Way Wire Length (in Feet) to Maintain 1% VRISE
#12
539
269
180
135
108
90
77
67
60
54
49
45
41
38
#10
860
430
287
215
172
143
123
108
96
86
78
72
66
61
#8
1371 686
457
343
274
229
196
171
152
137
125
114
105
98
#6
2172 1086 724
543
434
392
310
272
241
217
197
181
167
155
#4
3463 1732 1154 866
693
577
495
433
385
346
315
289
266
247
VDrop Calculation
The typical formula to calculate the percent of voltage drop for 240 VAC single-phase is:
% Vdrop =
(Amps/Inverter x # of Inverters) x (Resistance/feet x 2-way wire length)
240 Volts
Circuit Current Calculation
The formula to calculate the circuit current of a branch is:
Amps/Branch = Continuous Output Power ÷ 240 V x # of Microinverters
For example, if the continuous output power is 270 Watts AC and there are 14 microinverters, then:
270 W ÷ 240 V x 14 = 15.75 Amps/Branch
1
 2015 Enphase Energy Inc. All rights reserved.
November 2015
Circuit Calculations for the S280 Microinverter
Overcurrent Protection Calculation
Use the value of 1.25 and the result of the Circuit Current Calculation on page 1 to determine the overcurrent
protection value. For example, if the circuit calculation is 15.75 Amps, then:
15.75 X 1.25 = 19.69 Amps
Conclusions for 240 VAC Single-Phase
For less than 1% voltage drop in a fully populated branch circuit home run for 240 VAC single-phase:
•
•
•
•
Install 1 to 14 S280 Microinverters per branch circuit
Install a maximum 2-pole 20 Amp circuit breaker
Use a minimum 12 AWG wire size
Engage Cable required
208 VAC Three-Phase
External Branch (Home Run) Wiring Maximum Distance to Maintain 1% VDrop
for 208 VAC Three-Phase
Microinverters per Branch for 208 VAC Three-Phase
3
6
9
12
15
18
21
Maximum One-Way Wire Length (in Feet)
to Maintain 1% VRISE
AWG
#12
270
135
90
67
54
45
39
#10
431
215
144
108
86
72
62
#8
687
343
229
172
137
114
98
#6
1088
544
363
272
218
181
155
#4
1734
867
578
434
347
289
248
VDrop Calculation
The typical formula to calculate the percent of voltage rise for 208 VAC three-phase is:
% Vdrop =
(Watts/Inverter x # of Inverters) x (Resistance/feet x 1-way wire length) ÷ 208 Volts
208 Volts
Circuit Current Calculation
The formula to calculate the circuit current of a 208 VAC three-phase branch is:
Amps/Branch = Maximum Output Power ÷ 208 Volts ÷ √3 x # of Microinverters
For example, if the maximum output power is 270 Watts AC and there are 21 microinverters, then:
270W ÷ 208V ÷ 1.732 x 21 = 15.74A
Overcurrent Protection Calculation
Use the value of 1.25 and the result of the Circuit Current Calculation on page 2 to determine the overcurrent
protection value. For example, if the circuit calculation is 15.74 Amps, then:
15.74A X 1.25 = 19.67A
2
 2015 Enphase Energy Inc. All rights reserved.
November 2015
Circuit Calculations for the S280 Microinverter
Conclusions for 208 VAC Three-Phase
For less than 1% voltage drop in a fully populated branch circuit home run for 208 VAC three-phase:
•
•
•
•
Install 1 to 21 S280 Microinverters per branch circuit
Install a maximum 3-pole 20 Amp circuit breaker
Use a minimum 12 AWG wire size
Engage Cable required
Summary
To minimize any installation difficulties, please adhere to the tables in this document and the information in
the Enphase technical brief, Calculating AC Line Voltage Drop for S280 Microinverters with Engage Cables
on http://www.enphase.com/support).
3
 2015 Enphase Energy Inc. All rights reserved.
November 2015