178 lines
4.7 KiB
Markdown
178 lines
4.7 KiB
Markdown
---
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id:
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aliases: []
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tags:
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- destiny/fleeting
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- status/incomplete
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- topic/construction/electrical
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- type/encyclopedia
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- authorship/original
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title: Conductor Sizing
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dg-publish: true
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---
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# Conductor Sizing
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It is tempting to size circuit conductors
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based on overcurrent protection,
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but they are sized independently.
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Conductors are sized to be suitable for the load,
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overcurrent protection is sized to protect the conductors.
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Allowable conductor ampacity restrictions
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are not based on a wire's ability to carry current,
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they are intended to protect its insulation
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from damage due to excessive heating.
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## "The 80% Rule"
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> [!danger]
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> This description is provided _for reference only_.
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>
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> Like every NEC "rule"
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> that isn't preceded by a section reference,
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> it is _not_ code.
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>
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> It is my opinion that this one should never be repeated,
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> even as shorthand.
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"The 80% Rule" is a rule of thumb
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referring to a common convention of several articles
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including:
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* [[nfpa-70_210_branch-circuits#210.19(A)(1) General.]]
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* [[nfpa-70_215_feeders#215.2(A)(1) General.]]
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* [[nfpa-70_215_feeders#215.3 Overcurrent Protection.]]
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* [[nfpa-70_430_motors#430.22 Single Motor.]]
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* [[nfpa-70_430_motors#430.51 General.]]
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which paraphrased states:
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> ... the minimum conductor size shall have an ampacity
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> not less than the noncontinuous load
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> plus 125 percent of the continuous load
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When the rule is repeated,
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the noncontinuous load is ignored
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and it is stated that conductors are suitable
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for 80% their listed rating,
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since 80% is the reciprocal of 125%.
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The rule neglects important context and common exceptions,
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namely transformers, whose feeder conductors are sized at 100%[^1].
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[^1]: [[nfpa-70_215_feeders#215.2(B)(1) Feeders Supplying Transformers.]]
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## Branch Circuits
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### Receptacle Branch
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There is no maximum number of receptacles per circuit _in any occupancy_.
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It is a common misconception that the limit
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can be calculated with a formula like
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$$
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\frac{1.25(180VA)}{120V} = 1.875A, \quad \frac{20A}{1.875A} = 10.\bar{6}
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$$
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but the 180VA per yoke load specified in
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[[nfpa-70_220_load-calculations#220.14(I) Receptacle Outlets.|220.14(I)]]
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is specifically for calculating service and feeder sizing.
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Per [[nfpa-70_210_branch-circuits#210.19(A)(1) General.|210.19(A)(1)]]
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a receptacle branch circuit's load
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is the load of the equipment intended to be served by it.
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Where general-use receptacles are provided
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without specific equipment in mind,
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circuits will be engineered at the minimum load.
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If a receptacle circuit's load is a whole multiple of 180VA
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there's a good chance that's the number of devices,
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or at least was at some point in the design.
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## Feeders
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![[nfpa-70_250_grounding#250.122(A) General.]]
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Apparently in the 2026 NEC First Draft Meetings,
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Code Making Panel 5 clarified that the equipment grounding conductor (EGC)
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never needs to be larger than the largest ungrounded conductor in any raceway
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when installed in parallel.
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I can not find a source to verify this.
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Statements from other reputable sources including Mike Holt
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are in contradiction to this idea.
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***
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Given a minimum ampacity, find all valid configurations.
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[[nfpa-70_310_conductors_for_general_wiring]]
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> [!cite] NEC Article 310 (emphasis added)
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> #### 310.10(H) Conductors in Parallel.
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> ##### (1) General.
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> Aluminum, copper-clad aluminum, or copper conductors,
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> for each phase, polarity, neutral, or grounded circuit
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> shall be permitted to be connected in parallel
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> (electrically joined at both ends)
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> _only in sizes 1/0 AWG and larger_
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> where installed in accordance with 310.10(H)(2) through (H)(6).
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Rank by total cost of install.
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### Complexity to Ignore
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#### Conductor Material
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Tinned copper and copper-clad aluminum conductors
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can be assumed out of scope.
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### Complexity to Respect
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#### Equipment Grounding Conductor Material
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Wire and EGC conductors are usually assumed to match,
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but it is sometimes necessary to use a copper EGC with aluminum wires,
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either for spec requirements or conduit fill considerations.
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## Dwelling Unit Services
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![[nfpa-70_310_conductors_for_general_wiring#Table 310.12 Single-Phase Dwelling Services and Feeders|Table 310.12]]
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## Voltage Drop
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[[voltage-drop]]
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## "Bundling"
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[[nfpa-70_310_conductors_for_general_wiring#Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors]]
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It is permissible but uncommon to run more than three c
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## Transformers
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$$
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I = \frac{S}{ \sqrt{3} \times V \times E }
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$$
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* $I$ = nameplate current rating
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* $S$ = nameplate kVA rating
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* $V$ = feeder voltage
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* $E$ = efficiency (95--99%)
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## Motors
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$$
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1~\text{electric horsepower} \equiv 746~\text{watts}
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$$
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full-load current (FLC) / full-load amperes (FLA)
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minimum circuit ampacity (MCA)
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$$
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\text{MCA} = 1.25 \times \text{FLC}
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$$
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