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Old 04-21-2019, 07:53 AM   #1
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Default Motor switch/starter ratings

Got a used 5HP single phase 208v air compressor to hook-up, and the pressure switch on the unit is only rated 2HP 240V / 1.5HP 120V for single-phase. So I need to find a heavier duty pressure switch or add a motor starter.

Curious that motors are available in both 200V and 230V designs but the switches and starters only seem to have 120, 240, 480, 600V ratings. So I assume a pressure switch rated for 5HP at 240V is also suitable for 5HP at 208V? Probably splitting hairs, the FLC table currents in 430.248 for a 5HP is only a couple amps difference each between 230, 208, 200V.
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Old 04-21-2019, 08:46 AM   #2
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Take a couple pictures.

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Old 04-21-2019, 09:04 AM   #3
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Those pressure switches should only be used for pilot duty only. I don’t ever trust them for contactor duty. The surface area of those contacts are tiny and a DP contactor, IEC contactor, or NEMA starter is cheap insurance in my opinion.
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Old 04-21-2019, 01:09 PM   #4
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Is it a true 5hp motor? Compressor manufacturers like to exaggerate. What is the full load amperage on the motor?

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Old 04-21-2019, 01:51 PM   #5
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I don't use very many IEC starters so I don't know about their ratings but I do use a ton of NEMA starters.

Most, if not all, NEMA starters have HP ratings for 200 and 230 volts. I don't know about other manufacturers but the HP tables in the Allen Bradley book list the ratings as motor voltage, not system voltage.

Single phase motors are not listed at 200 volts, I imagine because;

1) As far as I know, no one makes single phase motors in 200 volts.

2) Only an idiot would specify a large single phase motor on a 208 3 system. But yes, it happens all the time.........
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Old 04-21-2019, 02:23 PM   #6
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Actual motor;


Actual pressure switch;


Example of a starter I was considering, and then realized they aren't putting 200/208V ratings on it;

Maybe go with the 7.5HP @ 240V version of that starter..

Yes I know that motor is more suited for a 240V nominal system. The site only has a 120/208 single-phase service, they got a good deal on the compressor and are willing to chance it's longevity running at the lower end of voltage spec.
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Old 04-21-2019, 05:31 PM   #7
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You can buy pressure switches on mcmaster Carr that are cheaper than a contactor and enclosure.

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Old 04-21-2019, 09:01 PM   #8
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Quote:
Originally Posted by L_S View Post
Actual motor;


Actual pressure switch;


Example of a starter I was considering, and then realized they aren't putting 200/208V ratings on it;

Maybe go with the 7.5HP @ 240V version of that starter..

Yes I know that motor is more suited for a 240V nominal system. The site only has a 120/208 single-phase service, they got a good deal on the compressor and are willing to chance it's longevity running at the lower end of voltage spec.
That's a 'definite purpose' contractor. It's made mainly for HVAC.

Hermetic HVAC compressors come up to speed quickly so the time spent with more than full-load current is less. Thus, this type of contractor has smaller contacts than other types.

If it is used for something that takes longer to start, the contacts will heat up and eventually burn up because of the longer time spent in high current.

I wouldn't use that type of contractor at its HP ratings, I would go at least one HP step higher and if it starts and stops often, I'd go double the HP.

Even then, it won't hold up as well as a NEMA starter.

One thing to remember about electrical stuff and motor control in particular.....you almost always get what you pay for.
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Old 04-22-2019, 05:39 AM   #9
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The motor already has built in overload protection so the additional overloads are unnecessary. Grab a heavy DP contactor or a good used NEMA 2 starter and remove and save the overloads for another project. Or use my method, hit Ebay and buy a good used combo starter of your brand choice and pull in all three phases for the replacement motor in the future.
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Old 04-23-2019, 12:09 AM   #10
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Quote:
Originally Posted by micromind View Post
That's a 'definite purpose' contractor. It's made mainly for HVAC.

...

I wouldn't use that type of contractor at its HP ratings, I would go at least one HP step higher and if it starts and stops often, I'd go double the HP.

Even then, it won't hold up as well as a NEMA starter.

...

That's what I ended up grabbing and using this morning, a 40A / [email protected] 2-Pole Definite-Purpose contactor. Kinda wish I would have gotten a "real" motor starter now that I read your info... Next time!
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