Over rated wiring?

So we have this de-facto standard at work for internal installs:

480VAC 3 phase motors (most < 3 hp) - 10 or 12 AWG Black (some of our vendor installs use Brn/Org/Yel of the same AWG rating)
120VAC controls - 14 AWG red
120VAC nuetrals - 14 AWG white
24VDC controls - 16 AWG blue
Grounding - Equivelant AWG of above mentioned with Green wire

In almost all cases this wiring is way over rated for what it is doing. The main reason that I can see that this is the de-facto internal standard is because this is analagous to the standards that multiple vendors and electrical contractors install our equipment with.

So copper can get kinda pricey along with larger conduit and accessories to support the larger gauges... why does this seem to be so common? Voltage drop shouldn't be an issue because in almost every case (except on conveyor runs) the distance is less than 50 ft.

Anyone else in the field see this and ever wonder why? I can't seem to fathom why a 1/2 HP motor 20 feet from the cabinet needs 10 AWG wiring.

The other thing I don't get is that the electricians are so finicky about matching up the ampacity requirements for the power feed to the gauge wire they are running, but then once they're in the cabinet, it's all over-rated.

The other thing I don't get is that the electricians are so finicky about matching up the ampacity requirements for the power feed to the gauge wire they are running, but then once they're in the cabinet, it's all over-rated.

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Don't mean to be rude, but have you asked the sparkies these questions? And what are their reasons, if so. Maybe then comments could be offered.

I personally see no reason to use any larger wire, or any other component for that matter, than is necessary to safely, legally do the job. We consistently run 14 AWG to our 1 and 2HP 480VAC motors--and the panel wire controlling those motors is the same size.

Rube said:

Don't mean to be rude, but have you asked the sparkies these questions?

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Honestly it really isn't something I've put much thought into until the last few weeks. I've been setting up new and moving existing systems over the last month and it just kinda dawned on me. We pretty much handle all this internally except on very large projects, so I don't cross paths with the sparkies that often.

I build the control systems where I work and we are the end user. My latest system has three AB racks, 14 Microscan Barcode readers, 28 diverter/pushers of my own design, and close to 80 motors, 60 of which are VFD controlled. The panel has 4200 wires (11 miles) inside it with another 1000 wires (6 more miles) to the devices. I did not use one 10AWG wire to any control.

Your electricians should wire things the way you want them to unless you're jacking with the Code or CFS (Common Frigging Sense). See what their thoughts are next time you see them.

Thanks for the advice Mark. I guess since I'm finally in a role where I can make those calls, I should.

I know in my upcoming projects, I'm definately going to push much more conservative wiring and try to get it out of my crew's heads that higher voltage does not mean larger wiring is required.

monkeyhead said:

Thanks for the advice Mark. I guess since I'm finally in a role where I can make those calls, I should.

I know in my upcoming projects, I'm definately going to push much more conservative wiring and try to get it out of my crew's heads that higher voltage does not mean larger wiring is required.

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You have to consider more than the size when determining what is to be used. In general the wire used will be capable of the maximum current possible on that circuit....ie if the circuit is fused at 40 amps then the wire must be able to carry 40 amps even if the only item used in that circuit is a 1HP motor that draws 5 amps or less.

A 3PH 480 motor would carry maybe 5 amps but start current could be 30-40 amps or more, the use of at least 12 gauge is appropriate with motor loads. The guideline is 125% of full rated load but there are more issues to consider in a manufacturing plant.

Wire (just like fuses etc) may be rated for different voltages.
Temperature is also a consideration.

Basically the "sparkies" seem to be following NEC/NFPA guidelines even with wire color. I will state that depending on situation the motor leads could be 14AWG and the control wiring 18AWG...it just depends, if not designated specifically then "sparkies" will use the standard.

