AC Wire Sizing

When designing a Grid-Tie Photovoltaic (PV) system, it is important to size your AC inverter output wires to carry a less than 1% voltage rise.  If these wires are undersize, the inverter will experience an AC Overvoltage Error and shut-down due to its sensitive “Anti-Islanding” protection required by UL-1741

What is Anti-Islanding and UL-1741?
Underwriter Laboratory’s listing UL-1741 requires that all grid interactive PV inverters disconnect from the grid during grid-outages, brown outs or other interruptions of normal service.  This is so that the current and voltage generated by PV Solar Modules cannot create or contribute to a dangerous situation.  UL-1741 requires the PV inverter to test for Utility voltage and frequency to be within a normal and safe range in order to supply current to the grid.  The allowed range for voltage @ 240Vac is 211-264V (+10/-12%) and frequency is 59.3 – 60.5 Hz.   If the inverter detects either of these to be outside of this range, it will shut down so as not to create an “island” of power on the utility grid.

What does UL-1741 have to do with wire sizing? 
Improper wire sizing will create unwanted resistance in the wire.  Too much resistance will result in a voltage rise (see below) measured at the inverter.  If there is too much resistance along the wire, the voltage will rise above the allowed maximum operating voltage of 264 Volts, causing the inverter to shut down.

Why does the voltage rise instead of drop? 
Because the voltage is provided by the utility and not the inverter, and the current is coming from the inverter and not the utility, the voltage (pressure) will actually rise at the inverter as current starts to flow.  This is like trying to blow air into a tube with the middle clamped.  The tube blows up like a balloon because the air cannot flow due to the resistance of the clamp.  The air is the current, the clamp is the resistance of the wire and resulting higher pressure is the voltage rise.

Why design for a 1% voltage rise if UL-1741 allows for 10%?
This is the most misunderstood design requirement and one of the most critical.  1% of 240 volts is only 2.4V while UL allows a 24 V rise.  But what isn’t discussed is the inverter’s voltage sensing circuit is not 100% accurate.  An inverter can only measure voltage to within a minimum tolerance.  Revenue grade meters have a tolerance of +/-2%, so let’s assume our voltage sensor carries the same tolerance.
Next, to comply with UL-1741, the tolerance on the voltage sensor cannot be above the limit of 264V, so the target voltage is lowered.  At +/-2%, a spread of 4% or 9.6V, the inverter is now looking for a voltage rise of at least 254V.  Maybe more…or maybe less!
Finally, Utilities are allowed to have a maximum voltage of as much as 5.8% or 252V.  It is common during the summer for the Utility to raise the voltage to handle the high demand of air conditioning loads.  Assuming your utility raised their voltage to 252V and your inverter is set to shut down at 254V, the resistance in the wire can only cause a 2V voltage rise before the system shuts town!  This is why we must size the wires for a 1% voltage rise.

How do you design for 1% voltage drop?
To design for a 1% voltage drop, you will need to know:

• d = 1-way wire distance
• I = Maximum continuous current rating of the inverter
• V = Nominal utility voltage (i.e. 240, 208 etc.)
• V_d = Percentage voltage drop, in this case 1% (0.1)

Remember to first use the Tables in the National Electric Code article 310 to determine the proper wire gauge first based on ampacity, taking into account the necessary derate factors that would apply such as wire temperature and conduit fill.  Use this wire gauge as a minimum starting point.  Then do the following calculation:

(d*2*I*k)/(V*0.1)=Wire Size in circular mills

k is the thermal conductivity, or the “K-Factor,” of a metal and for copper wire use 12.9.  You can convert circular mills to AWG by using NEC Chapter 9, Table 8 (example shown below).  Compare this value to the wire gauge you determined by the tables in NEC 310 and use the larger of the two.