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Old 06-21-2008, 04:02 PM   #16
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Most of the time there will be a buzz and the thing won't start. Although, it is possible that the breaker will trip. It is a possibility and wire size should be accounted for, it COULD be a factor here.
If it's been wired by an "electrician", a "buzz" should not be heard. Unless of course the electrician is totally non-technical (as most are). Bring on the flames.

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Old 06-21-2008, 05:20 PM   #17
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Are you serious??????
yeah, why?
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Old 06-21-2008, 11:16 PM   #18
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Where motors and refrigeration compressors are concerned, it's very common (and completely legal) to have a breaker much larger than the wire it's feeding. The reason is that every motor or compressor must have an overload protective device that is responsive to current in series with the conductors. This will not only protect the motor, but the wires that feed it as well. the only thing the breaker does is provide short-circuit protection. The overloads are precise, but slow acting, and won't react to a short (or ground fault) quick enough.

Here's an example; in a motor control center, or a combination motor starter that has a circuit breaker, the breaker will likely be a 'motor circuit protector'. This breaker has only an instantaneous trip unit in it. A regular breaker has both a short term and a long term trip unit. The motor circuit protector has an adjustable trip setting on it, a 15 amp one might have settings from 60 to 120 amps. This is to handle the starting surge of the motor, anything higher is assumed to be a short circuit or a ground fault. The long term overcurrent is in the motor overload device, not the breaker.

Motors and compressors are one of the very few times that undersize wire is allowed.

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Old 06-22-2008, 11:00 AM   #19
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I guess I do not understand. Where in the code does it state you can use #14 on a 30 amp load? You guys have got me real curious now.
Is the rule similar to the rule with welders for example. Non-continuos duty?.....Thanks John
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Old 06-22-2008, 11:42 AM   #20
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I guess I do not understand. Where in the code does it state you can use #14 on a 30 amp load? You guys have got me real curious now.
Is the rule similar to the rule with welders for example. Non-continuos duty?.....Thanks John

Not sure I can find this allowance and not sure that is what chris meant. He simply stated he has wired many ac units with #14 wire and used a 30 amp breaker. He did not mention the load spec. The OP does have a 30 amp rated load and I think this may be simply confusing the 2.

There may be situations where a #14 on a 30 amp load is acceptable but offhand I can't think of any and there are too many possibities in the code to start looking for such a blanket statement.

#14 wire is rated at 30 amps, and even greater, in certain situations so I would not be surprised to find such an allowance someplace.
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Old 06-22-2008, 11:45 AM   #21
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I guess I do not understand. Where in the code does it state you can use #14 on a 30 amp load? You guys have got me real curious now.
I

Sorry was not referring to the load.

I was referring to the MINIMUM circuit size and the MAXIMUM breaker size... You could easily have a MIMIMUM Circuit size of 23 amps, and a maximum breaker size of 40 amps. so #12 thhn and a 40 amp breaker would be legal.

Last edited by chris75; 06-22-2008 at 11:52 AM.
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Old 06-22-2008, 12:04 PM   #22
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Not sure I can find this allowance and not sure that is what chris meant. He simply stated he has wired many ac units with #14 wire and used a 30 amp breaker. He did not mention the load spec. The OP does have a 30 amp rated load and I think this may be simply confusing the 2.

.
Yeah, I missed the 30 amp rated load part of the op's question...
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Old 06-22-2008, 12:17 PM   #23
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I'll use motors, since I'm the most familiar with them, but refrigeration compressors are very similar. The provisions for motors are found in article 430. Compressors are in 440.

Basically, for a single motor, (not a group of motors), the minimum wire size must be not less than 125% of the full-load current of the motor. This is found in 430.22. This current is the larger of the value given in tables 430.248 and 430.250, or the actual nameplate current. Almost always the value in the tables is larger. This is a minimum, the wire can be larger.

Part III, beginning with 430.31 basically states that every motor must have some sort of overload protective device. In a nutshell, this is nothing more than a very precise circuit breaker that contains only a long term trip unit. (A normal circuit breaker has 2 trip units in it, long term and short term, also known as thermal and magnetic).

Part IV beginning with 430.51 covers short-circuit and ground fault protection. Overcurrent for both the motor and the wire that feeds it has already been dealt with in part III. 430.51 states that "These rules add to or amend the provisions of article 240." 240 is where we get the fact that a #14 wire must be protected by a 15 amp breaker. Table 430.52 states that the maximum rating of an inverse time breaker (the kind we commonly install in panels) cannot be more than 250% of the full-load current of the motor. This is a maximum, it can be less.

Now, for an example, lets use a 2HP single phase motor operating at 230 volts. Table 430.248 states the full-load current to be 12 amps. the actual wire size must be 125% of this value as stated in 430.22. This gives us 15 amps. article 110.14 (1) (a) (4) states that the 75C temperature rating is to be used. Table 310.16 states that a #14 copper is good for 20 amps. (The * by the #14 refers you to 240.4(D), and states that a #14 wire must be on a 15 amp breaker, but 430.51 amends this provision, so it doesn't apply.) Thus, a #14 wire is the minimum. Table 430.52 states that the maximum size circuit breaker cannot exceed 250% of the full-load current of the motor. 12X2.5=30. thus we can install any breaker of 30 amps or less, and land a #14 wire on it.

