Mike Dolt Enterprises
Mike Dolt Enterprises
User: Alwayslearningelec, Location: NJ
Asked on: Thursday, July 3, 2025 at 8:45 am EDT
Answered on: Thursday, July 3, 2025 at 8:47 am EDT
Question: Trying to figuring voltage drop some are more that 400' away. Not sure what I would use for current at end of run?If I had (4) outdoor receptacles feed from same circuit and 1st 400' away and others 50' apart further would I use 180 va per receptacle then divide by voltage?
You're on the right track, but let's break down the voltage drop calculation step-by-step for your outdoor receptacles. It's a bit more nuanced than just assuming 180VA per receptacle at the end of the run.
Understanding the Problem Voltage Drop
Electricity loses voltage as it travels through a wire due to resistance. Excessive voltage drop can cause equipment to malfunction, motors to overheat, lights to dim, and generally reduce the efficiency and lifespan of the connected devices.
Long Runs
The longer the wire run, the greater the voltage drop. That's why your 400' run is a concern.
Multiple Receptacles
You have several receptacles along the run. We need to consider how the current is distributed among them.
Steps to Calculate Voltage Drop1. Determine the Load (VA or Amps) per Receptacle
NEC Minimum
The NEC (National Electrical Code) requires you to assume a minimum load for general-use receptacles. For outdoor receptacles, it's generally 180 VA per receptacle (as you mentioned). This is a minimum.
Actual Load
If you know what equipment will be plugged into the receptacles, use the actual VA (Volt-Amps) or Amps rating of those devices. This is far more accurate and will prevent undersizing the wiring. Look at the nameplate of the intended appliance/tool.
Example
Using the NEC minimum: 180 VA per receptacle. Using an actual load: If you know someone will use a 1200W leaf blower, that's 1200VA. A 500W string trimmer is 500VA.2.
Convert VA to Amps
If you have VA, convert to Amps using the formula:
Assume a voltage of 120V for standard receptacles. Amps (I) = VA / Voltage (V)
Example
Using the NEC minimum: 180 VA / 120V = 1.5 Amps Using a leaf blower: 1200 VA / 120V = 10 Amps Using a string trimmer: 500 VA / 120V = 4.17 Amps3.
Calculate the Current at Each Point
This is crucial because the current flowing through each segment of the wire is different.
The Key
Work backwards from the last receptacle to the source.
Receptacle 4
Assume receptacle 4 is at the end of the run (400' + 50' + 50' + 50' = 550'). The current flowing after receptacle 4 is likely zero (or very minimal if you have other loads further downstream). The current flowing before receptacle 4 is the current drawn by receptacle 4. Let's use the 1.5A value for this step
Receptacle 3
The current flowing before receptacle 3 is the current drawn by receptacle 3, plus the current drawn by receptacle 4 (1.5A + 1.5A = 3A)
Receptacle 2
The current flowing before receptacle 2 is the current drawn by receptacle 2, plus the current drawn by receptacle 3, plus the current drawn by receptacle 4 (1.5A + 1.5A + 1.5A = 4.5A)
Receptacle 1
The current flowing before receptacle 1 is the current drawn by receptacle 1, plus the current drawn by receptacle 2, plus the current drawn by receptacle 3, plus the current drawn by receptacle 4 (1.5A + 1.5A + 1.5A + 1.5A = 6A)4.
Choose a Wire Size
You need to select a wire gauge before you can accurately calculate voltage drop. Start with a guess, and adjust if needed. Factors:
Ampacity
The wire must be rated to carry the total current (the current flowing before receptacle 1) safely. Consult the NEC ampacity tables (Table 310.16) for appropriate wire sizes based on the insulation type (e.g., THHN, THWN) and installation method (e.g., in conduit, buried).
Voltage Drop Considerations
Even if the wire is large enough for ampacity, it might not be large enough to keep voltage drop within acceptable limits. This is why we're doing this calculation.5.
