# How can I size a cable for a motor?

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#### mostafa0020

##### Member level 1
Hi all
How can i size a cable for a motor?i mean do i have to multiply the rated cuurent by a factor to compensate the starting current capacity heating for the cable?

#### Davood Amerion

motor cable size

Hi;
as i know. we must use one parameter for determining of wire diameter.
this parameter known as 'J'.
J is current density in (eg.) copper wire.
J for copper wire is about 3-4 A/mm2 in transformer.
but we can use grater value, say 5-6, for other purposes.
this means for 3.14 Ampere current we need a 1mm
diameter wire (for J=4).

If your motor started 1 time in each (for ex.) 1 hour , you dont need use
thiker wire. but if your motor frequently started, you must increase wire size.
motor current in start condition reach about 10 times of
working its normal current.

Regards
davood

#### flatulent

motor cable sizes

The general rule for mains cables is that for lighting the voltage should drop by 2% or less zero to full load. For other loads 5% drop is acceptable.

With the present rising cost of electricity generating, it is probably more economical to pay for the larger diameter cable and save on operating costs by having more like 2% cable voltage drop for your motor.

You should also check the local laws about building wiring in your area. They may have more stringent requirements including the insulation type and possibly metal conduit covering for the cable routing and how many cables can be in a conduit. The source you purchased the motor from should be able to help you on this, or possibly the manufacturer of the motor.

#### IanP

cable sizing for motor

Here is "Wire sizing: a practical guide": **broken link removed**
You can apply formula used in this guide:
VDI = (AMPS x FEET)/(%VOLT DROP x VOLTAGE)
VDI = Voltage Drop Index (a reference number based on resistance of wire)
FEET = ONE-WAY wiring distance (1 meter = 3.28 feet)
%VOLT DROP = Your choice of acceptable voltage drop (example: use 3 for 3%)

Regards,
IanP

#### khongai

##### Full Member level 4
motor cable sizing

you may contact to ABB, Siemens, ... seller in your region to request that.

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