In reality, a #14 wire would run a 2HP single phase motor at 230 volts forever, and never overheat, even if the motor was overloaded. The overload would trip before the wire overheated. A 15 amp breaker would very likely not start this motor. A 20 might, but a 30 would start it every time. If a short circuit or a ground fault developed anywhere in the line, the overload would be too slow to respond, (or the fault might be between the breaker and the overload), but the 30 amp breaker would trip.

Article 430 is somewhat confusing, in reality, you'll use only a very small part of it. In 18 years of being a comm'l/industrial electrician, I've likely installed over 1000 motors of all sizes, from little itsy-bitsy ones up to 15,000HP that ran on 13,800 volts. In any given installation, only about a half-dozen or so parts actually apply. Just pick out the ones that apply to your particular installation, and the others don't matter.

Rob
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Old 06-22-2008, 01:16 PM   #24
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Very good post micro! If, as an electrician you do mostly resi, then you may never have to use the articles listed above. If you do industrial, you use tha above mentioned articles all the time!
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Old 06-22-2008, 07:54 PM   #25
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If it's been wired by an "electrician", a "buzz" should not be heard. Unless of course the electrician is totally non-technical (as most are). Bring on the flames.
You are absolutely right. Unfortunately electricians don't wire 100% of the ACs out there.
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Old 06-23-2008, 03:16 AM   #26
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Before I can really comment on OP's part i will like to know more on the nameplate on the unit.

However if the unit is hard to start there is a kit to take care of it. that term will varies a bit depending on where your ear land on it.

They useally called either " hard start kit " or " soft start kit " but both basically are almost the same it will change the size of starting capaitor on single phase units.

Merci,Marc
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Old 06-23-2008, 10:34 AM   #27
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Has anyone noticed the OP has gone away? And he never did say there was a problem with start up. If he was able to measure the run current as stated in his post, I doubt the inrush was the problem. Just sayin.
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Old 06-23-2008, 10:24 PM   #28
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Here's an example; in a motor control center, or a combination motor starter that has a circuit breaker, the breaker will likely be a 'motor circuit protector'. This breaker has only an instantaneous trip unit in it. A regular breaker has both a short term and a long term trip unit. The motor circuit protector has an adjustable trip setting on it, a 15 amp one might have settings from 60 to 120 amps. This is to handle the starting surge of the motor, anything higher is assumed to be a short circuit or a ground fault. The long term overcurrent is in the motor overload device, not the breaker.
Rob-

You seem to have a lot of experience with this sort of thing and I wanted to share a story from the vault. Working on a power plant job, the spec. allowed motors of 0-200HP to be 480V 3-ph. Well, we had 2-200hp. 480V ind. motors that were running some pretty sizable secondary air fans. The starters were size 7 if I remember correctly and when we initially tried to commission them, the contactors would buzz like mad and the motors would trip on the O/L's. A couple of calcs revealed that the time to accelerate to full speed was ~25 seconds and not even the largest heater would allow this. We wound up having to retrofit the heater/Overload block with an electronic unit to allow the motors to come up to speed. Measured time to full running RPM was 27 seconds! At the end of it, we all lamented that these should have been spec'd as 4160V motors.

Take care,
Jimmy
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Old 06-23-2008, 10:30 PM   #29
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Has anyone noticed the OP has gone away?
You know John, et al, we often get into some pretty in-depth conversations about schtuff that are related to the OPs' questions but are really not doing anything to answer the question. (anyone remember the 240 range wiring thread where Speedy, Stubbie and 220 got rather, shall we say, argumentative?! ). I totally love this, but as a group, let's all admit that sometimes we forget that people are trying to learn or glean useful/helpful info for what they are trying to accomplish. Not bashing anyone, but when you think about it, it's funny the way electrical professionals can get to arguing (and learning!)!

Take care John and all the regulars,
Jimmy
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Old 06-24-2008, 05:51 AM   #30
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Rob-

You seem to have a lot of experience with this sort of thing and I wanted to share a story from the vault. Working on a power plant job, the spec. allowed motors of 0-200HP to be 480V 3-ph. Well, we had 2-200hp. 480V ind. motors that were running some pretty sizable secondary air fans. The starters were size 7 if I remember correctly and when we initially tried to commission them, the contactors would buzz like mad and the motors would trip on the O/L's. A couple of calcs revealed that the time to accelerate to full speed was ~25 seconds and not even the largest heater would allow this. We wound up having to retrofit the heater/Overload block with an electronic unit to allow the motors to come up to speed. Measured time to full running RPM was 27 seconds! At the end of it, we all lamented that these should have been spec'd as 4160V motors.

Take care,
Jimmy
ummm, maybe a soft start starter? A lot cheaper than 4160 gear.

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