Jimdaboe join:2001-01-16 Corpus Christi, TX TP-Link Archer C8 Linksys E2500
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Circuiit Breaker RequirementInstalling an air compressor. 15 H.P. electric motor 230 volt 3 phase, 41 amp FLA. Does anyone have a chart to determine the size of circuit breakers needed for this circuit? I know that the start up inductance for motor start up increases the load, I just don't know how much. Any help is greatly appreciated. |
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Rifleman Premium Member join:2004-02-09 p1a |
Rifleman
Premium Member
2006-Dec-14 1:28 pm
It's been a long long time but I think you would require a 50 amp breaker and number 8 wire. Start up amps are 300 % but this is only momentary and they don't rate the breaker for that. |
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opus74Deep Thought Premium Member join:2002-03-04 Coello, IL |
to Jimdaboe
Match the breaker to the wire ... match the wire to current needed and distance. There are several charts on-line to help. Rifleman sounds right to me. |
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Jimdaboe join:2001-01-16 Corpus Christi, TX |
Thanks for the replies. someone told me to go 125% of the FLA but a Master Electrician I talked to said because I am using a magnetic motor stater that it would limit the startup current and a 50 amp would be correct. Thanks again for your help! |
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Rifleman Premium Member join:2004-02-09 p1a 1 edit |
Rifleman
Premium Member
2006-Dec-16 1:39 am
Actually--both were correct. As soon as you mentioned the 125 percent it came back to me. 1HP=746watts 15HPx746=11190watts 11190watts/240=46.6amps 46.6ampsx.125=5.8amps 46.6+5.8amps= 52.4 amps
If I did that right technically you would need a 60 amp breaker and #6 wire. |
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EdrickI aspire to tell the story of a lifetime Premium Member join:2004-09-11 San Diego, CA |
to Jimdaboe
Thats a lot o' power for a air compressor. |
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Jimdaboe join:2001-01-16 Corpus Christi, TX |
Thanks again. It's pushing 50 CFM. Running a lot of air tools!!! |
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PolarBear03The bear formerly known as aaron8301 Premium Member join:2005-01-03 |
Damn, can you post a pic of this beast? Sounds like the industrial compressors they use in the fruit packing plants around here (they run on 3-phase 440v). |
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Jimdaboe join:2001-01-16 Corpus Christi, TX TP-Link Archer C8 Linksys E2500
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1 edit
1 recommendation |
to Jimdaboe
I assume this is a commercial building with three-phase power available.
First question, is your service 240V or 208V?
Most commercial / industrial buildings have 208V, three-phase service. A 230V motor is designed to run on 240V power. 230V motors may not even work at all on 208V. You may have bought the wrong compressor, or may need a transformer to use it without having problems. They will draw more current than if on 240V. Check it out, and post back.
Second, what is the motor's starting load amps, SLA?
If it will work, and you have 208V, the FLA rating at 208V will be 46.2A. The conductors need to be rated at 125% of the motor's FLA:
46.2 x 125% = 57.75A
Assuming the run is relatively short, using THHN in conduit with 75 degree terminals requires #6 AWG copper wire and a #10 AWG ground. You will need 1" size conduit, and a flexible connection to the motor; either flexible metal conduit (Greenfield) or liquidtight flexible conduit (Sealtite).
With a general branch circuit breaker (inverse time delay), you can use between 100% to 150% (wound rotor) or 100% to 250% (synchronous motor) multiplied by the FLA current circuit breaker. That would be 60A (next higher size) to 80A (wound rotor) or to 125A (synchronous motor) circuit breaker using commonly available sizes. However, the #6 AWG THHN wire is only rated to 65A, and the circuit breaker must also protect the wire. So unless you are interested in oversizing the wire for any particular reason, you should us a 60A breaker as others have recommended.
One very important point: the NEC, articles 430.31 & 430.32 require overload devices to protect the motor and motor branch circuit conductors from excessive heating due to motor overloads & failure to start. This must be done by one of the following methods:
- A separate overload device sensitive to motor heating. If the motor is on its own branch circuit with no other equipment, than you should be covered. Depending on the ratings of the motor (see the NEC) this is less than either 115% or 125% of the FLA rating; 66A or 72A. Your 60A breaker will do this, provided it lets through enough current to start the motor. (I think it will assuming it works properly on 208V.)
- Thermal protector integral with the motor (may or may not have this from the manufacturer).
