Transmission voltage and distance

Transmission voltage is chosen against current, not against distance. For a fixed power and power factor the three-phase line current is I = P / (√3 × V(L-L) × cos φ), and the resistive loss in the conductors is 3 × I² × R, so for a given power the load loss falls as the square of the voltage. Double the voltage and the loss quarters; quadruple it and the loss drops to one sixteenth.

That single relationship explains the whole ladder from generation through sub-transmission to bulk transmission. What it does not explain is how far a line can usefully deliver power, which is set by loadability limits — voltage drop on short lines, angular stability on long ones. Keep those two questions apart and most of the folklore around this topic evaporates.

Reviewed July 2026 by Sergey Syrvachev

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What it is (precise)

Start with the loss identity. Line current for a balanced three-phase circuit is I = P / (√3 × V(L-L) × cos φ), and total load loss in the three conductors is 3 × I² × R. Substitute one into the other and, at fixed power and power factor, loss is proportional to R / V². The US Energy Information Administration puts the same point in simplified single-phase, unity-power-factor form: "Power P = V*I.

Doubling of voltage (V), reduces the current (I) by half. Power loss L =I2*r= (P/V)2*r. Doubling of voltage, reduces the losses (L) by a factor of four." Apply that twice and quadrupling the voltage cuts load loss to one sixteenth. The Department of Energy states the consequence directly: transmission networks "are designed to minimize power loss over long distances by transmitting power at high voltage".

Now the caveats, because 1/V² is a scaling law rather than an accounting identity. The EIA formulation above is single-phase and assumes unity power factor; the real three-phase form carries the √3 and the cos φ. R is not constant either — it rises with conductor temperature and with skin effect at power frequency.

And two significant loss terms do not scale as (P/V)² at all: corona loss on the line, and no-load loss in the transformers at each end, both effectively fixed once the equipment is energised. A June 2023 ERCOT workshop deck concedes the point in its own wording, noting the advantage holds "since most transmission losses are load losses proportional to the square of current". Most, not all.

Distance enters through a different door. Higher voltage cuts loss per unit length, but what caps the power a line of a given length can carry is loadability. The formal treatment is the St. Clair curve, which MISO describes as setting "the safe loading limit of an AC transmission line based solely on the length of the line in miles and the surge impedance loading calculated for the line".

Its binding criteria are explicit — voltage drop above 5.0%, or loading above 70% of the maximum power transfer limit at roughly 44.5° of angular displacement — with voltage drop dominating short lines and angular displacement long ones. Cables are limited differently again: EIA notes that with long AC cables "the reactive power flow resulting from the large cable capacitance will limit the maximum possible transmission distance".

Why it matters in a real grid-scale project

A battery plant does not pick its interconnection voltage from a catalogue; it takes what the network offers within reach of the site. The Department of Energy's description of the US ladder is the map. Power plants "generally produce electricity at low voltages (5–34.5 kilovolts (kV))"; step-up substations raise that for transmission over long distances at typical voltages of 115, 138, 230, 345, 500 and 765 kV; sub-transmission networks "used to transmit power over shorter distances" run at 34, 46 or 69 kV; and distribution systems are typically rated below 34 kV.

A BESS sits at the generation end of that chain — it produces at low voltage, collects at medium voltage, and steps up once to whatever class the point of interconnection occupies. That class then fixes the study, the equipment ratings and the grid code.

Inside the fence the distances are short and the driver is current rather than loadability, which is precisely why the collection network runs at medium voltage instead of at converter output voltage — the 1/V² relationship is the entire argument for stepping up before the cable run. Three published benchmarks are worth carrying for sanity-checking loss numbers.

EIA estimates US transmission and distribution losses averaged about 5% of the electricity transmitted and distributed over 2018 through 2022. CEER reports European transmission losses between 0.99% and just under 3.96% for 2022. The IEA frames efficient grid losses as around 5% against as much as 18% in some regions. CEER's stated reason is the physics of this entry: higher voltages "result in lower current, which leads to lower technical losses".

Past a certain distance the AC answer runs out, and the crossover is a design decision rather than a rule. ENTSO-E, summarising the literature rather than publishing its own analysis, reports break-even distances "on the order of 500-800 km" for overhead lines and 40–150 km for cables, and warns that "break-even distances are highly project-dependent".

EIA, quoting ABB as "a rough rule of thumb", gives shorter figures — 60 km (37 miles) submarine and 200 km (124 miles) overhead — and separately a loss-based break-even of 300 km (186 miles) for a 1,200 MW overhead line. A break-even exists because HVDC pays a fixed penalty at each end: National Grid gives conversion losses per converter end of 0.7% to 0.8% of transmitted power for current source converters and 1% for voltage source converters.

