DC Current Circuit Breaker Sizing: NEC 690.8 Calculations

Introduction

Selecting the correct DC current circuit breaker amperage rating is a precise engineering calculation—too small and nuisance tripping disrupts operations, too large and wires overheat before protection activates. Unlike voltage ratings where oversizing provides safety margin, current ratings must match the specific load and wire capacity within tight tolerances.

This sizing-focused guide provides electrical designers and system engineers with comprehensive methodology for DC current circuit breaker selection. We cover NEC Article 690 solar calculations, continuous load derating factors, wire ampacity verification, load type considerations, and the critical distinctions between overload and short-circuit protection requirements.

For professionals designing solar PV systems, battery energy storage, DC microgrids, or industrial DC distribution, proper current rating selection ensures safe, code-compliant installations that protect equipment without false trips.

💡 Sizing Priority: The DC current circuit breaker protects the WIRE, not the load. Wire ampacity (after temperature derating) determines the maximum allowable breaker rating—never exceed this limit regardless of load requirements.

NEC Article 690.8 Solar PV Overcurrent Protection

The 1.56 Multiplier Explained

NEC 690.8(A)(1) requires solar PV string overcurrent devices rated:

I_ocpd ≥ I_sc × 1.56

This 1.56 factor represents two sequential 125% multipliers:

First 125% – High Irradiance Condition:
– Solar irradiance can exceed Standard Test Conditions (STC: 1000 W/m²)
– Edge-of-cloud effects, ground reflection, and snow reflection increase irradiance to 1250 W/m²
– Module I_sc increases proportionally: I_sc_actual = I_sc_STC × 1.25

Second 125% – Continuous Operation Derating:
– NEC 210.20(A) requires continuous loads (>3 hours) derated to 80% of breaker rating
– Inverting: breaker must be rated 125% of continuous load
– I_ocpd = I_load / 0.80 = I_load × 1.25

Combined Effect:
1.25 × 1.25 = 1.5625 ≈ 1.56

Step-by-Step Solar String Calculation

Example System:
– Module: 400W, I_sc = 11.24A (from datasheet)
– String configuration: 20 modules in series

Step 1 – Module I_sc Verification:
Always use datasheet I_sc value, not calculated from power rating.

Step 2 – Apply NEC 690.8 Multiplier:
I_ocpd_min = 11.24A × 1.56 = 17.53A

Step 3 – Select Standard Rating:
Standard DC breaker ratings: 10A, 16A, 20A, 25A, 32A…
Selected: 20A (next size above 17.53A)

Step 4 – Verify Wire Ampacity (Critical):

Wire Size	Ampacity at 30°C	Derated at 60°C	20A Breaker OK?
14 AWG	20A	11.6A	❌ NO
12 AWG	25A	14.5A	❌ NO
10 AWG	30A	17.4A	❌ NO
8 AWG	40A	23.2A	✅ YES

Temperature correction factor at 60°C: 0.58 (from NEC Table 310.15(B)(2)(a))

Critical Finding: 10 AWG insufficient! Must upsize to 8 AWG wire to support 20A breaker.

⚠️ Common Mistake: Selecting breaker based only on NEC 690.8 calculation without verifying wire ampacity. This violates NEC 240.4(D) and creates fire hazard—breaker allows current that overheats wire.

Array-Level Main dc current circuit breaker Sizing

For combiner output feeding inverter:

Formule:
I_main = (N_strings × I_sc × 1.25) ÷ 0.80

The 0.80 divisor ensures breaker operates in its optimal range (80% loading).

Example – 8 String System:
– Strings: 8 parallel
– I_sc per string: 11.24A
– Calculation: (8 × 11.24A × 1.25) ÷ 0.80 = 140.5A
– Selected: 160A DC breaker

Verification Against Inverter:
– Inverter max DC input: 150A (from manual)
– 160A breaker protects inverter input ✓
– If inverter limit was 120A, use 125A breaker instead

Temperature Derating Factors and Wire Ampacity

NEC Table 310.15(B)(2)(a) Correction Factors

Wire ampacity decreases at elevated temperatures:

I_derated = I_ampacity_30C × Correction_Factor

Common Correction Factors:

Ambient Temp	Correction Factor	Application
30°C	1.00	Reference temperature
40°C	0.91	Indoor conditioned spaces
50°C	0.82	Attics, indoor unconditioned
60°C	0.58	Roof-mounted conduit (common)
70°C	0.41	Direct sun exposure, desert
80°C	0.29	Extreme conditions