I stated above there could be other issues in a plant, especially if machinery is moved around alot. NOTE: This is my opinion/method from OBSERVATION over many years.
1. Its useless to run conduit smaller than 1/2" in most situations.
2. If using 1/2 conduit then run 12ga wire (minimum), for inductive or motor loads.
3. If machinery is moved constantly then attempt to "pattern" the conduit, from the supply, so it does not have to be removed, possibly boxed off to create a junction for a later date.
4. Document/label disconnects/breakers and conduit runs....this will always be needed.
5. Maintain the same size wire that is used in the control panel..ie if 16AWG is used for 24vdc then use 16AWG all thru the circuit especially maintaing the same color.

In general the idea is to develop a system that works for you and your location and always use that system. The "sparky" standard may cost a few dollars more but it is less likely to have a problem, if properly done, at a later date.

Six Reasons to Use Larger Wire

monkeyhead,

1. As Ron said, when powering motors, larger wire is often needed to handle the starting currents, and to match the circuit breaker or fuse sizes.

2. Larger wire is often a result of trying to minimize the number of different sizes that have to be stocked by the installer and the maintenance department.

3. Larger-than-needed wire will always handle the current, but smaller-than-needed will not. I am guilty of using larger sizes than required in some circumstances, where experience has shown the wisdom of using a larger size (air compressors and other heavily loaded motors).

4. If the plant is one where modifications and upgrades occur often, where equipment is changed out regularly, then the larger wires can result in a cost savings over time. Many times, I have been relieved to find that I needed to up-size the motor starter, and the old motor power wires were one size too large before, but now are just right. Tearing out the old wire and conduit and replacing with larger wire and conduit will cost a LOT more than the small original difference between larger and smaller wire.

5. Another factor is voltage drop due to the length of the wires. Where the runs exceed 200 feet, usually the wire must be one size larger at least.

6. For control circuits and instrumentation, very small wire CAN be used, but there are cases where the mechanical strength of the wire comes into play. Larger sizes are mechanically stronger, and will resist vibration more than the small sizes. I ran some #20 LAN network cables, attached to a wall in a building that had an overhead trolly crane. The wires broke several times over a period of 2 years, due I think to vibration caused by the crane movements. I finally replaced the #20 cable with a #14 cable and the problem went away. For this reason, many plants only use #14 control cable, even though the currents are low enough to use #16 or #18.

If the Job is large enough then the engineers get involved..after that its out of our hands!! I once had an engineer make me pull a 750MCM ground to ground a 200amp panel..Stupid..seeing as the building was only bonded with a 4 ought..But hey he gets paid the big bucks..

That being said i never run anythings less than 3/4" and #12..This is for expansion..Sure the #14 is good enough for a 10HP motor at 600v but what happens when something changes and that motor is upgraded to a 15Hp?..Then the #14 isn't big enough..You could just swap a motor and overloads or swap the motor, overloads, wire then whoops the pipe size is to small now..Have to run another conduit..

That is usually why we oversize certain things, Also there are other factors to consider..conduit fill for one..if the pipe has anymore than 3 current carrying conductors you then have to start derating the ampacity of the wire..I admit most of us cant be bothered with all the math so we oversize to be safe..

Color codes seem to be standard..red #14 or #16 for 120v control etc.etc..

Hope this clears a couple of things up for you..

D

The basic idea monkeyhead is the "sparkies" do this because its an old tried and true method....its better to oversize sometimes regardless of the extra cost...in the long run the extra cost may be less cost in the future.

Another reason larger diameter cores are used is enclosure type. I'm not sure how it is in the USA or Europe, but in Australia the copper core diameter needs to be increased if the cable is buried or enclosed in tight conduit, since the small amount of cable heating is magified by the lack of ventilation.

However, be aware practices vary widely around the world. Sites my company has worked on in the USA tend to use greatly oversized cables, often specifying a minimum size of 1mm2 (very hard to fit into tight terminals and valves!). In China, normal practice is to use normal core sizes but enormously thick insulation. In Europe, clients often require detailed cable schedules with motor FLA included so cabling contractors can select just the right cable for the job.

All of these seem to work, provided your electricians or installers are consistent (most countries have standards that are very conservative). I have certainly had more problems with poor terminations, non-existent core and cable labelling and random cable colours than overheated cores!

