Sections 690 and 705 of the National Electric Code have specific rules for sizing the DC and AC conductors associated with grid-tied PV systems. With these requirements, there are minimum conductor sizes that will allow safe operation in any installation.
The code doesn’t dwell on voltage drop considerations for PV inverters–there is no mention in either section; however, this is an important consideration for any installation, and particularly those requiring long cable runs on either the DC or AC side of the inverter.
PV inverters have a mandated normal operating voltage window, and excessive voltage drops in cabling that effectively moves the nominal operating voltage seen at the terminals of the inverter to one end of this window can result in nuisance tripping of the inverter and an associated loss of generation.
Basic wire sizing
The NEC calls out different requirements for determining the base ampacity of a conductor depending on its location in the PV system:
- NEC Section 705.60 states the base wiring ampacity for AC conductors used to connect the inverter to grid must be based on 125% of the inverter nameplate current rating.
- NEC Section 690.8 states the base wiring ampacity for DC conductors carrying current generated by PV modules be based on 125% of the STC short circuit current capability of the string/array.
Once base ampacity values are determined, they must be further compensated to account for conditions of use per Section 310.15 of the NEC. These conditions of use include adjustments for ambient temperature, number of conductors in a raceway (conduit) and height of a conduit above a rooftop. Each of these factors, when applicable, increases the base ampacity to generate a minimum required ampacity value. The minimum wire size required can be found using Table 310.15(B)(16) in the NEC, depending on whether 75°C or 90°C rated copper or aluminum wire is specified.
Note that most inverters do not have the wiring terminals rated for aluminum wire, and so copper conductors must be specified.
Voltage drop considerations
The minimum wire size obtained above doesn’t take into account voltage drops due to long runs between the array and inverter and between the inverter and the grid. For inverters, this is important: Every percentage of voltage drop results in a percentage of power loss from the inverter. While this may not seem like much, the cumulative energy (kWh) lost over the life of the system can be significant.
Most inverter manufacturers recommend a maximum of 5% voltage drop for the system— typically 2.5% on either side of the inverter. On large systems, many designers specify an even tighter value of 3% total or less, to maximize the energy harvest.
Once the wiring size per NEC requirements is determined, the expected voltage drop in the wiring must be calculated and compared to the desired limit.
Percent voltage drop in a two wire circuit is given by:
%ΔV = 2 x (IMP/VMP) x R’ x (L/1000)
where:
IMP= current flowing in the conductor (for a string it will be the max power current rating of the PV module
VMP= reference voltage, e.g., for a string, it is the voltage driving the current flow, for a PV array this is the maximum power voltage
R’ is the resistance/kft for the wire size determined via NEC considerations
L = the one way length of wire in the circuit
As an example, consider a PV output conductor that carries current from five strings, each of which has an ISC = 9.2A and an IMP = 8.5A. The length of the run is 400’, and the voltage across the string is 500V.
Then the basic NEC ampacity required would be (5 x 9.2) = 46A. Assume the ambient temperature surrounding the wire is 35C, there are only two conductors in the conduit and the conduit is raised above the rooftop to a height of 6”:
- Since the conduit runs across a rooftop and is spaced 6” above the surface, Table 310.15(B)(3)(c) indicates the ambient temperature must have a 17C adder, or for the wiring we must assume local ambient of 35+17 = 52C
- From Table 310.15(B)(2)(a), for 90C wire, based on an ambient of 52C, the derating factor for ambient temperature is 0.76.
- Since there are only two conductors in the raceway, the conduit derating factor is 1.0 (no derating)
Thus, the NEC minimum ampacity is AMPNEC = 46.0/[(0.76)*(1)] = 60.52A, and using Table 310.15(B)(16), the minimum (90C/Cu) conductor to meet the requirement would be #6AWG.
From NEC Table 8, the resistance/kft for #6AWG wire is 0.510Ω/kft. For the voltage drop calculation use IMP in the voltage drop equation, or IMP would be 5 x 8.5 = 42.5A. Calculating the voltage drop,
%ΔV = 2 x (IMP/VMP) x R’ x (L/1000) = 2 x (42.5/500) x 0.51 x (400/1000) x 100 = 3.47%
Based on a desired 2.5% voltage drop for the DC side, a larger wire must be specified:
- For #4AWG, r’ = 0.321 Ω/kft and results in a %ΔV = 2 x (42.5/500) x 0.321 x (400/1000)x 100 = 2.18%
- For #3AWG, r’ = 0.254 Ω/kft and results in a %ΔV = 2 x (42.5/500) x 0.254 x (400/1000)x 100 = 1.73%
Thus, using #4AWG achieves the 2.5% max voltage drop figure; going to #3AWG more than meets the objective, but this choice also adds additional cost, which must be evaluated.
This article is contributed by Roy Allen, technical sales engineer at ABB Solar Inverters.