Why it matters
A PV module’s voltage rises as it gets colder. On a clear, freezing morning a string can exceed its nameplate open-circuit voltage by 10% or more. NEC 690.7 requires the maximum DC voltage to be calculated at the site’s lowest expected temperature, and that number has to stay under the inverter’s maximum input and under the code limit for the building: 600 V for one- and two-family dwellings, and 1,000 V for other buildings.
How to calculate it
690.7(A) allows three methods: the manufacturer’s temperature coefficient, the crystalline-silicon correction factors in Table 690.7(A), or, for systems of 100 kW or more, an engineered design. The coefficient method is the usual one:
βVoc is the module’s Voc temperature coefficient as a fraction per °C (a negative number), and Tmin is the lowest expected ambient temperature. The code’s informational note points to ASHRAE’s Extreme Annual Mean Minimum Design Dry Bulb Temperature for the nearest weather station as a source; many jurisdictions publish the value they expect you to use.
Worked example
A module with Voc 49.5 V and a Voc coefficient of -0.26%/°C, at a site with a design low of -15 °C, on a house with a 600 V inverter:
600 V ÷ 54.65 V = 10.98 → at most 10 modules per string
10 modules give 546 V. An 11th would reach 601 V, over the 600 V limit, even though the nameplate total (544 V) looks safe.
The other end: voltage in the heat
The code sets the maximum. The minimum is about performance: in summer the string voltage drops, and if it falls below the inverter’s MPPT window or start-up voltage the inverter clips or stops. Check Vmp at a hot cell temperature, commonly the design high plus 25–35 °C for a roof-mounted array.
41.6 × [1 + (70 − 25) × (-0.0034)] = 35.24 V per module
200 V MPPT minimum ÷ 35.24 V → at least 6 modules per string
Here γ is the Vmp (or Pmax) coefficient and the cell temperature is a 40 °C day plus 30 °C. So this array wants strings of 6 to 10 modules.
Common mistakes
- Using the average winter low instead of the extreme design minimum
- Using the Pmax or Isc coefficient for the Voc calculation
- Forgetting that optimizers and some inverters have their own string limits, set by the manufacturer’s design tool rather than simple multiplication
- Checking the maximum but not the minimum, and ending up with strings too short for summer
- Mixing module counts between strings on one MPPT input
How Ballast Solar checks it
Ballast Solar checks every string against the inverter’s limits under the adopted NEC edition, and a string that doesn’t fit is caught before the set is released. See also rapid shutdown and the 120% rule.
Questions
How do you calculate maximum string voltage for solar?
Multiply the module Voc by [1 + (lowest expected temperature − 25 °C) × the Voc temperature coefficient], then by the number of modules in the string. The result must stay under the inverter maximum and the NEC 690.7 limit.
What temperature do I use for the NEC 690.7 calculation?
The lowest expected ambient temperature. The NEC informational note points to ASHRAE’s Extreme Annual Mean Minimum Design Dry Bulb Temperature; many jurisdictions publish the value they expect.
What is the maximum PV voltage on a house?
600 volts DC for one- and two-family dwellings under NEC 690.7, and 1,000 volts for other buildings.
Why is my string too short in summer?
Heat lowers module voltage. Check Vmp at a hot cell temperature against the inverter’s MPPT window and start-up voltage, and add modules if it falls below.
This guide explains the code for planning purposes. The adopted edition, local amendments and the AHJ’s interpretation govern; the contractor of record is responsible for the design.