Wire Guage

[Gummi Bear]

I realize you probably have quite a bit of training/education at this, I have not, and don't wish to argue, just to state what I read in the code. This appears to be a change in the 2005 code from previous versions (looking at the page markings), and this may be the cause of the misunderstanding. I too would think you would want to use actual nameplate numbers for calculating values but the code state specifically otherwise (as I read it). I have omitted text in exceptions 1 and 2 that is irrelevant to the discussion. Exception No. 3 could carry the day, but I suspect that most large compressors are not "listed appliances", being built of off the shelf component pieces. Note that none of this discussion pertains to portable, plug in compressors, but rather to hardwired compressors on dedicated branch circuits.

Note that 90.5(A) defines shall as mandatory.

2005 NEC
Article 430 - Motors, Motor Circuits, and Controllers
430.6(A)(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not in horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Exception No. 1: Multispeed motors …………………..

Exception No. 2: For equipment that employs a shaded pole or permanent-split capacitor-type fan or blower motor that is marked with the motor type…………

Exception No. 3: For a listed motor-operated appliance tha is marked with both motor horsepower and full-load current the motor full-load current marked on the nameplate of the appliance shall be used instead of the horsepower rating on the appliance nameplate to determine the ampacity or rating of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection and any separate overload protection.

You are correct. For a motor that is loose, or a part of a non UL (or other listing agency) appliance (machine, pump, conveyor or whatever) you do use the NEC Tables.

It's that listed part that is a source of confusion sometimes. What that's telling us, as the end installer, is that the machine has been engineered, and that these values given on the nameplate are what is necessary. Kind of a 'stamp of approval' if you will.

Good reference.

 

[Charles (in GA)]

And indeed, I went out and looked a second time at my compressor's data decal on the side of the tank and there was a circular CSA logo (which I missed the first time I looked), which is one of the "listing agencie's" logo. Mine is a Homey Despot Husky made by Campbell-Hausfeld. I'm willing to bet some brands are not listed. Anyone have an Eaton? Does the compressor data plate or decal (NOT the motor dataplate) have a listing agency logo?

The reference I cited in the NEC would also (technically) apply to any compressor that has been altered or modified, motor or pump replaced, etc, unless the parts were exact replacements for the original.

Well, I've beat this horse enough.......... thanks for the info..........

Charles

 

[ron in sc]

Well I got my compressor today and Charles was right the 32 amp figure I was given was not accurate. The plate on the motor says amps 20.6 and where it says usable at 208 it says 22.7 amps.

Also my unit has a magnetic starter, I don't know if that makes any difference as far as amps go.

I did use 8 gauge wire from the wall to the fusebox and I'll just go ahead and do 8 gauge to the magnetic starter too.

 

[z28toz06]

Found this on Eatons web site.

It takes approximately 5 amps per horsepower to make a horsepower--running horsepower, NOT starting horsepower. To be a true 5 HP motor, it takes at least 23 amps or it is not a 5 hp motor. They are giving you "Starting" HP which means nothing. It is not what you are actually getting. This motor is all copper windings with cast iron ends. This unit is wired in conduit and meets all NEMA Wiring Codes.

 

[ron in sc]

To be a true 5 HP motor, it takes at least 23 amps or it is not a 5 hp motor.

Really, if that's the case I didn't get what I though I did. Seriously though I think I did get a 5 hp. I don't know whether Eaton is the last word on motors and compressors. I did speak with them when I was shopping for a new compressor and they were very nice but their compressor pumps were made in China and I think their motors were too. Not saying Chinese compressors or motors are bad, just not my preference. Could it be that Chinese motors need more amps to make the same HP as a Baldor.

I have a Baldor L1430T. Baldor represents it as a 5 HP. It's full load amp rating is 20.6 Below is the link to their website which gives all the specific data on this motor.

http://www.baldor.com/products/specs.asp?1=1&catalog=L1430T&product=AC+Motors&family=General+Purpose|vw_ACMotors_GeneralPurpose&winding=36WGT077&rating=40C+AMB-CONT

 

[Gummi Bear]

You can't really break down horsepower by amperes, and expect that figure to hold for all given voltages.

Ohms Law.

Break it down to Watts, and then you can carry that over.


One Horsepower is equal to 746 Watts.

Here's a good explanation of what Horsepower really is:
http://www.web-cars.com/math/horsepower.html

 

[Gummi Bear]

Really, if that's the case I didn't get what I though I did. Seriously though I think I did get a 5 hp. I don't know whether Eaton is the last word on motors and compressors. I did speak with them when I was shopping for a new compressor and they were very nice but their compressor pumps were made in China and I think their motors were too. Not saying Chinese compressors or motors are bad, just not my preference. Could it be that Chinese motors need more amps to make the same HP as a Baldor.

I have a Baldor L1430T. Baldor represents it as a 5 HP. It's full load amp rating is 20.6 Below is the link to their website which gives all the specific data on this motor.

http://www.baldor.com/products/specs.asp?1=1&catalog=L1430T&product=AC+Motors&family=General+Purpose|vw_ACMotors_GeneralPurpose&winding=36WGT077&rating=40C+AMB-CONT

This is an interesting argument.

Can one 5 HP motor require more power to operate than another 5 HP motor at a given voltage?

Yes. It all comes down to efficiency. Tolerances, materials, construction method, bearings and even the climate can all affect how much power a motor needs to operate.

With your Chinese vs American debate, usually you'll get a better motor made here, just due to the tighter tolerances, better materials and better craftsmanship that goes into making it. The tradeoff: you pay the difference in dollars for that pride in craftsmanship.

Motors manufactured today, often times run significantly more efficent than motors made just 20 years ago. Technology has helped us out a lot there, bringing smarter design, and more precise construction methods. There is a lot of time, thought and effort that goes into a more efficient motor.

Companies that value production, are always looking for ways to cut production costs, and I've seen more than one work order to re-fit a machine with a more modern, higher efficiency motor.

DFW Airport just did a huge bunch of work like this a couple of years back, replacing a whole bunch of baggage conveyors with modern equipment. One of the guys in my Masters test prep class was working on that job. Apparently they had a ton of cool stuff to work with.

I've done re-fits in large industrial machine shops; food plants for mixers, pumps, and conveyors; a culvert manufacturing facility, and even a lead smeltering plant. It's neat taking an amp reading before and after.

 

[cc_rider]

I just browsed the thread, so maybe it was discussed already, but any motor-driven device is required to have an electrical disconnect within sight of the device (and some distance limitation also). A plug is considered a suitable disconnect for compressors, table saws, et al. Central HVAC compressors usually have a 'breaker box' on the wall where the feed comes out to the unit, but the purpose is the same: provide a quick way to verifiably disconnect power to the device, just in case some poor schmuck gets his hand caught in it. I'm pretty sure a wall switch doesn't qualify though.

Electricity is one area where it's hard to be TOO careful. We can usually just learn from our missteps, but some lessons on electricity don't allow second chances... Which is why most inspectors are hardcases when it comes to electricity, so when they go over your DIY wiring with a fine-toothed comb don't get all bent. I've seen electrical hack jobs that were disasters waiting to happen; it's a wonder more houses don't burn down!

Happy Wiring!

c.