CABLE SIZING AND OVERCURRENT PROTECTION
The procedure presented below for cable sizing and overcurrent protection of that
cable is based on NEC requirements in Sections 690.9, 690.8, 110.14(C),
210.20(A), 215.2, 215.3, 220.10, 240.3(B), and 240.6(A). ( Let’s call this method
“Ampacity forward”)
1.Circuit Current.
For circuits carrying currents from PV modules,( PV source circuit) multiply the
short-circuit current by 125% and use this value for all further calculations. For PV circuits in the following examples, this is called the CONTINUOUS CURRENT calculation. In the Code, this requirement has been included in Section 690.8,( but
also emphasized in UL 1703 This “up-front” multiplier should not be applied twice). For dc and ac inverter circuits in PV systems, use the rated continuous currents. AC
and dc load circuits should follow the requirements of Sections 210, 220, and 215.
2. Overcurrent Device Rating. The overcurrent device must be rated at
125% of the current determined in Step 1. This is to prevent overcurrent
devices from being operated at more than 80% of rating. This calculation,
in the following examples, is called the 80% OPERATION rule. (This second use of 1.25 effectively results in PV source circuits x 1.56 for the OCPD)
3. Cable Sizing. Cables shall have a 30°C ( i.e. table basis) ampacity of 125% of the current determined in Step 1 (continuous current) to ensure proper operation of connected overcurrent devices. (There are no additional deratings applied yet-this is just applying the 80% rule to the cable and finding a wire size and insulation type in the relevant NEC tables..
4. Cable Derating. Based on the determination of Step 3 and the location of
the cable (raceway or free-air), a cable size and insulation temperature
rating (60, 75, or 90°C) are selected from the NEC Ampacity Tables
310.16 or 310.17. Use the 75°C cable ampacities to get the size, then use
the ampacity from the 90°C column—if needed—for the temperature deratings. The cable is then derated for temperature, conduit fill, and other requirements.
*compare step 4 (derated ampacity) result to step 1 ( calculation before derate)
The resulting derated ampacity must be greater than the value found in Step 1.
If not greater, then a larger cable size or higher insulation temperature must be selected. (The current in Step 3 is not used at this point to preclude over sizing the cables.) Again, if “derated ampacity” ends up less than the original continuous current calculation, use a larger diameter wire. ( then redo deratings calculations to check)
(Note: The following check in Step 5 may result in an even larger conductor size than just determined in Step 4..you’re not done yet).
5. Ampacity vs. Overcurrent Device.
( check the revised step 4 to original step 2 calculation)
The derated ampacity of the cable selected in Step 4, must be equal to or greater than the overcurrent device rating determined in Step 2 [240.4]. ( i.e. fuses should blow before wires burn) If the derated ampacity of the cable is less than the rating of the overcurrent device, then a larger cable must be selected. The next larger standard size overcurrent device may be used if the derated cable ampacity falls between the standard overcurrent device sizes found in NEC Section 240.6
6. Device Terminal Compatibility. Since most overcurrent devices have
terminals rated for use with 75°C (or 60°C) cables, compatibility must be
verified [110.3(B)]. If a 90°C-insulated cable was selected in the above
process, the 30°C ampacity of the same size cable with a 75°C (or 60°C)
insulation must be greater than or equal to the current found in Step 1
[110.14(C)]. This ensures that the cable will operate at temperatures below
the temperature rating of the terminals of the overcurrent device. If the
overcurrent device is located in an area with ambient temperature higher
than 30°C, then the 75°C (or 60°C) ampacity must also be derated
[110.3(B)].
RECAP ( treat as if there are more Amps than labeled)
-PV source circuit is “continuous current”, and is calculated from
PV module Isc x 1.25
- rated ampacity of conductors and OCPD reflects the “80%” rule
by use of an additional factor of 1.25 ( total calculation=Isc x 1.56);
(note that multipling by 1.25 gives same answer as dividing by .8)
-Using the calculated ampacity required, shop the wire table for possible conductor size and insulation type.
-recoginize that “conditions of use” result in derating this ampacity goal,
And check that resulting values still measure-up to original continuous current
Needs. Keep increasing wire size until it works.
----BUT WAIT, THERE’S MORE…
-Once ampacity seems okay, then check “voltage drop” based on chosen wire’s “round trip” length and resistance. ( E=IR; convert value to % of Vmp and check
Use larger wire if % is greater than 2 %)