Real-World Temperature Estimation

Rooftop Conduit Temperature:
T_conduit = T_ambient + T_solar + T_wire

Où ?
– T_ambient = outdoor air temperature
– T_solar = solar heating (20-30°C for black conduit in sun)
– T_wire = I²R heating (5-15°C depending on current)

Example – Phoenix Summer:
– Ambient: 45°C
– Solar heating: 25°C (black metal conduit)
– Wire heating: 10°C
– Total: 80°C

Ampacity Impact:
– 10 AWG at 30°C: 30A
– 10 AWG at 80°C: 30A × 0.29 = 8.7A

A 10 AWG wire loses 71% of its ampacity in extreme heat!

Conduit Fill Adjustment

NEC Table 310.15(B)(3)(a) requires derating when >3 current-carrying conductors in conduit:

Nombre de conducteurs	Facteur d'ajustement
1-3	1,00 (pas d'ajustement)
4-6	0.80
7-9	0.70
10-20	0.50
21-30	0.45
31-40	0.40

Combined Derating:
I_final = I_ampacity × f_temp × f_fill

Example – 6 Conductors at 60°C:
– 10 AWG ampacity: 30A
– Temperature (60°C): 0.58
– Conduit fill (6 cond): 0.80
– Final: 30A × 0.58 × 0.80 = 13.9A

🎯 Design Practice: For rooftop solar installations, assume 60°C ambient minimum. For desert climates or black conduit, use 70°C. Always verify actual installation conditions during site survey.

Continuous vs Peak Load Considerations

Load Duration Categories

Continuous Loads (NEC Definition):
– Operate for 3 hours or longer
– Examples: Solar PV generation, battery charging, HVDC transmission
– Requirement: Breaker rated ≥ 125% of load current

Non-Continuous Loads:
– Operate <3 hours – Examples: Motor starting, short-term testing, intermittent equipment – Requirement: Breaker rated ≥ 100% of load current

Solar PV as Continuous Load

Solar generation during midday operates continuously for 5-8 hours:

Sizing Requirement:
I_breaker ≥ I_load_continuous × 1.25

This is already included in NEC 690.8’s 1.56 multiplier (1.56 = 1.25 × 1.25).

Common Confusion:
❌ Some designers apply 1.25× to NEC 690.8 result:
– I_sc = 10A
– NEC 690.8: 10A × 1.56 = 15.6A
– Incorrect: 15.6A × 1.25 = 19.5A (double-counting continuous factor)
– Correct: 15.6A → select 16A or 20A breaker

Peak vs RMS Current for Pulsating Loads

DC Microgrid with Inverter Loads:

Inverters draw pulsating DC current with high crest factor:
– Average (RMS) current: 50A
– Peak current: 100A (2:1 crest factor)

Breaker Sizing:
– Thermal trip responds to RMS heating: Size for RMS current
– Magnetic trip responds to peak: Ensure peak doesn’t cause nuisance trips

Selection:
– I_RMS = 50A → Select 63A breaker (continuous: 50A × 1.25 = 62.5A)
– Verify magnetic trip: 63A C-curve trips at 315-630A
– Peak 100A well below magnetic threshold ✓

Load Type Matching and Trip Curve Selection

Resistive vs Inductive vs Capacitive Loads

Load Type Impact on Breaker Selection:

Type de charge	Characteristics	Inrush Current	Recommended Trip Curve
Resistive (Heaters, LED lighting)	Steady current No inrush	1.0-1.2× I_rated	B-Curve (3-5× In)
Solar PV (Photovoltaic arrays)	Current-limited by module physics	1.0-1.15× I_sc	C-Curve (5-10× In)
Battery (Li-ion, Lead-acid)	Surge during charge/discharge	2-3× I_rated	C or D-Curve
Inductive (Motors, transformers)	High starting actuel	5-10× I_rated	D-Curve (10-20× In)
Capacitive (DC link capacitors)	Massive inrush during charge	10-50× I_rated (brief)	D-Curve + Inrush limiting

Motor Load Sizing Example

DC Motor Specifications:
– Rated power: 5 kW
– Rated voltage: 250V DC
– Rated current: 22A
– Starting current: 6× rated = 132A
– Starting duration: 3 seconds

Breaker Selection Process:

