What Decides Cable Size?

A cable is never sized on current alone. The final size is the largest size that satisfies every one of these factors:

• Design (load) current — how much current the circuit will actually carry
• Protective device rating — the breaker or fuse protecting the circuit
• Derating factors — ambient temperature, grouping, and installation method
• Current-carrying capacity (ampacity) — the rated current of the chosen cable
• Voltage drop — must stay within the allowed limit over the run length
• Conductor material and insulation — copper vs aluminium, PVC vs XLPE
• Short-circuit / earth-fault withstand — for protection coordination

The Cable Sizing Formulas

1. Design (Load) Current

Single phase: I = P ÷ (V × pf)

Three phase: I = P ÷ (√3 × V_L × pf)

Where P is power in watts, V is the phase voltage (e.g. 230 V) or V_L the line voltage (e.g. 400/415 V), and pf is the power factor. For motors, also divide by the efficiency (η).

2. Voltage Drop

Three phase: Vd = √3 × I × L × R ÷ 1000

Single phase: Vd = 2 × I × L × R ÷ 1000

Where L is the length in metres and R is the cable resistance in Ω/km. As a percentage: %Vd = (Vd ÷ V) × 100.

3. Minimum Area from Voltage Drop

If voltage drop governs, find the minimum conductor area directly:

A = (√3 × ρ × L × I) ÷ Vd_allowed (three phase)

Where ρ ≈ 0.0225 Ω·mm²/m for copper at operating temperature (use ≈ 0.036 for aluminium).

The Golden Design Rule

Every cable must satisfy:

Ib ≤ In ≤ Iz

That is: the design current (Ib) ≤ the protective device rating (In) ≤ the cable’s effective capacity (Iz) after derating.

How to Calculate Cable Size: Step-by-Step

1. Calculate the design current (Ib) from the load using the formulas above.
2. Select the protective device (In) so that In ≥ Ib (e.g. the next standard breaker rating).
3. Apply derating factors — ambient temperature (Ca), grouping (Cg), and installation method. The required tabulated rating is It ≥ In ÷ (Ca × Cg × …).
4. Select the cable size from the manufacturer’s or standard ampacity tables so its rated current meets the requirement.
5. Check the voltage drop over the actual run length; if it exceeds the limit, increase the size.
6. Take the largest size from the ampacity check and the voltage-drop check — that’s your final cable.

Worked Example

Let’s size the cable for a 30 kW, three-phase, 415 V load with a power factor of 0.85, a run length of 50 m, installed at 40 °C ambient and grouped with other circuits. The voltage-drop limit is 3 %.

Step 1 — Design current:
I = 30000 ÷ (√3 × 415 × 0.85) = 30000 ÷ 611 ≈ 49 A. So Ib ≈ 49 A.

Step 2 — Protective device:
Choose In = 50 A (next standard rating), satisfying Ib ≤ In (49 ≤ 50).

Step 3 — Derating:
For a 70 °C PVC cable at 40 °C, Ca ≈ 0.87; grouped with others, Cg ≈ 0.80. Combined factor = 0.87 × 0.80 = 0.696.
Required tabulated rating It ≥ 50 ÷ 0.696 ≈ 72 A.

Step 4 — Select cable:
A 16 mm² copper cable has a tabulated rating of roughly 76 A, which exceeds 72 A. So its derated capacity Iz ≈ 76 × 0.696 ≈ 53 A, comfortably above both In (50 A) and Ib (49 A).

Step 5 — Voltage drop check (16 mm² Cu):
Using R ≈ 1.38 Ω/km at operating temperature:
Vd = √3 × 49 × 50 × 1.38 ÷ 1000 ≈ 5.9 V → %Vd = 5.9 ÷ 415 × 100 ≈ 1.4 %, well within the 3 % limit.

Step 6 — Final size: Both checks pass, so the cable is 16 mm² copper, protected by a 50 A device.

Note that on the voltage-drop check alone, a smaller 10 mm² cable would have been enough — but ampacity governs here, which is why we always take the larger result. You can verify any of these numbers instantly with the free MEPBase Electrical Cable Size Calculator.

Typical Voltage Drop Limits

Limits depend on your local code, but common values are:

• Lighting circuits: around 3 %
• Power circuits: around 5 %
• Total (supply origin to final point): often capped at 4–5 %

Always confirm the exact figure required by the standard you design to (IEC, BS 7671, NEC, or local regulations).

