Selecting Conductor/Wiring Size

• "Where I am from, we would size motor conductors at a minimum of 125% of the full load rating of the motor. But, that is just where I am from."

Technically that is incorrect (because I can see you are in the US of A). We are required to size conductors per the charts in the NEC, article 430, not the nameplate FLC on the motor. Most people do not see that clearly and ASSUME those charts are just guidelines for when you do not know the motor nameplate FLC, but in reality, those are hard and fast rules! The only time you use the FLC is when you KNOW that the FLC on the nameplate is actually HIGHER than the appropriate chart value. If it is lower, you MUST still use the chart value.

But to Noudge79, Nof60 is still very much correct in stating that how you size conductors is strictly based on local codes and regulations where the installation is happening. You do not state where you are, so we cannot help. You indicate that you have an interest in the Philippines, but for all we know you are a Philippino American in Podunk Alaska, in which case the NEC would apply to you. But if you are in fact located in the Philippines, then I don't know what your local code says, you have to look that up yourself.

Hey JRaef... I am not in the US by the way, I am in Canada, where we use the nameplate values.

Yes,

Do a search for wire sizing on the internet, or consult NEMA or NFPA, or the local code book, or a text book

Fortunately, this is the most popular topic in the Electrical Engineering section. Try "Search this Forum" on the right.

Bedtime reading of British Standard 7671 would be a good starting point, as has been pointed out here countless times before. So often, in fact, it has become tiresome.

As others say, wire rating is for continuous load. And the rule is the circuit breaker protects the wire. Motors require protection via some electrical switch that prevents the smoke from being released. How long of a run is the motor power wiring? I'm assuming this is for home owner construction. How far from the main service entrance panel is the motor connection? Is your service 100amp, 200, 400? How close are you to the low voltage transformer, that your main service connects too. Think in terms of lengths, wire sizes, during surge current type loads. Do you see incandescent light bulbs dim? This indicates IR drops in your power system.

LRA is to me typically 7-9 times run current.

So I'm guessing your running a 10amp motor. And estimating 1.5HP@120V, or 3HP at 240V. If these are out on a long extension cord, powering a motor operated machine, for example running a cement mixer, the IR drop needs to be addressed. The IR drop at connectors, duplex, with male plug can get mighty hot. Are you fighting this sort of motor problem with line loss?

See CR4

How to Calculate Locked Rotor Current 11/24/2009

This is covered there

Good Luck, Old Salt

If you're just looking for a conductor that will carry / survive a 75 amp locked rotor then you are looking at 6AWG to 3AWG depending on the wire and temperature rating.

I hope you don't have to run a long way because that size wire is $$$$$$$$

If your looking for a fast rule of thumb here it is ( not to be used by anyone to engineer a system for sure, but for trouble shooting only).

1.5 amps per HP; gives you the amp rating of motor to 10 HP. Then reduce to 1.3 amps per after. 3 Phase motors @ 480 volt. 3 conductors in a raceway or conduit.

12 Gage THHN is 20 AMPs

10 Gage THHN is 25 AMPs

#4 is good to about 85 amps.

Typical 100 amp service to homes is feed with #2 THHN copper wire. Just for reference.

Covers most motors used in smaller industries. But if you really need to figure this up and have not done voltage drops and such for a wiring run, you need to seek out expert advise and direction. That can be on site to see what your trying to do and adivse properly.

Just look at the motor name plate and see what is the FLC rated and get a conductor that can carry 1.5 time of the motor FLC or a conductor 2 x time the motor FLC rated.