Step 1 – Continuous Rating:
I_breaker ≥ 22A × 1.25 = 27.5A
Select: 32A breaker

Step 2 – Trip Curve Check:
– 32A D-curve: Magnetic trip at 320-640A
– Starting current 132A well below magnetic threshold ✓
– If C-curve used: 32A × 10 = 320A maximum magnetic trip
– Starting 132A might nuisance trip—D-curve better

Step 3 – Thermal Verification:
– 132A for 3 seconds won’t trip thermal element
– Thermal trip typically requires 1.45× In for 60 minutes
– 132A / 32A = 4.1× for only 3s—safe

Final Selection: 32A D-Curve DC Breaker

Capacitive Inrush Mitigation

Problème:
DC bus capacitors (common in inverters, VFDs) can draw 1000-5000A inrush for 1-10ms when energized.

Solutions:

Option 1 – Pre-Charge Resistor:

Main Breaker --[Pre-charge Resistor]--[Bypass Contactor]-- Capacitor
                                           (closes after
                                            capacitor charged)

Limits inrush to 10-50A, then bypassed for normal operation.

Option 2 – Soft-Start Circuit:
Electronic circuit gradually increases voltage to capacitor over 100-500ms.

Option 3 – Oversized D-Curve Breaker:
Size breaker for 2× continuous current, D-curve tolerates 20× inrush.
– Continuous: 50A → Select 100A D-curve
– Magnetic trip: 1000-2000A
– Inrush: 500A (10×) won’t trip

Trade-off: Oversizing reduces protection quality—wire must support larger breaker.

Common Sizing Errors and Corrections

❌ Error #1: Ignoring Temperature Derating

Scénario:
– Designer selects 20A breaker per NEC 690.8 calculation ✓
– Specifies 12 AWG wire (25A at 30°C) ✓
– Installs in rooftop conduit (60°C actual)

Problème:
– 12 AWG at 60°C: 25A × 0.58 = 14.5A
– 20A breaker exceeds wire capacity by 38%

Correction:
– Upsize wire to 10 AWG: 30A × 0.58 = 17.4A (still insufficient!)
– Upsize to 8 AWG: 40A × 0.58 = 23.2A ✓

Lesson: Always apply temperature correction BEFORE comparing to breaker rating.

❌ Error #2: Using Motor Formula for Solar PV

Scénario:
– Designer familiar with motor circuits
– Applies NEC 430 motor formula: 125% of FLA
– For 10A solar string: 10A × 1.25 = 12.5A → Selects 16A breaker

Problème:
– Solar requires NEC 690.8: 10A × 1.56 = 15.6A → Need 16A minimum
– 16A breaker marginal (exactly at minimum)

Correction:
– Use NEC 690.8 formula specifically for solar
– Result: Select 20A breaker for proper margin

Lesson: Different NEC articles apply different sizing rules—verify correct article.

❌ Error #3: Oversizing for “Safety Margin”

Scénario:
– NEC calculation: 17.5A required
– Standard sizes: 16A, 20A, 25A
– Designer selects 25A “to be safe”

Problème:
– 10 AWG wire (specified): 17.4A at 60°C
– 25A breaker allows wire to carry 25A before trip
– Wire overheats at 17.4A → fire hazard

Correction:
– 20A breaker maximum for 10 AWG at 60°C
– If 25A desired, upsize wire to 8 AWG minimum

Lesson: “Safety margin” in breaker sizing means ensuring wire supports breaker, not oversizing arbitrarily.

❌ Error #4: Single dc current circuit breaker for Multiple Load Types

Scénario:
– Circuit feeds both continuous (30A solar) and motor (20A, 100A starting)
– Designer sizes: (30 + 20) × 1.25 = 62.5A → 63A breaker

Problème:
– 100A motor inrush may trip 63A C-curve breaker (magnetic at 315-630A)
– Marginal—likely nuisance trips during motor starts

Correction:
Option 1: Use D-curve breaker (magnetic at 630-1260A)
Option 2: Separate circuits:
– Solar: 40A C-curve (30A × 1.25 rounded up)
– Motor: 25A D-curve (20A × 1.25)

Lesson: Mixing load types in single circuit requires careful trip curve selection.

❌ Error #5: Failing to Account for Parallel Strings

Scénario:
– Array: 4 strings, I_sc = 10A each
– Designer sizes each string breaker: 10A × 1.56 = 16A ✓
– Main breaker: Also 16A ❌

Problème:
– Combined current: 4 × 10A = 40A
– Main breaker should be: (4 × 10A × 1.25) / 0.8 = 62.5A → 63A

Correction:
– String breakers: 16A each (correct)
– Main breaker: 63A or 80A

Lesson: String-level and array-level breakers have different calculation formulas.

