Q: What is the maximum voltage drop allowed per IS 3961?

Maximum 5% voltage drop from origin to load point per IS 3961. For long runs (>30m), voltage drop often governs cable selection rather than current rating.

Question 1

How do I calculate cable size for a 3kW motor in India?

Accepted Answer

FLA = 3000÷(1.732×415×0.85) ≈ 4.9A. Apply 125% for starting current = 6.1A design current. IS 3961: 2.5mm² rated 19A adequate for most runs. Check voltage drop does not exceed 5% for runs over 30m.

Question 2

What cable is required for a hospital under NBC 2016?

Accepted Answer

HFFR IS 17048:2018 mandatory. HCl < 0.5%. FR-LSH (HCl < 15%) does not comply for hospitals, airports, or metro.

Question 3

What is the maximum voltage drop allowed per IS 3961?

Accepted Answer

Maximum 5% voltage drop from origin to load point per IS 3961. For long runs (>30m), voltage drop often governs cable selection rather than current rating.

Question 4

What derating factor applies for cable in conduit?

Accepted Answer

IS 3961 Part 2 specifies 0.8 derating for cables in conduit or trunking. For 7+ cables bunched in a tray, derate to 0.6.

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IS 3961 Ampacity Tables ⚡

NBC 2016 Compliance Matrix 🏙