Calculate Voltage Drop
Formula
The voltage drop formula depends on whether you're using single-phase or three-phase power. Since you are using 120V receptacles, this would be single phase. There are variations of this formula, but a common one is:
Where: `VD` = Voltage Drop (in volts) `2` = Accounts for the two directions of the circuit (out and back) `K` = Direct-current constant. The NEC allows for using a K of 12.9 ohms for copper conductors `I` = Current (in Amps) for that specific segment of the wire (see Step 3) `L` = Length of the wire run (in feet) for that specific segment. `CM` = Circular Mils of the wire (from a wire size chart – see below) VD = (2 K I L) / CM
Circular Mils
Circular mils (CM) is a unit of area used to describe the cross-sectional size of a wire. You need to look up the CM for the wire gauge you've chosen. Here are some common sizes: 14 AWG: 4,107 CM 12 AWG: 6,530 CM 10 AWG: 10,380 CM 8 AWG: 16,510 CM
Apply the Formula to Each Segment
You must calculate the voltage drop for each segment of the wire separately.
Segment 1
From the breaker to Receptacle 1 (400 feet, current = 6A in our example).
Segment 2
From Receptacle 1 to Receptacle 2 (50 feet, current = 4.5A in our example).
Segment 3
From Receptacle 2 to Receptacle 3 (50 feet, current = 3A in our example).
Segment 4
From Receptacle 3 to Receptacle 4 (50 feet, current = 1.5A in our example).
Total Voltage Drop
Add up the voltage drop calculated for each segment to get the total voltage drop for the entire circuit.6.
Determine Percentage Voltage Drop% Voltage Drop = (Total Voltage Drop / Source Voltage) 100
Example
If your total voltage drop is 6 volts, and your source voltage is 120V:
7. % Voltage Drop = (6 / 120) 100 = 5%
Is the Voltage Drop Acceptable? NEC Recommendation
The NEC recommends limiting voltage drop to 3% for branch circuits and 5% overall (branch circuit + feeder). This is a recommendation, not a hard requirement, but it's a good guideline.
Your Equipment
Consider the voltage requirements of the equipment you'll be using. Some devices are more sensitive to voltage drop than others.
If the Voltage Drop is Too High
You have a few options:
Increase Wire Size
A larger wire gauge has lower resistance and will reduce voltage drop.
Reduce the Load
If possible, use less power on the circuit (but this might not be practical).
Reduce the Run Length
Shortening the wire run is always beneficial. Consider adding a subpanel closer to the loads.
Increase the Voltage
Although not practical for existing 120V receptacles, increasing the voltage would decrease the current, reducing voltage drop.
Example Calculation (Simplified)
Let's do a quick example using 12 AWG copper wire and the NEC minimum load (1.5A per receptacle) to see the impact of wire size on the last receptacle: Segment 1
(400 ft, 6A, 12 AWG) VD = (2 12.9 6 400) / 6530 = 9.47 Volts
Segment 2
(50 ft, 4.5A, 12 AWG) VD = (2 12.9 4.5 50) / 6530 = 0.89 Volts
Segment 3
(50 ft, 3A, 12 AWG) VD = (2 12.9 3 50) / 6530 = 0.59 Volts
Segment 4
(50 ft, 1.5A, 12 AWG) VD = (2 12.9 1.5 50) / 6530 = 0.29 Volts
Total Voltage Drop
9.47 + 0.89 + 0.59 + 0.29 = 11.24 Volts
Percentage Voltage Drop
(11.24 / 120) 100 = 9.37%In this simplified example, using 12 AWG wire results in a 9.37% voltage drop, exceeding the recommended 3% for a branch circuit. You would need to increase the wire size (e.g., to 10 AWG or 8 AWG) to reduce the voltage drop to an acceptable level.
Key Considerations and Recommendations Safety First
If you are not comfortable working with electricity, consult a qualified electrician. Voltage drop calculations are important for safety and proper equipment operation.
NEC Compliance
Always comply with the latest edition of the National Electrical Code (NEC) and local codes.
Overestimate Loads
It's better to overestimate the potential load on the receptacles than to underestimate it. This will give you a safety margin.
Use a Voltage Drop Calculator
There are many online voltage drop calculators that can simplify the calculations. Search for "voltage drop calculator" on the internet. However, understand the principles behind the calculations before relying solely on a calculator.
Consider Future Needs: Think about any potential future uses for the receptacles and factor that into your load calculations.By carefully calculating the voltage drop and choosing the correct wire size, you can ensure that your outdoor receptacles operate safely and efficiently. Good luck! Flag for review