Copper vs Aluminium

Copper has lower resistivity, higher ampacity for the same size, and is easier to terminate — ideal for smaller circuits and tight spaces. Aluminium is lighter and cheaper, so it’s common on large feeders and long runs, but it needs about one or two sizes larger than copper for the same current and requires proper terminations. Choose based on cost, run length, and space.

Common Cable Sizing Mistakes

• Forgetting derating factors — a cable in a hot, crowded conduit carries far less than its tabulated rating.
• Ignoring voltage drop on long runs — ampacity may pass while voltage drop fails.
• Sizing on the breaker only without checking the actual load current.
• Mixing units — keep length, resistivity, and current consistent.
• Overlooking protective device coordination (Ib ≤ In ≤ Iz, and I2 ≤ 1.45 × Iz).

Frequently Asked Questions

How do I calculate cable size from load?
Find the design current from the load, choose a protective device of equal or higher rating, apply derating factors, select a cable whose rated current meets the requirement, and finally confirm the voltage drop is within limits.

What is the formula for three-phase cable current?
I = P ÷ (√3 × V_L × pf), where P is power in watts, V_L is the line voltage, and pf is the power factor.

What voltage drop is acceptable?
Commonly around 3 % for lighting and 5 % for power, but always follow your local code.

Should I use copper or aluminium cable?
Copper for smaller circuits and tight spaces; aluminium for large feeders and long runs where weight and cost matter. Aluminium needs a larger size for the same current.

Is there an online cable size calculator?
Yes — the MEPBase Electrical Cable Size Calculator does the full calculation (current, derating, and voltage drop) in your browser.

Conclusion

Cable sizing always follows the same logic: find the load current, protect it, derate the cable, check the ampacity, and confirm the voltage drop — then take the largest size. Keep the rule Ib ≤ In ≤ Iz in mind and you’ll size cables safely every time.

Want a fast, accurate result without the manual maths? Try the free MEPBase Electrical Cable Size Calculator.

This guide is for general educational purposes. Always design to your applicable electrical code (IEC, BS 7671, NEC, or local regulations) and have designs verified by a qualified engineer.

This guide explains the most common psychrometric processes used in HVAC design, with the practical formulas engineers use daily and where each method is applied.

Key psychrometric properties (quick reference)

Before jumping into processes, here are the key properties you will see again and again:

• Dry Bulb Temperature (DBT): normal air temperature measured by a standard thermometer.
• Wet Bulb Temperature (WBT): temperature measured with an evaporating wet wick; useful for evaporative cooling and moisture evaluation.
• Dew Point Temperature (DPT): temperature at which condensation starts (air becomes saturated).
• Relative Humidity (RH): how “full” the air is with moisture relative to saturation.
• Humidity Ratio (ω): mass of water vapor per mass of dry air (moisture content).
• Enthalpy (h): total heat content of moist air (sensible + latent).

Common HVAC relationship (Imperial): Sensible heat is often estimated using:

• Q_s = 1.08 × CFM × ΔT

Latent heat (moisture change) estimate (Imperial):

• Q_L = 0.68 × CFM × Δω

Note: The constants 1.08 and 0.68 are commonly used in Imperial HVAC calculations and are based on standard air conditions. For high accuracy (different altitude/air density), use psychrometric software or corrected air properties.

Sensible heating (temperature increases, moisture stays the same)

Process: Heat is added to air without adding moisture. This means the air gets warmer but the moisture content does not change.

Typical formula (Imperial):

• Q_s = 1.08 × CFM × ΔT

What changes during sensible heating:

• DBT increases
• Humidity ratio (ω) stays constant
• RH decreases (warm air can hold more moisture, so RH drops)
• Enthalpy (h) increases

Where it’s used:

• Winter space heating
• Preheat coils (before humidification)
• Reheat coils (after dehumidification to control supply air temperature)

Sensible cooling (temperature decreases, moisture stays the same)

Process: Heat is removed from air without removing moisture. Air becomes cooler, but moisture content remains unchanged.

