An electrical question for y'all electrical folks

My buddy has two Ingersol Rand two stage compressors with 5 hp motors on them. They were each on a 30 amp 240 volt circuit breaker with #8 ga solid copper wiring. They were about 75 feet from the breaker panel. One of them would trip the circuit breaker on startup occasionally. This week we moved them to a new building. He put them on 40 amp breakers, with 8 ga wire, and there is no problem now.They are only 30 feet from the breaker panel now. I checked them both with an Amprobe. They both go up to 152 amps on startup but run on 22 amps after initial startup. This is on just one pole/wire of the 240 volt breaker. I can't understand why the one compressor would trip the 30 amp breaker but not the other compressor would not. It seems to me since they are both pulling 152 amps on startup both would trip the breaker. I'm guessing the breakers are slow to trip to account for startup. John T and some other folks on here always give clear explanations so I told my friend I would ask about it. Thanks in advance for your replies. Rick

P am just an old dummayzz but I believe some of these breakers get weak after many trips.

It's a lot better for the compressor too having the closer to the panel. I had a similar compressor about 70 feet from the panel and it had a lot of trouble starting in cold weather. Once I moved the compressor about 40 feet from the panel I didn't have anymore problems.

The breaker should be rated for the wire and 30 amp is for 10ga. wire. The 40 amp breaker is more appropriate for the 8ga wire.

Much less voltage drop on the short run, you more than halved the distance which will result is much less voltage drop.

Breakers can get weak over time, but they are built to allow for the motor starting amps. Also if there is a bad connection it can cause a larger draw. As someone already said the compressors require a minimum of #10 wire and that is good up until about 100 feet(just a rule of thumb i don't want to take the time to do math), then you need to upsize the wire. Accorfing to the NEC you are allowed to put an oversize breaker on a wire for a motor if it is tripping the existing one, however you are limited on how big of a breaker. To put it simply the compressors on the #8 should have been fine so there was probably something else causing it.

Hello kito 169,

I second the breaker is weak. Just sawp the wires on the panel. All other readings appear the same, other then a tight bearing?

Guido.

There can be a number of factors.

Could be a weak capacitor or the centrifugal switch contacts are arced, not always making good connection.

Could be an intermittent short to ground in the motor, pressure switch, or wiring. Compressors shake around a lot, can wear through the insulation.

And can also be a mechanical problem with the compressor itself, unloader not always unloading, leaking valves or carboned valves letting pressure bleed back to the low pressure cylinder.

Running them on 30a breakers is pushing the limit for a heavy motor starting load, which compressors are. Now that they are on 40a breakers there should be no problem as long as everything else is good, mechanical and electrical. And yes, breakers do have a time delay lag to prevent false tripping. It is normal for motors to draw huge amounts of amps during start up. They are drawing much more than your meter is reading, the meter is just not quick enough to catch it. The reason this is acceptable is it happens so quickly, nothing has time to heat up enough to cause damage.

That the motors are running right at the motor full load amp rating is a good sign. That means the motors are matched to the compressor and the compressors are working as they should.

I checked both a Square-D motor circuit selector rule, and a Cutler-Hammer rule. Both specify for single-phase, 230-volt 5 Hp motor, a motor full load current = 28 amps. Cutler-Hammer instantaneous-trip circuit breaker type GMCP /HMCP = 50 amps. Both rules specify an ordinary circuit breaker trip rating = 60 amps. Minimum THHN/THWN/etc. wire size = No. 8 copper. Smaller circuit breakers will offer more protection, and are not bad idea, but that could be the cause of tripping.

Again I want to thank all of y'all for your replies. I have learned a little more about this stuff. I love to learn. Merry Christmas to all.