vault backup: 2025-10-03 15:08:33

conductor-sizing.md

@@ -3,10 +3,56 @@ id:
 aliases: []
 tags:
   - destiny/fleeting
+  - status/incomplete
   - topic/electrical
   - type/encyclopedia
 ---
-# Feeder Sizing
+# Conductor Sizing
+
+## "The 80% Rule"
+
+"The 80% Rule" is a rule of thumb
+referring to a common convention of several articles
+including:
+
+- [[nfpa-70_article-210_branch-circuits#210.19(A)(1) General.]]
+- [[nfpa-70_article-215_feeders#215.2(A)(1) General.]]
+
+which paraphrased states:
+
+> ... the minimum conductor size shall have an ampacity
+> not less than the noncontinuous load
+> plus 125 percent of the continuous load
+
+When the rule is repeated,
+the noncontinuous load is ignored
+and it is stated that conductors are suitable
+for 80% their listed rating,
+since 80% is the reciprocal of 125%.
+
+## Branch Circuits
+
+### Receptacle Branch
+
+> [!important]
+> There is no maximum number of receptacles per circuit _in any occupancy_.
+
+It is a common misconception that the limit
+can be calculated with a formula like
+
+$$
+\frac{1.25(180VA)}{120V} = 1.875A, \quad \frac{20A}{1.875A} = 10.\bar{6}
+$$
+
+but the 180VA per yoke load specified in
+[[nfpa-70_article-220_load-calculations#220.14(I) Receptacle Outlets.|220.14(I)]]
+is specifically for calculating service and feeder sizing.
+
+Per [[nfpa-70_article-210_branch-circuits#210.19(A)(1) General.|210.19(A)(1)]]
+a receptacle branch circuit's load
+is the load of the equipment intended to be served by it.
+
+## Feeders
 
 > [!cite] 250.122 Size of Equipment Grounding Conductors
 > **(A) General.** Copper, aluminum, or copper-clad aluminum
@@ -37,15 +83,49 @@ Given a minimum ampacity, find all valid configurations.
 
 Rank by total cost of install.
 
-## Complexity to Ignore
+### Complexity to Ignore
 
-### Conductor Material
+#### Conductor Material
 
 Tinned copper and copper-clad aluminum conductors
 can be assumed out of scope.
 
-## Complexity to Respect
+### Complexity to Respect
+
+#### Equipment Grounding Conductor Material
 
 Wire and EGC conductors are usually assumed to match,
-but it is sometimes preferable to use a copper EGC
-with high-amp aluminum parallel feeds.
+but it is sometimes necessary to use a copper EGC with aluminum wires,
+either for spec requirements or conduit fill considerations.
+
+## Voltage Drop
+
+$$
+V_d = \frac{ I \times R \times L \times M }{ P }
+$$
+
+where
+- $V_d$ = Voltage Drop in volts ($V$)
+- $I$ = Current in Amperes ($A$)
+- $R$ = Linear resistance in ohms per foot ($\Omega\text{ft}^{-1}$)
+- $L$ = Length of wire one way in feet ($\text{ft}$)
+- $M$ = Multiplier
+  - $2$ for 1-phase
+  - $\sqrt{3}$ for 3-phase
+- $P$ = Number of parallel runs
+
+It is often more useful to know the maximum length
+a certain wiring configuration is suitable for.
+
+$$
+L = \frac{ V_d }{ I \times M } \times \frac{ P }{ R }
+$$
+
+- $L$ = Max length of wire one way in feet ($\text{ft}$)
+- $\frac{ V_d }{ I \times M }$ = Max circuit resistance in ohms ($\Omega$)
+- $\frac{ P }{ R }$ = ??? in feet per ohm ($\text{ft}\Omega^{-1}$)
+
+> [!info] Ohm's Law
+> $$
+> V = I \times R, \quad R = \frac{ V }{ I }, \quad I = \frac{ V }{ R }
+> $$