132 lines
3.7 KiB
Markdown
132 lines
3.7 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/electrical
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- type/encyclopedia
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---
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# Conductor Sizing
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## "The 80% Rule"
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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_article-210_branch-circuits#210.19(A)(1) General.]]
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- [[nfpa-70_article-215_feeders#215.2(A)(1) 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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## Branch Circuits
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### Receptacle Branch
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> [!important]
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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_article-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_article-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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## Feeders
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> [!cite] 250.122 Size of Equipment Grounding Conductors
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> **(A) General.** Copper, aluminum, or copper-clad aluminum
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> equipment grounding conductors of the wire type
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> shall not be smaller than shown in Table 250.122,
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> but in no case shall they be required to be larger
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> than the circuit conductors supplying the equipment...
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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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> [!cite] 310.10(H) Conductors in Parallel.
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> **(1) General.** 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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## Voltage Drop
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$$
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V_d = \frac{ I \times R \times L \times M }{ P }
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$$
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where
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- $V_d$ = Voltage Drop in volts ($V$)
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- $I$ = Current in Amperes ($A$)
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- $R$ = Linear resistance in ohms per foot ($\Omega\text{ft}^{-1}$)
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- $L$ = Length of wire one way in feet ($\text{ft}$)
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- $M$ = Multiplier
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- $2$ for 1-phase
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- $\sqrt{3}$ for 3-phase
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- $P$ = Number of parallel runs
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It is often more useful to know the maximum length
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a certain wiring configuration is suitable for.
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$$
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L = \frac{ V_d }{ I \times M } \times \frac{ P }{ R }
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$$
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- $L$ = Max length of wire one way in feet ($\text{ft}$)
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- $\frac{ V_d }{ I \times M }$ = Max circuit resistance in ohms ($\Omega$)
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- $\frac{ P }{ R }$ = ??? in feet per ohm ($\text{ft}\Omega^{-1}$)
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> [!info] Ohm's Law
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> $$
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> V = I \times R, \quad R = \frac{ V }{ I }, \quad I = \frac{ V }{ R }
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> $$
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