You did a much better job of explaining this than I ever could.
Steve
Member
You did a much better job of explaining this than I ever could.
Steve
Member
I have been reading this thread with interest as I recently installed an Ingersoll Rand air compressor with an Emerson continous duty motor that is 5hp (22.5 fla listed) and I am not sure that it meets code from what I am seeing. IR recommended a 60 amp breaker, to handle the starting load, with #10 wire on its own circuit (no plug). 10 feet of wire from CB box to motor. From what I see here it seems as though #8 wire might be required or is that one wire size larger than code to remain conservative? Is #10 suitable since it is on its own circuit and such a short run? Is the 60 amp breaker out of line? It works fine so far but I would like to be covered legally.
Allen
Member
My 220V circuits are wired the same as Steve Wilson described his were. Not a problem ever nor any indications of one. I would think that a very high percentage of shops are. This is the first time I have seen reccomendation for a non-commercial shop wiring be wired with this heavy a amp circuit breaker. Are you folks sure that those ciruit breaker ared sized correct for a non-commercial application where twist type plugs are to be used?
Leo
Member
Cut the Crap!
When I first saw these replies I thought they were jokes, but it appears they are dead serious. What I've read in the last couple of pages (no names) is the most infuriating misinterpretation of information I have seen in a long time. This is a case of taking information from one area and applying it to another area, and it does not fit here!
The NEC goes into a lot more information about motors than the limited explanations above, and for something like a tablesaw, none of this massive overkill is required.
When you misquote the NEC like this, the urban myths will last for months on the internet, and that is why I am coming down so hard on this.
As was already stated, the 3 hp tools need 20 amp circuits and the 5 hp tools need 30 amp circuits.
Member
So Rick, What you are saying is that a 5hp motor, even if it starts under load such as an air compressor, should be fine with a 30 amp breaker? I was told that a 30 amp breaker would trip every time that it started. I could go buy more breakers of various sizes and keep going up if it didn't work but if the 60 amp breaker that Ingersoll Rand recommended is safe I will just leave it alone. #10 wire and 60amp breaker, 5hp running and less than 10 feet of wire. Allen
Last edited by Allen Bookout; 04-13-2006 at 7:13 PM.
Member
My Curtis Toledo has been running on a 30 amp breaker for over 10 years without ever tripping, and I have even had it on a pretty short duty cycle with 3 DA sanders running simultaneously. It is the short duty cycles with repetative restarts under load that can pre-heat a circuit breaker (and the wiring) to the point that they will trip prematurely.Originally Posted by Allen Bookout
I don't know who at IR recommended a 60 amp breaker for this application, but they need a swift slap.
Member
I always thought that circuit breakers sized for running amperage would accommodate inrush current upon starting a motor. Higher amperage requirements than are usually experienced in a small shop need fancy stuff like “heaters” that we’d best have spec’d by an electrician.
True or not true?
Frank
Member
I wish that I had not involved the Ingersoll Rand tech people as they were a big help to me in other areas and trying to help me on this issue. So please, just forget that I said that they recommended anything as I do not want to cause problems for anyone at that company.
Thank you for your response!!! Allen
Member
Although I would not express the same sentiment as Rick has, something does not sound right with suggesting that that NEC would allow 8 guage conductors to be on the load side of a 70 amp breaker. I have worked for years as an electrician before becomming a Electrical Engineer and I have not come across this before.
lou
Member
Per the calculations in article 430, it could be absolutely appropriate to have a 70 amp inverse time circuit breaker protecting #8 THHN.
Use an example of a 5HP, 230v, single phase motor with a nameplate current rating of 24 amps and service factor of 1.25.
From 420.22(A), the conductors will be at least 125% of the FLA rating from the tables, 28 amps * 1.25=35 amps. #8 THHN is rated for 40 amps, using the 60 degree terminal column.
From 430.52(C)(1), the branch circuit short-circuit and ground-fault protection can be up to 250% of the FLA rating, 28 amps *250% = 70 amps.
So - indeed - we can have a 70 amp breaker providing short-circuit and ground-fault protection for said motor circuit.
We're not done yet.
We also need an overload. That would be sized per 430.32 at 125% of the nameplate current rating or 1.25 * 24 amps = 32 amp overload.
It's important to note that the above calculations are for continuous duty motors. It's also important to note that the above calculations are - most likely - not how I would wire the circuit for a 5 HP, 230V compressor in my shop.
I'd wire a circuit with #8 to help reduce voltage drop at startup and protect it with a 30 amp breaker. In this scenario the circuit breaker is serving 3 purposes - short-circuit protection, ground-fault protection and motor overload protection. If that tripped, I'd go to a 40 amp breaker but would also add a separate motor overload because I could now have a situation where the motor was continuously drawing 167% percent of it's nameplate rating and that's a potentially hazardous situation.
The NEC rules for motors are different than the rules for normal branch circuits. The rules for normal branch circuits are more conservative in terms of breaker sizing in relation to conductor gauge. That's because the circuit breaker serves more purposes in a normal branch circuit than in a dedicated motor circuit where the motors have separate overload protection. It's not just about protecting the circuit conductors, it's also about protecting the motor.
