Editing Three-phase electric power (section)

== Single-phase loads ==
Except in a [[high-leg delta]] system and a corner-grounded delta system, single-phase loads may be connected across any two phases, or a load can be connected from phase to neutral.<ref name="IAEI" /> Distributing single-phase loads among the phases of a three-phase system balances the load and makes most economical use of conductors and transformers.

In a symmetrical three-phase four-wire wye system, the three phase conductors have the same voltage to the system neutral. The voltage between line conductors is [[square root of 3|{{sqrt|3}}]] times the phase conductor to neutral voltage:<ref>The boy electrician by J. W. Sims M.I.E.E. (p. 98).</ref>
:<math>V_\text{LL} = \sqrt{3} V_\text{LN}.</math>

The currents returning from the customers' premises to the supply transformer all share the neutral wire. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently.

If the supply neutral is broken, phase-to-neutral voltage is no longer maintained. Phases with higher relative loading will experience reduced voltage, and phases with lower relative loading will experience elevated voltage, up to the phase-to-phase voltage.

A [[high-leg delta]] provides phase-to-neutral relationship of {{math|1=''V''<sub>LL</sub> = 2 ''V''<sub>LN</sub> }}, however, LN load is imposed on one phase.<ref name=Fowler>{{cite book|last=Fowler|first=Nick|title=Electrician's Calculations Manual |edition=2nd|year=2011|publisher=McGraw-Hill|isbn=978-0-07-177017-0|pages=3–5|url=https://books.google.com/books?id=9QBWHJdPGM0C}}</ref> A transformer manufacturer's page suggests that LN loading not exceed 5% of transformer capacity.<ref>{{Cite web|url=http://www.federalpacific.com/university/transbasics/chapter3.html|archive-url=https://web.archive.org/web/20120530122049/http://www.federalpacific.com/university/transbasics/chapter3.html|url-status=dead|title=Federal pacific|archive-date=May 30, 2012}}</ref>

Since {{sqrt|3}} ≈ 1.73, defining {{math|''V''<sub>LN</sub>}} as 100% gives {{math|''V''<sub>LL</sub>}} {{math|1=≈ 100% × 1.73 = 173%}}. If {{math|''V''<sub>LL</sub>}} was set as 100%, then {{math|''V''<sub>LN</sub> ≈ 57.7%}}.

=== Unbalanced loads ===
When the currents on the three live wires of a three-phase system are not equal or are not at an exact 120° phase angle, the power loss is greater than for a perfectly balanced system. The method of [[symmetrical components]] is used to analyze unbalanced systems.

=== Non-linear loads ===
With linear loads, the neutral only carries the current due to imbalance between the phases. [[Gas-discharge lamp]]s and devices that utilize rectifier-capacitor front-end such as [[switch-mode power supplies]], computers, office equipment and such produce [[Harmonics (electrical power)#Third-order harmonics|third-order harmonics]] that are in-phase on all the supply phases. Consequently, such harmonic currents add in the neutral in a wye system (or in the grounded (zigzag) transformer in a delta system), which can cause the neutral current to exceed the phase current.<ref name="IAEI">{{cite web|last=Lowenstein|first=Michael|title=The 3rd Harmonic Blocking Filter: A Well Established Approach to Harmonic Current Mitigation|url=http://www.iaei.org/blogpost/890108/159506/The-3rd-Harmonic-Blocking-Filter-A-Well-Established-Approach-to-Harmonic-Current-Mitigation|publisher=IAEI Magazine|access-date=24 November 2012|url-status=dead|archive-url=https://archive.today/20130908001044/http://www.iaei.org/blogpost/890108/159506/The-3rd-Harmonic-Blocking-Filter-A-Well-Established-Approach-to-Harmonic-Current-Mitigation|archive-date=8 September 2013}}</ref><ref>{{cite web|last=Enjeti|first=Prasad|title=Harmonics in Low Voltage Three-Phase Four-Wire Electric Distribution Systems and Filtering Solutions|url=http://www.pserc.wisc.edu/documents/general_information/presentations/pserc_seminars/pserc_seminars0/enjeti_slides.pdf|publisher=Texas A&M University Power Electronics and Power Quality Laboratory|access-date=24 November 2012|archive-date=13 June 2010|archive-url=https://web.archive.org/web/20100613051835/http://www.pserc.wisc.edu/documents/general_information/presentations/pserc_seminars/pserc_seminars0/enjeti_slides.pdf|url-status=dead}}</ref>