Line loss against distance for three voltages carrying the same power on the same conductor — the 1/V² relationship drawn out.

Key facts

Loss scaling: At fixed power, current ∝ 1/V and load loss ∝ 1/V². Double the voltage, quarter the loss; quadruple it, one sixteenth
Working formulas: I = P / (√3 × V(L-L) × cos φ); total conductor load loss = 3 × I² × R
US classes (DOE): Generation 5–34.5 kV; transmission 115/138/230/345/500/765 kV; sub-transmission 34/46/69 kV; distribution below 34 kV
UK classes (DESNZ, 2022): Transmission 275/400 kV in England and Wales, 132/275/400 kV in Scotland; distribution 132 kV down to 6.6 kV plus below 1 kV
AEP "reach" at 1,500 MW: 765 kV ≈ 550 miles; 500 kV ≈ 140 miles; 345 kV double circuit ≈ 110 miles; 345 kV single circuit ≈ 50 miles
St. Clair multipliers (MISO): 3.00 × SIL at 50 miles, 1.00 at 300 miles, 0.72 at 450 miles; criteria are 5.0% voltage drop or 70% of maximum power transfer
HVDC break-even: 500–800 km overhead and 40–150 km cable (ENTSO-E, from the literature); 200 km / 60 km as ABB's rule of thumb via EIA; project-dependent
Loss benchmarks: ≈5% US T&D 2018–2022 (EIA); EU transmission 0.99–3.96% in 2022 (CEER); HVDC ≈3.5% per 1,000 km vs 6.7% AC at similar voltage (EIA, citing Siemens 2017)
IEC 60038 and distance: IEC 60038:2009+AMD1:2021 defines voltage bands only; the document contains no distances in kilometres or miles

Typical values and standards

Voltage classes are published per jurisdiction, so name the jurisdiction whenever you quote them. United States, per the Department of Energy: generation 5–34.5 kV; transmission 115, 138, 230, 345, 500 and 765 kV; sub-transmission 34, 46 or 69 kV for shorter distances; distribution typically below 34 kV.

California, per the CPUC's transmission fact sheet: transmission carries electricity "over long distances" at "voltages of over 200 kV", with "220 kV to 500 kV" typical; sub-transmission below 200 kV, typically 66 kV or 115 kV; distribution lines "cover much shorter distances" at 16, 12 or 4 kV. The two US sources do not agree on where transmission begins, which is exactly why the governing document matters more than the remembered number.

United Kingdom, per the DESNZ networks modelling appendix (August 2022): transmission is 275 kV and 400 kV in England and Wales, and 132 kV, 275 kV and 400 kV in Scotland; distribution is 132, 66, 33, 11 and 6.6 kV plus below 1 kV in England and Wales, and 33, 11 and below 1 kV in Scotland. The 132 kV overlap is a real classification difference, not an error — transmission in Scotland, distribution in England and Wales.

Primary distribution is 6.6 kV and above; secondary is below 1 kV. The international standard, IEC 60038:2009+AMD1:2021, defines bands only: clause 4.1 covers 100 V to 1,000 V, 4.2 traction, 4.3 above 1 kV to 35 kV, 4.4 above 35 kV to 230 kV, 4.5 above 230 kV, 4.6 below 120 V AC or 750 V DC. It contains no distances.

Distances that are genuinely attributable come from loadability work rather than from voltage tables. AEP publishes transmission "reach" — the distance over which a stated power can be delivered. For 1,500 MW that is roughly 550 miles at 765 kV, 140 miles at 500 kV, 110 miles on a double-circuit 345 kV line and 50 miles on a single-circuit 345 kV line, with AEP's own qualifier that "for short distances, these relationships can differ to some extent reflecting thermal capacities".

MISO's St. Clair multipliers map the same decay continuously — 3.00 at 50 miles, 2.05 at 100, 1.60 at 150, 1.30 at 200, 1.10 at 250, 1.00 at 300, 0.88 at 350, 0.80 at 400 and 0.72 at 450 — applied to surge impedance loadings of 2,435 MW at 765 kV and 426 MW at single-circuit 345 kV.

How it shows up in specs, studies and contracts

The interconnection agreement fixes the point-of-interconnection voltage, and that one number decides which class you are in, which study the transmission planner runs and which equipment standards the substation is built to. When a planner says the network cannot take your export, get the binding criterion in writing, because there are several and they have different fixes. A thermal limit is about conductor rating. A voltage-drop limit — the St.

