I'm hooking up a 5hp 220v 15amp motor to an exhaust fan for a car spray booth. I'm looking for advice as to what gauge wire I need to power it about 100' from the panel. Also can I get by with a heavy duty switch of some type or to I need to add a relay/contractor controlled by a switch? Thank you

A #12 Copper conductor will limit the voltage drop to 2.18% or less when supplying 15.0 amps for 100 feet on a 240 volt system

See voltage calculator.
Voltage Drop Calculator (http://www.southwire.com/support/voltage-drop-calculator.htm)

On your motor's label, is the 15 amps shown as "FLA" or "FLC"

You need to know the "FLC" (Full Load Current) to properly size the motor conductors and protection for the motor.

Just using the numbers you have given us, here is what I come up with.

According to the NEC table for single phase motors. The FLC for a 230 V 5 HP motor is 28 amps.

1) Branch circuit conductors are sized at 125% of the "FLC". So 28 * 1.25 = 35 Amps. Which means that you would need a # 8/2 with ground cable.

2) Branch current Short Circuit/Fault protection is sized at 250% of the "FLC". So, 35 X 2.5 = 87.5 amps - You would need a 90 amp Dual Pole Breaker.

3) Overcurrent protection is sized by using the "FLA" value of 28 amps and then multiplying by a percentage based on the "SF" value off the motor's label. For example if the "SF" is 1.15 or higher, you would multiply the "FLA" value 28 (?) by 1.25 to get the amperage needed to select the O/L protection breaker or fuse. If the "Temp Rise" (also on the motor's) label is 40 (C) or LESS, also multiply by 1.25. If neither value matches, than you would multiply the "FLA" by 1.15.

Now if the amperage does not match one of the standard sizes of breakers you are allowed to round-up to the next standard size. Also, if the motor fails to start, you can go up buy a higher percentage for both the Branch circuit protection and/or the overload breaker sizes.

"car spray booth"??
Do you mean paint booth? If so please realize you have created a "classified" area and the fan and motor should be explosion proof!
What are you spraying?

Thanks for the catch! I forgot about "Vapors & Fumes".

Quick question, associated with this, would you not use the O/L protection to make sure that the fan continues to pull the vapor / fumes out of the area?

Oh, by the way a distance of 100 ft. is not a problem with respect to voltage drop.

Thanks for the catch! I forgot about "Vapors & Fumes".

Quick question, associated with this, would you not use the O/L protection to make sure that the fan continues to pull the vapor / fumes out of the area?


Are you suggesting setting the O/L to keep the fan running? I wouldn't recommend that...

We have set-up paint booths controls so "the door must be closed" and "the air compressor must be running" and the "exhaust fan must be running" for the paint sprayer to operate...
This was a classified explosion proof area with a paint type that when atomized would easily ignite.

This gentlemen needs to be sure he's familiar with the materials he'll be using and the relevant safety precautions.

No, I was thinking that the O/L protection be eliminated. I'm trying to think of where I read that the O/L should be eliminated if there was a danger created by the fan stopping. I think it's time to revisit Article 430, part III.

By the way, did I do the motor calc's correctly, since they were just guesswork. I'm always willing to learn, especially now that I bored out of my ever-loving mind. Laying in bed is such a "PIA"! Oh well, at least I have potty privileges again. And my knee is holding me up again so I can now stagger around and bounce off walls like a pin-ball.

I saw the statement in the 2011 code. Article 430.31 - General

"These provisions shall not require overload protection where a power loss would create a hazard, such as in the case of fire pumps."

Would the need to ventilate the area carry that level of concern?

In a paint booth... I wouldn't think so, the interior of a booth is fairly well contained from the outside. If we were talking about ventilation of storage where gas or vapors build up over time then, yes...