Typical formula (Imperial):

• Q_s = 1.08 × CFM × ΔT

What changes during sensible cooling:

• DBT decreases
• Humidity ratio (ω) stays constant
• RH increases (cooler air holds less moisture, so RH rises)
• Enthalpy (h) decreases

Where it’s used:

• Sensible cooling coils (when coil surface temperature is above dew point)
• Night cooling strategies
• Pre-cooling in dry climates (when dehumidification is not required)

Cooling and dehumidification (remove heat and remove moisture)

Process: Air is cooled below its dew point temperature so water vapor condenses out. This is the most common process in air-conditioning because it reduces both temperature and humidity.

Total cooling:

• Q_total = Q_sensible + Q_latent

Component formulas (Imperial):

• Q_s = 1.08 × CFM × ΔT
• Q_L = 0.68 × CFM × Δω

Sensible Heat Ratio (SHR): SHR tells how much of the total cooling is sensible (temperature change) versus latent (moisture removal).

• SHR = Q_s / Q_total

Typical SHR range: ~0.70 to 0.85 (depends on building and climate).

What changes during cooling & dehumidification:

• DBT decreases
• Humidity ratio (ω) decreases
• RH may increase toward saturation near the coil (then controlled by leaving conditions)
• Enthalpy (h) decreases significantly (sensible + latent removed)

Where it’s used:

• AHU / FCU cooling coils
• Fresh air treatment units
• Summer air-conditioning and humidity control

Engineer tip: If the coil leaving air is too cold, systems often add reheat to control supply temperature while still removing humidity.

Engineer shortcut: To calculate SHR, enthalpy change, and moisture removal accurately for cooling coils, use the MEPBase Psychrometric Calculator instead of manual chart plotting.

Humidification (add moisture to air)

Process: Moisture is added to air, either with steam (adds heat) or with adiabatic methods such as water spray (evaporative, often cools air slightly).

Steam humidification

Process: Adds moisture and typically increases temperature slightly.

Moisture addition (rule-of-thumb form):

• ṁ_w = (CFM × Δω) / 60

Adiabatic humidification (spray/fog)

Process: Water evaporates into air. DBT can drop slightly, moisture increases, and WBT is often close to constant.

Where it’s used:

• Winter conditioning in dry climates
• Clean rooms and laboratories (humidity control)
• Museums and hospitals (comfort + preservation)

Mixing of air streams (fresh air + return air)

Process: Two air streams mix (example: outdoor air and return air). The mixed condition is a mass-weighted average of the incoming streams.

Mixed air temperature (concept)

• T_mix = (ṁ₁T₁ + ṁ₂T₂) / (ṁ₁ + ṁ₂)

Mixed air humidity ratio

• ω_mix = (ṁ₁ω₁ + ṁ₂ω₂) / (ṁ₁ + ṁ₂)

Where it’s used:

• AHU mixing box (return + outdoor air)
• Economizer mode (free cooling)
• VAV systems and fresh air compliance

Engineer tip: Always check mixed-air condition before coil selection. In hot climates, outdoor air can dominate the coil load.

Evaporative cooling (cooling by evaporation)

Process: Water evaporates into air. This reduces DBT while increasing humidity ratio. Wet-bulb temperature is approximately constant (especially for direct evaporative cooling).

Temperature drop estimate:

• ΔT = ε × (DBT − WBT)

Typical effectiveness (ε): ~0.7 to 0.9 (varies by equipment type).

Types:

• Direct evaporative cooling: typically 70%–80%
• Indirect evaporative cooling: typically 50%–70%
• Two-stage systems: often 90%–95%

Best for:

• Hot, dry climates
• Low RH conditions (for better temperature drop)

Dehumidification (remove moisture from air)

Process: Moisture is removed from air either by cooling below dew point (condensation) or using desiccants (chemical absorption/adsorption).

Cooling-based dehumidification

Air is cooled below dew point so water condenses and drains away. This is the most common method in comfort AC.

Desiccant dehumidification

Moisture is removed using a desiccant wheel or chemical media. Often used when very low humidity is required.

Moisture removal rate (Imperial style):

• ṁ_w = 0.68 × CFM × Δω

Where it’s used:

• Swimming pools (high latent loads)
• Ice rinks
• Supermarkets and cold rooms (humidity control)

Quick checks HVAC engineers should remember

• If DBT changes but ω is constant, it’s a sensible-only process (heating or cooling).
• If ω decreases, moisture is being removed (dehumidification / cooling below dew point).
• If ω increases, moisture is being added (humidification / evaporation).
• High SHR means mostly sensible load; low SHR means high latent load (humidity problem).