- Separate motor controller. These typically have a manual on & off push-buttons, and have a thermal element that you must specify. For 230 Volts, this is a #2 size starter, and 48.3Amp heater elements.
Your big issue is whether or not the motor will properly run on 208V (and ideally 5% less than nominal voltage - 197 V as your system may have voltage drops, or you can experience utility voltage sags). You should check with the manufacturer of the motor.
Questions:
What is the voltage available (240 or 208)?
Get the following from the motor's nameplate:
- Voltage? (May have two ratings shown as a 'slash value' like 230/460 or 208/240 for example.) Also give us the FLA if it has a 'slash' rating.
- rated temperature rise or insulation class?
- winding type
- thermally protected or not? (Would be marked 'thermally protected' or 'T.P.')
- Starting current (or SLA). May or may not be marked. |
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Jimdaboe join:2001-01-16 Corpus Christi, TX TP-Link Archer C8 Linksys E2500
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I'm on Christmas vacation so I don't have all the info. Voltage available 208 3 ph Motor has Multi Voltage rating ( a few terminal changes at the pecker head). 41 FLA at 208 V. SLA Unknown. Unknow rated temp & winding. Comes with a Simplex magnetic starter so start up current is probably limited. Separate branch circuit (currently #6 THHN , 50 AMp Breakers)
Thanks for all your input! Looks like I need to go up to 60 amp breakers. |
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telcotechIBEW 2222 Boston, MA Premium Member join:2004-09-02 united state |
A circuit breaker is designed to be loaded at 80% of rating. Thus, a 50 amp breaker is used on a load of 40 amps MAX. NOT 41 amps. As pointed out above, a 60 amp breaker is correct, being capable of sustaining a 48 amp load. The wire should be matched to the circuit breaker - in most cases a #6 copper type would good. The insulation on the #6 copper conductor along with the types of locations (temperatures & moisture levels, etc.) it passes through between the panel and the compressor determines its ampacity. The NEC (or local electrical code) should be consulted to determine what is correct for your application.
Don't be confused about the uses of 125% and 80% - they are basically the same - but viewed from two different directions - one from load the other from supply. An example:
A device with a nameplate of 80 amp load requires 80 amps x 125% = 100 amp breaker. A 100 amp breaker can supply 100 amps x 80% = 80 amps.
Hope this helps |
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Steve, you forgot one thing...
A circuit breaker, that is not rated for continuous use, is designed to be loaded up to 80% of its rating. Many panels and breakers are rated for continuous use, which allows their loading up to 100% of their ampacity rating. This is not normally the case for residential equipment, or products picked up at the local hardware store.
That being said, uses of 125% and 80% are not basically the same (if viewed from opposite directions). Even if the breakers are rated for continuous use, the NEC still requires the 125% ampacity rating of the breaker for motors (as in this case).
The insulation on the #6 copper conductor along with the types of locations (temperatures & moisture levels, etc.) it passes through between the panel and the compressor does determine its ampacity, but more specifically which column to use in the NEC ampacity tables. Don't forget, of course, the rating of the terminals, panelboards, and equipment housings, that the wire will be attached to at both ends. In most cases, the terminals will be rated for 75 degrees C, so the 90 degree ampacity column can not be used. If derating must be used, (for example high ambient temperatures, or multiple circuits in one conduit), then the derating is applied from the 90 degree column (for 90 degree wire such as THHN, etc.) |
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telcotechIBEW 2222 Boston, MA Premium Member join:2004-09-02 united state |
Thanks for the clarification. You are of course - correct.
You have done me one better than what I was trying to get across - that being that there is no "standard answer". EVERY situation is different. Just because your neighbor had his [device] installed with [abc] and [xyz] does NOT mean that is the way it should be done at your house.
I should have done exactly what you did, as I may have suggested that what I added was all that was needed to do the job correctly, which of course - is not true. Terminal temperature ratings, number of conductors in a raceway, insulation, etc., ALL come into play
Nice catch. |
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to Jimdaboe
I'm a bit rusty on t his all, but it's not 3 phase delta is it? If so, your screwed, as most utilities are "phasing" that out. |
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said by Anonuser:3 phase delta ... most utilities are "phasing" that out. Sorry, but I have no clue what you're talking about, Anonuser. Three phase delta has to be the most prevalant configuration supplied by utilities everywhere in the world. No need for utilities to incurr the expenses of running a fourth (the neutral), when all you need is a transformer (which is there anyway). |
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