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Discussion Starter · #1 · (Edited)
I don't have much experience when it comes to determining conductor size and OCPD for motors.

The unit is a 5hp Ingersoll-Rand air compressor. The nameplate FLA is 21.4 amps at 230v.

The single phase table in the NEC says 28 amps for a 5hp. Which should I use?

I can use #10 for both FLA's correct? The difference would be in the breaker used. The lower FLA would use a 60 and the larger FLA would use a 70.

Is the pressure switch considered a disconnect? It can manually break both lines.

Does the "red button" on the motor count as overload protection or do I need to install fuses?
 

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You need a conductor rated for 35 amps.

28 x 125%=35

You don't need additional overload protection if the motor already has a thermal overload. I would still use a starter though, you don't want to run the motor leads through the pressure switch. It'll burn the contacts out in short order. Use the PS to fire the starter coil.
 

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Discussion Starter · #3 ·
You need a conductor rated for 35 amps.

28 x 125%=35

You don't need additional overload protection if the motor already has a thermal overload. I would still use a starter though, you don't want to run the motor leads through the pressure switch. It'll burn the contacts out in short order. Use the PS to fire the starter coil.
So use the 28 amp FLA per the NEC and #8 awg wire? The unit is designed to use the pressure switch as the main set of contacts.
 

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So use the 28 amp FLA per the NEC and #8 awg wire? The unit is designed to use the pressure switch as the main set of contacts.
Is this new equipment? If so, just follow the manufacturers instructions.

You don't need a starter if the motor has integral overload protection. If anything you would use a contactor controlled by the pressure switch. You say the pressure switch contacts are rated for that size motor so you don't anything else.
 

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Bilge Rat
motors and controls.........
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Cow has it right.

430.6 (A) (1) states that table values are to be used for wire and breaker size. 430.6 (a) (2) states that nameplate values are to be used for overloads.

110.14 (C) (1) (a) (4) states that the 75º column is to be used for motors.

240.4 (D) and Table 240.4 (G) remove the 15, 20 and 30 amp restrictions for #14, 12 and 10s. The actual values in Table 310.16 apply.

Table 430.52 allows a breaker to be sized up to 250% of the table value.

One of the funniest cases allows #14s to be used with a 40 amp breaker serving a 1HP 1ø motor operating on 115 volts.
 

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The only air pressure switch I know of that is rated at 5HP single phase is the old Furnas (now owned by Hubbell Industrial Controls) 69HA or HB. Those are rated as a controller, but are NOT a disconnect switch, there is no locking mechanism (430.102). You might be able to use the breaker in the panelboard as the disconnect by adding a locking mechanism, IF you meet the other necessary criteria in the exceptions, but it's still a better idea to put one next to the compressor.
 

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Bilge Rat
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Also, the red reset button qualifies as a the overload protective device. The only other protection you need is short-circuit and ground-fault. The breaker in the panel qualifies, and in your case, provided it is not over 70 amp.

If it were me, I'd use a 50 and either #8s or #10s.
 

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Nice point on the disconnect. I skipped right over that. I also recommend putting a disconnect close to the machine, regardless if the breaker meets the criteria. A lot of facilities require one for LOTO anyways.
 

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Discussion Starter · #10 ·
Nice point on the disconnect. I skipped right over that. I also recommend putting a disconnect close to the machine, regardless if the breaker meets the criteria. A lot of facilities require one for LOTO anyways.
Panel is out of sight. Was going to recommend a disco. But my question here is this, if the motor leads are connected to the pressure switch how would you hook it up any other way than THROUGH the switch?
 

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Panel is out of sight. Was going to recommend a disco. But my question here is this, if the motor leads are connected to the pressure switch how would you hook it up any other way than THROUGH the switch?
There's nothing wrong with it being AHEAD of the switch. Think about a combo starter mounted on the wall next to a machine. The disconnect device is AHEAD of the motor starter. All the code says is that it has to be there, nothing about the order.
 

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Discussion Starter · #12 ·
There's nothing wrong with it being AHEAD of the switch. Think about a combo starter mounted on the wall next to a machine. The disconnect device is AHEAD of the motor starter. All the code says is that it has to be there, nothing about the order.
So #10 on a 60 amp breaker with a non fused disco before the pressure switch would be a compliant install?

Sorry about the confusion Jraef, I misread your post.
 
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