Advanced Sizing Scenarios

Scenario 1: Mixed String Currents in Combiner

Système:
– 3 strings: I_sc = 11A each
– 2 strings: I_sc = 9A each (different module type)

Individual String Breakers:
– 11A strings: 11A × 1.56 = 17.2A → 20A breakers
– 9A strings: 9A × 1.56 = 14.0A → 16A breakers

Main Breaker:
Total: (3 × 11A + 2 × 9A) × 1.25 / 0.8 = 82.8A → 100A breaker

Wire Sizing:
– 20A string wiring: 8 AWG minimum (23.2A derated)
– 16A string wiring: 10 AWG acceptable (17.4A derated) if voltage drop OK
– Main bus: 2 AWG (115A derated at 60°C)

Scenario 2: Battery System with Charge/Discharge Asymmetry

Battery Bank:
– Voltage: 48V nominal
– Max discharge: 200A continuous, 400A peak (10s)
– Max charge: 100A continuous

Breaker Selection:

Discharge Protection:
– Continuous: 200A × 1.25 = 250A
– Peak 400A acceptable for 10s
- Sélectionner : 250A or 315A C-curve
– C-curve magnetic: 1250-2500A (400A peak won’t trip)

Charge Protection (if separate):
– Continuous: 100A × 1.25 = 125A
- Sélectionner : 125A C-curve

Bi-Directional Circuit (common in ESS):
– Use higher rating: 250A or 315A
– Must handle both charge and discharge
– Verify inverter/charger doesn’t produce transients >magnetic trip

Scenario 3: DC Microgrid Bus Protection

Bus Configuration:
– Multiple sources: 50kW solar, 30kW battery, 20kW genset
– Multiple loads: 40kW HVAC, 30kW manufacturing, 20kW lighting
– Bus voltage: 400V DC

Bus Current Calculation:
Max source: 50kW + 30kW + 20kW = 100kW
I_bus = 100,000W / 400V = 250A

Main Bus Breaker:
I_breaker = 250A × 1.25 = 312.5A → 400A breaker

Selectivity Consideration:
– Source breakers: 100-200A range
– Load breakers: 50-100A range
– Main bus breaker: 400A
– Ensure coordination: smaller breakers trip before main

Frequently Asked Questions (Sizing Focus)

Why does NEC 690.8 use 1.56 multiplier instead of standard 1.25?

NEC 690.8 accounts for two distinct conditions: (1) Solar irradiance can exceed STC by 25% due to edge-of-cloud effects and reflected radiation, increasing module I_sc proportionally; (2) Solar generation is continuous (>3 hours), requiring 125% derating per NEC 210.20(A). These multiply: 1.25 × 1.25 = 1.5625 ≈ 1.56. This is NOT double-counting—first factor is environmental (actual current increase), second is electrical code requirement (breaker thermal management). Using only 1.25× undersizes protection by 25%.

Can I use a smaller dc current circuit breaker if my wire is oversized?

No—NEC 690.8 establishes minimum dc current circuit breaker rating based on solar array I_sc, regardless of wire size. Undersizing breaker below I_sc × 1.56 means breaker may trip during normal high-irradiance conditions (midday summer with cloud enhancement). Oversized wire allows voltage drop reduction and future expansion but doesn’t permit smaller breaker. Example: I_sc = 10A requires 16A minimum breaker even if you install 6 AWG wire (65A capacity). The breaker must protect against array maximum output, not wire capacity.

How do I size dc current circuit breaker for battery systems with surge currents?

Batteries can surge 2-5× continuous rating during discharge/charge transitions. Size breaker for continuous rating (I_cont × 1.25), then verify trip curve tolerates surge: C-curve magnetic trip at 5-10× In handles most battery transients. Example: 100A continuous, 250A surge (10s): select 125A C-curve (magnetic 625-1250A). If surges cause nuisance trips, options: (1) D-curve breaker, (2) electronic breaker with programmable I²t characteristics, (3) separate surge path with contactor. Never simply oversize breaker—this reduces wire protection.

What if calculated dc current circuit breaker size falls between standard ratings?

