# How much cable voltage drop is allowed in a rooftop solar plant, and what does it cost in units?

Canonical: https://senseible.earth/answers/rooftop-solar-cable-voltage-drop-2-percent

The Ministry of New and Renewable Energy (MNRE) specification keeps voltage drop under 2% on the direct current side and limits it to a maximum of 2% on the alternating current side. On the same current, my reading is that each 2% drop costs about 2% of the plant's units.

## What the cable clauses say

These are clauses from the technical specification in Annexure 3 of MNRE's guidelines for its residential rooftop solar scheme, Pradhan Mantri Surya Ghar: Muft Bijli Yojana. A factory is not bound by them, but they are a published checklist to hold a quotation against.

- Clause 9.6: cable sizes between array interconnections, array to junction boxes and junction boxes to the inverter shall be so selected to keep the voltage drop less than 2% (direct current (DC) cable losses).
- Clause 9.7: the size of each type of alternating current (AC) cable shall be based on minimum voltage drop, and the maximum drop shall be limited to 2%.
- Clause 9.4: multi-strand annealed high conductivity copper on the DC side, and copper or flame retardant low smoke (FRLS) type aluminium on the AC side. For DC cabling, multi-core cables shall not be used.
- Clause 9.5: operating temperature range of -10 to +80 degrees Celsius and voltage rating of 660/1000 volts.
- Clause 9.13: cable should be compatible up to the life of the panels, that is 25 years.
- Clause 9.3: armoured cable with polyvinyl chloride (PVC) type A or cross-linked polyethylene (XLPE) insulation for protection against ultraviolet light.

## Why a voltage drop is lost energy

My reading, from basic electrical law and not printed in the guideline: at the same current, the share of power lost in a cable equals its voltage drop as a share of the voltage. A 2% drop is therefore about 2% of the power passing through. The DC and AC runs lose in turn, so a DC drop at the 2% ceiling and an AC drop of 2% compound to a little under 4%. Clause 9.6 says less than 2%, while clause 9.7 limits the AC drop to a maximum of 2%, so 2% is only reachable on the AC side. Clause 9.6 also applies its limit to several DC segments (array interconnections, array to junction boxes, junction boxes to the inverter). If the limit is read per segment, the total DC drop can be higher; ask for the total.

## What to ask the designer

- The longest DC run and the AC run from the inverter to the main panel, in metres, with the cable size chosen for each.
- The calculated voltage drop for each run at full output. A figure under 2% on paper is the evidence.
- The cable type and standard on the drum label: IS 694 or IS 1554 (Indian Standard, IS), or IS 17293:2020 for 1,500 volt DC.
- That cables run in a rigid PVC pipe or galvanised iron cable tray, tagged, with clamps at intervals not exceeding 50 centimetres (clauses 9.9 and 9.11).

## Both runs at the 2% ceiling (example plant)

- Plant output before cable losses: 140,000 kilowatt hours (kWh) a year (an example)
- DC run at the 2% ceiling (clause 9.6 says less than 2%): 140,000 x 0.02 = 2,800 kWh lost
- Both runs at the 2% ceiling: 1 minus (0.98 x 0.98) = 3.96%, so 140,000 x 0.0396 = 5,544 kWh lost
- Grid carbon dioxide (CO2) for 5,544 kWh bought instead: 5,544 x 0.675 = about 3,742 kilograms

Plant output is an example. 0.675 kilograms of CO2 per kWh is the Central Electricity Authority grid factor for financial year 2025-26.

## What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so your monthly grid units are on file. It does not size cables or measure losses.

- [Performance ratio to demand](https://senseible.earth/answers/rooftop-solar-performance-ratio-75-percent-guarantee)
- [Payback of rooftop solar for a factory](https://senseible.earth/answers/rooftop-solar-payback-for-a-factory)
- [Earthing: pits and resistance](https://senseible.earth/answers/rooftop-solar-earthing-5-ohm-three-earth-pits)

Sources: [Ministry of New and Renewable Energy (MNRE), Guidelines for PM-Surya Ghar: Muft Bijli Yojana, Central Financial Assistance to Residential Consumers, with Annexures 2 to 4](https://mnre.gov.in/static/uploads/2025/08/f20f6d61628866e66b612810b49abd78.pdf), [Central Electricity Authority (CEA), CO2 Baseline Database, India grid factor for financial year 2025-26](https://cea.nic.in/cdm-co2-baseline-view?mappingId=234&lang=en)

Last reviewed 2026-11-06. Re-check clause numbers and figures against the current version of the guidelines before you rely on them. Clause numbers are from Annexure 3, section 9. Cable sizing is for a qualified electrical designer to confirm.