'sparkies' ...I hate that term...

Most electricians install equipment from a set of schematics that were drawn by an engineer. The oversizing must be a preference. When installing, I oversize on both conduit & # of wires, but not sizes. When adding common controls, I use either # 16 or #14 THHN stranded. For fractional HP motors, I use the same size/rating wire,color coded for low (BRB) and high (BOY). All other motors are sized at FLA & wire/starter/etc are sized accordingly.

The ampacity of a circuit is determined by the load. The wire is sized from the amperage needed by the load (+/- other factors). Other devices are sized from there. Oversizing the wire will not add any benefit if an overcurrent device fails. Instead, it tends to be a liability as the wire can withstand a greater # of amps before failing.

darrenj,

Where did you pull the ground from? Ideally it should go back to a grounding electrode of some type.

I once had an engineer make me pull a 750MCM ground to ground a 200amp panel..Stupid..seeing as the building was only bonded with a 4 ought..But hey he gets paid the big bucks..

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I am one of those engineers but I haven't made more than $35/hour (big bucks?) since 1992. My NEC code book says that a 200 Amp panel should have a #4 grounding electrode conductor if it is a "service entrance" panel, and a #6 if it is not a service entrance panel. Notice that the panel ground size doesn't depend on the size of the building steel "bond" (NEC Tables 250-66, 250-122).

This was from a sub panel (200amp 48VDC) to the main building ground bar..This bar was bonded to the main water pipe with a 4 Ought (The main building service was 800amp 600v

This is Major over kill..everyone including the engineer knew this..However i cant blame the engineer who told me to do it..This idea sprang from a High teck company here (Nortel)..They have there own spec..What happend is they had noise issues in one of there labs.. They tried to correct this by doing about a thousand things including a massive ground wire, The other things they tried were isolated oversised neutrals and whole buch of opther things that made sence..When they powered the lab back on the noise was gone..Now no one knew which of the one thousand things they had done actully fixed the problem so the "Nortel" spec was born..every lab they built after that one was built the same specs..

BTW it wasnt a personal dig at all engineers..Just some of them...

Lancie1 said:

darrenj,

Where did you pull the ground from? Ideally it should go back to a grounding electrode of some type.

I am one of those engineers but I haven't made more than $35/hour (big bucks?) since 1992. My NEC code book says that a 200 Amp panel should have a #4 grounding electrode conductor if it is a "service entrance" panel, and a #6 if it is not a service entrance panel. Notice that the panel ground size doesn't depend on the size of the building steel "bond" (NEC Tables 250-66, 250-122).

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One of the considerations for oversizing at that leve (14awg or so) is the physical strength of the wire. More able to stand up to physical punishment, so to speak.

rsdoran said:

The basic idea monkeyhead is the "sparkies" do this because its an old tried and true method....its better to oversize sometimes regardless of the extra cost...in the long run the extra cost may be less cost in the future.

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Valid point and I agree with you if there's the possibility of upsizing loads...

...but I covered that with my CFS remark. Make every effort to build properly and prevent changes that would require additional conduit or wiring. We took a long hard look at the conduit runs in our new building--for which I built the panel I spoke about in my earlier post. We DID use larger diameter conduit than was required. I happen to think most conduit runs in anything less than 3/4" is useless.

And Lancie1 said using oversize wire may keep electricians (is that better, ha-ha) from having to stock different wire sizes. I argue that my contractors should install the wire I need them to install and as long as I legal and safe, what do I care if they have to get rolls of 14 wire when they pick up their rolls of 12 wire?

I know most of this is personal taste, and CFS, but I'm paying the bill. I always take the advice mentioned by Ron and Lancie1 (is there a Lancie2?) before I spec the project to the electricians--look down the road and see what might happen. If needed, go bigger. If not, save your pennies, uh, copper.


On an unrelated note--What ever happened to Allen Nelson? Anyone know? His last activity was in February I think. I hope all is well.