Always round UP to next standard rating. If calculation gives 17.5A and standards are 16A/20A, select 20A. Then verify wire ampacity supports 20A after derating—if wire insufficient, upsize wire (don’t downsize breaker). Example: 17.5A calculated, 10 AWG wire (17.4A derated) insufficient for 20A breaker. Options: (1) Upsize to 8 AWG (23.2A derated) with 20A breaker, (2) Use 16A breaker ONLY if 16A ≥ I_min from code calculation. Never interpolate or use non-standard ratings.

How does altitude affect current rating selection?

Altitude primarily affects voltage ratings (dielectric strength decreases), not current ratings. Current rating relates to thermal management (I²R heating), which is minimally affected by altitude below 2000m. Above 2000m, reduced air density slightly decreases convective cooling, but NEC doesn’t require current derating for altitude. Some manufacturers specify 1-3% current derating per 1000m above 2000m, but this is conservative. Voltage derating (10% per 1000m above 2000m) is far more critical. Focus altitude corrections on voltage specification, not amperage.

Can one dc current circuit breaker protect multiple loads with different current requirements?

Yes, but size breaker for sum of loads: I_breaker ≥ ΣI_loads × 1.25 (if all continuous). Each load must have wire sized for breaker rating (not individual load). Example: 20A and 30A loads on common circuit → Total 50A × 1.25 = 62.5A breaker. Both wires must handle 63A (next standard) after derating. Problem: 30A load could use smaller wire if separately protected. Solution often is separate circuits: more protection optimization, easier troubleshooting, better load management. Shared circuit makes economic sense only when loads operate simultaneously and wire routing is identical.

How do I account for future expansion when sizing breakers?

Calculate current requirements for maximum planned configuration, not just initial installation. Example: 4 strings now, space for 8 total. Options: (1) Size main breaker for 8 strings now, install as planned capacity; (2) Size for 4 strings, document upgrade procedure requiring main breaker replacement when expanding. Option 1 costs more initially but avoids future modification. Ensure wire also sized for planned capacity—undersized wire requires conduit replacement (expensive). For string-level breakers, install only what’s needed now (easy to add more). Balance: known expansion plans (oversize), speculative expansion (size for current, document upgrade path).

Conclusion

DC current circuit breaker sizing demands precise calculation integrating code requirements, wire ampacity after environmental derating, load characteristics, and trip curve matching. Unlike voltage selection where conservative oversizing provides safety margin, current ratings must precisely balance protection against nuisance tripping—too small causes operational disruption, too large allows wire overheating before breaker activation.

Critical Sizing Principles:

NEC Compliance: Solar PV applications must apply 1.56 multiplier (NEC 690.8) accounting for high irradiance and continuous operation. Array-level breakers use (N × I_sc × 1.25) / 0.8 formula. Battery and motor applications follow respective NEC articles (480, 430). Never apply incorrect calculation method—each load type has specific requirements.

Déclassement de la température: Wire ampacity at 30°C must be corrected for actual installation temperature (NEC Table 310.15(B)(2)(a)). Rooftop conduit commonly reaches 60-70°C, reducing ampacity 42-58%. Breaker rating must never exceed derated wire ampacity—this is non-negotiable fire safety requirement.

Load Matching: Trip curve selection must accommodate load inrush characteristics. Resistive loads use B-curve, solar PV uses C-curve standard, motors require D-curve for starting current tolerance. Capacitive loads need pre-charge circuits or specialized protection—oversized breakers alone don’t solve inrush issues.

Wire Protection Priority: The breaker exists to protect conductors from thermal damage. All calculations must verify breaker rating ≤ wire ampacity after all derating factors. When conflicts arise between code minimum breaker size and wire capacity, upsize wire—never compromise wire protection.

For electrical designers and system engineers, mastering current rating selection ensures installations that protect personnel and equipment while maintaining operational reliability. The systematic methodology presented here—from code-compliant calculation through temperature correction to load-type matching—provides the foundation for professional DC protection system design.

Related Sizing Resources:
– DC Circuit Breaker Selection – Comprehensive breaker specifications
– DC Voltage Rating Guide – Voltage specification methodology
– Solar System Design – Complete PV protection design

Engineering Consultation: SYNODE provides current rating analysis and load study services for complex DC systems. Contact our applications engineering team for multi-source coordination studies, custom trip curve selection, or NEC compliance verification for commercial installations.

Dernière mise à jour : Octobre 2025
Auteur : SYNODE Applications Engineering Team
Examen technique : Licensed Professional Engineers, NABCEP-Certified Specialists
Références du code : NEC Article 690:2023, NEC Article 240:2023, NEC Article 310:2023