The simplest thing to do is stick with normal branch circuit sizing. If the 30 amp breaker works for your compressor, there's no reason to go larger. If you need to go to a larger breaker, you need to understand the rules that go along with increasing breaker size and motor overload protection is part of that.
Rob
Addy protocol: not a licensed electrician
Member
Well this should be interesting...
It would help if you were more specfic as to what exactly is "crap", what is "infuriating misinterpretation", and what is a "misquote".
For a plug-and-cord connected tool, yes my post agreed with that. For a direct-wired circuit, the local AHJ may not agree that this is always the case.As was already stated, the 3 hp tools need 20 amp circuits and the 5 hp tools need 30 amp circuits.
I am reluctant to accept your opinions without justification; partly because I took a quick glance at your "Electricity in the Woodshop" page and spotted a few errors. For example, in your section 1.6.3:
There are numerous paragraphs in the NEC which contradict this statement; e.g.,, 240-5(B) & 422.11 are two.A circuit breaker is not intended to protect the appliance, only the wire between the breaker and the outlet.
No it doesn't. Common trip is only required when multiwire branch circuits feed both line-to-line and line-to-neutral loads (210-4(C) Exception No. 2). Otherwise it is not a requirement.National Electric Code mandates that ALL Hot wires going to a load must, not only have a circuit breaker, but ALL circuit breakers feeding that device must trip together.
Member
Lou,
Take a look at this thread on Mike Holt's NEC forum for additional example:
http://www.mikeholt.com/codeForum/viewtopic.php?t=24713
The modoerators on that forum are on the committee that write the NEC, so I'm inclined to take their word for it.
Also, if you can look at a copy of the NEC Handbook, look at the explanatory text for 430-52. it has a few paragraphs just on this topic. I only have a hardcopy of the Handbook, so I can't easily paste the section here.
Member
Thanks for the link... There does seem to be a dispute about this same topic over there as well. The way I read the post that you linked to was that for short distances a motors OCPD located very close to the motor could be higher than the wiring code would allow, but the OCPD for the entire circuit was governed by the wiring tables and would represent a more traditional view that the OCPD should never allow for excessive loads to be sustained and thus risk fire. This was the moderators position as best as I can read it. But just for completeness, and now that you have gotten my attention I will make it a point to fully understand / and or correct / this issue.Lou,
Take a look at this thread on Mike Holt's NEC forum for additional example:
http://www.mikeholt.com/codeForum/viewtopic.php?t=24713
The moderators on that forum are on the committee that write the NEC, so I'm inclined to take their word for it.
Also, if you can look at a copy of the NEC Handbook, look at the explanatory text for 430-52. it has a few paragraphs just on this topic. I only have a hardcopy of the Handbook, so I can't easily paste the section here.
regards
lou
Member
Barry, the issue I have with this is that these postings wrongly lead the casual reader to believe that they need huge circuits for the tools listed. Neither you nor the other poster answered the original person's question; you simply quoted code for nonapplicable situations. You both misapplied what the code will permit you to do, and presented it as though it is what the code requires you to do. Regardless whether you can backpeddle fast enough to claim you were only stated what was permissible, this is not how the information was presented.
In several places you used the word "requirements", which implies to the reader that it is information that they are mandated to follow. And the implication that "it would be a code violation" to put a 3 hp (e.g. Unisaw) motor on anything less than a 50 amp plug is the most nearsighted thing I have read in years. It doesn't matter that you prefaced this with "supposedly", you deliberately and knowingly left the impression it is a requirement.
You also didn't take into account the duty cycle of the machines listed. All of these machines have a very low duty cycle, and none of them are automatically controlled. Nowhere in your dissertation did you address the code variations with duty cycle for motors.
Your discussion on cord and plug connections leaves me stymied as to how you could even come up with this. Does this mean I need to go down to the shop and cut all the power cords off my routers? By the way, if you think a NEMA 14-60 is the biggest connector available, you definately need to get out in the real world a little more.
I don't pretent to be an expert on code and I know my limitations. However, you think you know the code more than you actually do, and that is far more dangerous.
As for my article, if those are the only pedantic errors you can come up with, it falls a little short of knocking holes in my credibility. That article has been on the Internet for over 10 years and read by hundreds of thousands of people, including quite a few electricians, inspectors, and engineers (I know because they email me). Do you honestly think that after 10 years you will be the first person to find a significant mistake in it? Surprisingly, when I type your name and electricity into Google, I get no applicable hits.
Member
I am not really a part of this discussion, thank goodness, however I did get concerned about using a 60 amp breaker and #10 wire for my 5 running hp, 22.5 FLA, aircompressor motor so for anyone that is interested here is what I did. I replaced the 60 amp breaker with a 30 amp breaker (due to some information here) and ran the unit through five cycles of one minute running and 15 seconds off and it did not trip the breaker one time. I realize that the test may not be valid for a real world situation but it appears as though I may not have any trouble with it and I feel a lot better. Allen
Posting Permissions
Reply With Quote