Frequently asked questions (FAQ)

What is the difference between sensible and latent heat?

Sensible heat changes temperature (DBT), while latent heat changes moisture content (humidity ratio ω).

Why does RH drop during heating?

Because warm air can hold more moisture. If moisture content stays the same (ω constant) and temperature increases, RH decreases.

What does SHR tell you in HVAC?

SHR shows how much of total cooling is sensible. It helps you understand whether your load is mostly temperature-driven or moisture-driven.

What Is HVAC Load Calculation?

HVAC load calculation is the process of estimating the total amount of heat that must be removed or added to a building to maintain indoor design conditions. It forms the basis for sizing HVAC systems for air conditioning and heating equipment.

A complete building heat load calculation includes:

• Sensible heat load (temperature change)
• Latent heat load (moisture removal)
• Ventilation and infiltration air loads
• Transmission losses during heating

External Cooling Loads

External loads are heat gains that enter the building from outdoors through walls, roofs, windows, and air leakage.

1. Wall and Roof Heat Transfer (Conduction Load)

Q = U × A × CLTDcorr

Where:

• U = Overall heat transfer coefficient
• A = Surface area
• CLTD_corr = Corrected Cooling Load Temperature Difference

The CLTD/CLF method accounts for solar exposure, wall orientation, roof color, latitude, and time of year.

2. Window Heat Gain – Conduction

Q = U × A × (To − Ti)

Window U-values are taken from NFRC tables. High-performance glazing significantly reduces HVAC cooling load.

3. Window Solar Heat Gain

Q = A × SHGC × SC × SCL × CLF

Solar heat gain through windows is often the largest contributor to cooling load in commercial buildings.

Infiltration Heat Gain

Infiltration occurs due to uncontrolled outdoor air entering the building. It adds both sensible and latent heat loads.

Sensible Load:

Qs = CFM × 1.08 × ΔT

Latent Load:

Ql = CFM × 0.68 × ΔW

CFM can be calculated using crack method or air changes per hour (ACH).

Internal Cooling Loads

Occupant (People) Heat Load

Occupants generate both sensible and latent heat.

Qs = N × qs × CLF

Ql = N × ql

Typical ASHRAE values:

• Seated occupants: 245 sensible, 205 latent BTU/hr
• Office work: 250 sensible, 200 latent BTU/hr
• Light activity: 250 sensible, 250 latent BTU/hr

Lighting Load Calculation

Q = W × 3.41 × Fu × Fsa × CLF

Lighting load depends on fixture type. LED lighting produces lower heat gain compared to fluorescent lighting.

Equipment and Appliance Load

Q = Power × Fu × Fr × CLF

Motor heat gain:

Q = HP × 2545

Manufacturer data should always be used where available.

Ventilation Load Calculation (ASHRAE 62.1)

Ventilation load is calculated based on required outdoor air as per ASHRAE Standard 62.1.

Voz = Rp × Pz + Ra × Az

Sensible Ventilation Load:

Qs = 1.08 × CFM × (To − Ti)

Latent Ventilation Load:

Ql = 0.68 × CFM × (Wo − Wi)

Duct Heat Gain and Safety Factor

Duct heat gain or loss must be considered when ducts pass through unconditioned spaces.

Q = Uduct × Aduct × ΔT

A HVAC safety factor of 10–20% is added to account for uncertainties, future equipment, and distribution losses.

Heating Load Calculation

Transmission Heat Loss

Q = U × A × (Ti − To)

Infiltration Heat Loss

Q = CFM × 1.08 × (Ti − To)

Basement and Slab Heat Loss

Q = F × P × (Ti − To)

Additional pickup load of 10–40% is applied for morning warm-up and building thermal mass.

Total HVAC Load Summary

Qtotal = Qsensible + Qlatent

Cooling Capacity (TR) = Qtotal / 12,000

Diversity factors are applied since all loads do not peak simultaneously.

