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WireSizer User Guide

Thank you for using WireSizer, the most intuitive DC wire size and voltage drop calculator in the App Store!


WireSizer is a general reference tool that cannot anticipate all conditions. Juggernaut Technology, Inc. makes no representation or warranty that this application will be suitable to address your particular situation or requirements.

The wire gauges suggested by this app assume you are using good quality wire, appropriately select derating factors presented, have good, clean connections as needed, and are not operating in an "extreme" environment.

Do not use for AC wiring. This app is for DC only.

How to Use

You may specify the circuit voltage (DC only), current (in amps), and total circuit length (round trip, from the electrical source, to the load, and back) by "flipping" through the options in the top spinner. In the bottom spinner you can see the gauge of wire needed for different percentages of voltage drop. Wire gauge recommendations are for commonly available SAE, AWG, and ISO/Metric wire sizes (see tables below)

This application's output is suitable for most DC applications on boats, RVs, automobiles, radios, solar, wind, remote DC pumps/motors, and other uses that do not exceed 50V. However, there are variables that it does not take into account, such as extreme temperature variation, or hostile environments. The wire gauge calculated is for copper wire. Whether it is tinned, stranded, or solid usually does not matter, although your particular application may call for a specific construction of wire.

In most cases, a 1% to 3% voltage drop is acceptable for critical uses, and up to 10% may be acceptable for non-critical uses, such as area lighting. Rarely are drops above 10% considered acceptable, but they are included for informational purposes.

You may get additional information on most items in WireSizer by simply tapping the field on the main screen.

Materials List

So you don't have to find a pencil and paper to remember your wire selections, you can add them to the built in "Materials List" by tapping the "+" button on the main screen, or from the material list itself. You can review and edit your material list by tapping the notebook button on the main screen.


Settings are available by tapping the Settings icon in the upper left corner of the main screen.

Circuit Length: You may set the complete circuit length you specify to be in meters or feet. If you select meters, the list will be in 1/2 meter increments. WireSizer will attempt to preserve the equivalent selected length when this setting is changed.

Wire Gauges: You may specify your results in any combination of AWG, SAE, or ISO/Metric wire sizes. Due to space constraints, you are limited to 2 options on iPhones and similarly sized devices.

Show Fractional Amps: If selected, WireSizer will list amperages in 1/10 increments from 0 to 2 amps, in 1/2 increments from 2 to 10 amps, and in whole amps from there on up. If not selected only whole amps will be listed. Fractional amps can be useful for calculating wire sizes for low load circuits such as LED lighting.

Show Watts: If selected, WireSizer will display the equivalent watts value alongside the amperage value. This makes no difference in the wire gauges calculated, but can be a useful shortcut so you don't have to convert between amps and watts yourself.

Ampacity Limitations

Ampacity is a term used for a wire gauge's safe current carrying capacity. In some circumstances, calculating the voltage drop alone will result in a recommendation of a wire gauge that if used might exceed its safety factors. Ampacity limitations may come into play with shorter wire lengths, especially if the wire is derated by running through an engine compartment, sheathed, or has lower temperature insulation. A wire's current carrying capacity goes down as the temperature rises. WireSizer will suggest the larger of the calculated voltage drop or the wire with adequate ampacity. If a wire size that has been calculated by voltage drop percentage exceeds the wire's safe current carrying capacity, WireSizer will automatically increase the wire size to a safe size, and will indicate it has done so on the iPad version of this app as "Ampacity Limited", or marked with an "*" on other versions. If a wire is indicated as such, you do not need to do anything, it has already been done for you.

What does the Wire Insulation setting do?

Higher rated wire insulation means higher current carrying capacity of the wire. Wire or its packaging should be labeled with its temperature insulation rating. Some wire may have multiple insulation ratings depending on how it is used, such as "dry", "wet" or "oil". Please select whichever is appropriate for your use.

Wire Insulation Ratings
Rating Comments
60°CNot recommended for Engine Spaces.
75°CCommon derating for 105°C wire in Wet environments.
105°CCommon "Marine Grade" wire.
200°CNo derating applies in Engine Spaces.

What does the In sheath, bundle, or conduit? switch do?

If a wire is included in a bundle, is sheathed, in conduit, or otherwise tightly contained (limiting free air flow), its current carrying capacity is limited. For systems less than 50V DC there are no additional limiting factors for size of the bundle.

What does the Inside engine compartment switch do?

Normally, WireSizer calculates wire gauges for a 30°C (86°F) ambient environment - which works fine for many uses. However, temperatures inside engine spaces are usually warmer than those outside. The current carrying capacity of the wires decrease as the temperature rises. For conditions inside engine spaces, it is assumed that the ambient temperature is 50°C (122°F). You may use this switch for calculating other operating environments that have similar temperature requirements. At 50°C, the ampacity of copper wire is approximately 15% less than at 30°C. You will see more circuits being ampacity limited if this switch is set to Yes, but many wire size recommendations will remain unchanged.

Technical Notes

For you technical minded folks out there, the formula used by this app to calculate voltage drop is:

VD = [K x L x I] / CM
VD=Voltage Drop allowed
K=mil foot resistance of the conductor
L=Length of circuit (in feet, roundtrip)
I=Current (in Amps)
CM=Circular Mil area of Conductors

For K, WireSizer uses 10.75 Ohms, which is the the approximate mil-foot resistance of copper at 30 degrees Celsius or 86 degrees Fahrenheit.

The maximum amps that this app will suggest for a given wire gauge (tables show rated 105°C insulation, WireSizer will calculate others as appropriate) and temperature (inside or outside the engine compartment), and the circular mils used for calculations are as follows:

Gauge 30°C 50°C CM Area

Gauge 30°C 50°C CM Area

ISO / Metric / mm2
Gauge 30°C 50°C CM Area

All AWG values (for all insulation ratings) used by WireSizer are from ABYC E11, SAE values are derived by multiplying the AWG wires by 0.92 to account for its smaller size, and ISO/Metic values are taken from ISO/FDIS 10133. ISO values are generally a little more conservative than ABYC values. ISO values are used for 60°C, 90°C, 105°C, 120°C, 200°C rated wire insulation. ISO does not specify values for 75°C or 80°C - WireSizer calculates derived values for those.

The wire gauge calculated with the voltage drop formula is compared with the maximum ampacity of the wire in the table. If the wire meets ampacity requirements, then the voltage drop result is returned. If it is not, the wire gauge will be bumped up in size until it meets ampacity requirements.

AWG (American Wire Gauge) wires are 3-12% larger diameter than SAE (Society of Automotive Engineers), with an accompanying larger carrying capacity. The larger the number, the smaller the wire.

ISO/Metric wire is commonly denoted as mm2. The larger the number, the bigger the wire.

Used appropriately with a little bit of common sense, WireSizer's output corresponds to ABYC E-11 standards. ABYC standards generally meet or exceed NEC standards where applicable, as well as ISO/FDIS 10133.

About Us

Juggernaut Technology, Inc. has been in business for over 12 years, and specializes in complex database and Internet enabled business applications, and is branching out to applications for mobile devices.

We developed this application because we wanted an easy to use, keyboard-free way to help people quickly determine the appropriate wire gauge needed for when they are rewiring or adding new systems to boats, RVs, automobiles, radios, or other uses. Let us know what you are using it for, and we would appreciate your feedback! Please email us at info@WireSizer.com.

For more information about this app or other projects, please visit as at www.WireSizer.com.

Juggernaut Technology is a member of ABYC.

Member ABYC


Copyright © 2012-2017 Juggernaut Technology, Inc. All rights reserved.

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