Clair criterion of 5.0% — is often addressable with reactive support. An angular-stability limit, the criterion at 70% of maximum power transfer and roughly 44.5° of displacement, is a different problem, because it turns on the electrical length of the line. Asking which limit binds turns an apparently flat refusal into a scoped engineering problem, and tells you whether reactive plant on your side of the fence is worth pricing.

Loss factors appear in the commercial documents as a deduction between the plant meter and the settlement point, and they are worth arguing about because they compound across the life of the offtake. Hold them against about 5% for average US transmission and distribution losses over 2018 through 2022 (EIA) and 0.99% to just under 3.96% for European transmission in 2022 (CEER).

Be careful with distribution comparisons: CEER's 2022 distribution range of 1.95% to 22.63% is driven substantially by non-technical losses — energy delivered but not metered or billed — and by inconsistent national definitions, so the top of that range is not an I²R number and must never be quoted as an engineering benchmark.

Comparison tables in planning and vendor decks need their conditions read before their numbers get quoted. The June 2023 ERCOT workshop deck's 345 kV versus 765 kV table is the standard example: 100 miles, 5,000 MW assumed flow, capacity losses of 81 MW against 46 MW, annual energy losses of 710,374 MWh against 403,628 MWh.

That is a 43% reduction, and it is not a voltage-only result. The two columns use 12 circuits versus 2, conductor resistance of 4.63 Ω versus 2.16 Ω and thermal capacity of 21,504 MVA versus 13,250 MVA — and per-circuit phase current is higher in the 765 kV column, 1,889 A against 697 A, because there are only two circuits. Quote it with those conditions attached or not at all.

Common pitfalls

The rule you will be offered first is some form of "one kilovolt per mile" — that a line's length in miles should not exceed its voltage in kilovolts. As of mid-2026 it could not be traced to any standards body, regulator, transmission system operator, IEEE or CIGRE document; it survives on discussion forums and on commercial blogs.

Keep it out of studies, feasibility notes and board papers. The defensible substitutes are AEP's reach figures and MISO's St. Clair multipliers: both are attributable, both carry stated criteria, and both will give you a different answer from the folklore because they turn on conductor bundling, circuit count and surge impedance loading rather than on voltage alone.

The related pitfall is expecting a clean voltage-to-distance table to exist somewhere. As of mid-2026 no standards body, transmission system operator or regulator publishes one. Standards define voltage bands — IEC 60038, the DOE ladder, the DESNZ levels — while loadability is defined separately, from line length and surge impedance loading.

Only fragments connect the two: DOE's qualitative note that sub-transmission serves shorter distances, and an Asian Development Bank annex on rural distribution in Madhya Pradesh, India, observing that an 11 kV feeder from a 33/11 kV primary substation "may exceed 10 km". Typical low-voltage feeder length limits in metres could not be sourced to a standard or regulator at all.

Two smaller traps close this out. First, aggregate network length is not transmission distance. A circa-2013 National Grid technical note records approximately 7,200 km of overhead line and 1,400 km of underground cable at 275 kV and 400 kV, which is the total length of a meshed national network, not a point-to-point figure, and says nothing about how far power actually travels.

Second, check the vintage of anything you cite from a system operator: that same note describes National Grid as operating the system throughout Great Britain, which stopped being true when NESO took over the GB electricity system operator role on 1 October 2024, though National Grid Electricity Transmission still owns the England and Wales transmission system.

Sources & standards

Common misconception

There is a voltage-to-distance rule of thumb — roughly one kilovolt per mile — so you can read the transmission voltage a project needs straight off the line length.

In reality: As of mid-2026 that rule could not be traced to any standards body, regulator, transmission system operator, IEEE or CIGRE document; it lives on discussion forums and commercial blogs. Voltage is chosen against current and loss, since load loss falls as 1/V² for a fixed power, while distance is limited by loadability — voltage drop on short lines, angular stability on long ones. The attributable numbers are AEP's reach figures (1,500 MW travels about 550 miles at 765 kV but only about 50 miles on a single-circuit 345 kV line) and MISO's St. Clair multipliers, falling from 3.00 × surge impedance loading at 50 miles to 0.72 at 450 miles. Both turn on conductor bundling and circuit count, not on voltage alone.

Visuals & further reading

MV, LV, HV Glossary
Point of interconnection Glossary
BESS Single-Line Diagram: Reading the One-Line Article

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