HVAC Load Calculation Standards and Methods

• RTS Method (Radiant Time Series)
• CLTD / CLF Method
• Transfer Function Method (TFM)
• Manual J (Residential Load Calculation)

Related HVAC Tools on MEPBase

• Duct Sizer Pro
• Chilled Water Pipe Calculator
• AC Condensate Drain Calculator
• HVAC Unit Converter

Conclusion

A proper HVAC load calculation is essential for efficient HVAC design. By correctly estimating cooling and heating loads using ASHRAE methods, engineers can select optimal equipment capacity, reduce energy consumption, and ensure long-term system reliability.

Introduction – What Is AC Condensate Drain Calculation?

Air conditioning systems remove moisture from indoor air as part of the cooling process. This moisture becomes condensate water, which must be drained safely. Correctly calculating the condensate water production and sizing the drain line ensures proper operation and prevents water damage or clogs.

In this post you’ll find:

• A free Excel condensate drain calculator
• A link to an online calculator tool
• Condensate calculation formulas
• Pipe sizing guidelines

Online AC Condensate Drain Calculator (Free Tool)

For fast condensate drain sizing based on system capacity, you can use the online calculator here:

👉 AC Condensate Drain Calculator – Drain Pipe Sizing Tool:
🔗 https://tools.mepbase.com/ac-condensate-drain-calculator

This tool helps you determine the recommended drain pipe diameter for split units, AHUs or FCUs based on cooling capacity.

Free Excel Condensate Drain Calculation Sheet

You can also perform your own condensate calculations using a downloadable Excel sheet. This spreadsheet lets you:

• Calculate condensate production based on airflow and humidity differences
• Estimate annual condensate volume
• Size condensate lines and plan drain routes

Condensate Drain Calculation Formula

To estimate the amount of condensate produced in an AC system, use this common HVAC formula:

Condensate flow (GPM) = (M_air × ΔW_air) / (V_air × 8.33)

Where:

• M_air = Airflow rate in CFM
• ΔW_air = Change in specific humidity
• V_air = Specific air volume
• 8.33 = Conversion to gallons per pound of water

This gives you gallons per minute of condensate, useful for sizing drains and traps.

How to Size a Condensate Drain Pipe

A condensate line must be sized properly to prevent pooling and provide smooth gravity drainage. The online tool linked above helps with pipe recommendations based on cooling capacity:

• Up to ~5 TR → 3/4″ pipe
• 5–10 TR → 1″ pipe
• 10–20 TR → 1-1/4″ pipe
• 20–40 TR → 1-1/2″ pipe
• 40–80 TR → 2″ pipe

Note: These sizes are general engineering suggestions; always check local codes and conditions.

Best Practices for AC Condensate Drains

• Slope the drain line at least 1/8″ per foot for proper flow
• Use traps and cleanouts where appropriate
• Insulate horizontal lines in humid environments
• Avoid routing drains across finished ceilings without protection

AC Condensate Drain Calculator Excel

Conclusion

Proper calculation of air conditioning condensate drain requirements is essential for reliable HVAC performance and avoiding moisture problems. Use the online calculator tool linked above for quick pipe sizing, or the Excel sheet for more detailed analysis. Both tools support better system design and planning for MEP professionals.

Whether you’re working on residential, commercial, or industrial projects, this tool supports better planning, lower power losses, and enhanced safety.

What This Tool Does

• Calculate cable size based on load (kW or Ampere)
• Supports single-phase or three-phase systems
• Accounts for cable length and voltage drop
• Outputs standard copper/aluminum cable sizes
• Includes safety margin for derating and protection

File Details

Why Use This Cable Size Calculator?

Cable oversizing wastes money. Undersizing risks fire or voltage loss. This calculator provides a balanced and accurate result based on real design inputs.

• Helps in distribution panel design
• Useful for load scheduling and feeder sizing
• Ideal for project submittals and quick reference

Best For:

• Electrical Draftsmen
• Site Engineers
• Panel Builders
• Junior Electrical Designers
• Students preparing load sheets

Related Downloads

Residential Electrical Load Calculator (Excel)

Download Cable Sizing Tool

Click below to download the electrical wire sizing calculator in Excel format.

Electrical Cable Size Calculator

Final Words

Correct cable sizing is a foundational part of any safe and efficient electrical design. This calculator saves time, reduces human error, and follows standard formulas trusted by professionals worldwide.

Stay updated with MEPBase.com to access more free MEP calculators, templates, and technical drawings.