Wire oversizing

Originally Posted By: jmyers
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I have yet another question for all those that are electrically inclined.


You have #6 aluminum wire on a two pole 30 amp breaker. Is it ok to oversize the wire?

Joe Myers


Originally Posted By: dhartke
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I’m going to jump in here before the heavyweight sparkies and treat this like a quiz. Yes it is OK. The conductor is more than adequately protected. Maybe the equipment requires protection of only a 30 amp breaker.


Originally Posted By: Bob Badger
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Yes you can use a larger wire than required and we do this all the time to overcome voltage drop. icon_smile.gif


The limiting factor will be the terminals on the breaker, 6 is the top end for most 30 amp and below breakers and some are not made for larger than 10 AWG.

We have used 4 AWG copper for 20 amp roof outlets, and from 2 awg to 8 awg is often called for at parking lot lights, still protected by a 20 or 30 amp breaker.


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Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: Bob Badger
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Sorry David icon_redface.gif I am home from work sick and very bored, your post was not up when I started mine.


Bob


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Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: dhartke
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Bob,


Ok, so I was half right. Is voltage drop much of a problem in an average home? If so, to what equipment, under what circumstances?


Dave


Originally Posted By: Bob Badger
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dhartke wrote:

Ok, so I was half right.


Dave, I would not say you are half right, I would say you are entirely right ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif)

Quote:
Is voltage drop much of a problem in an average home? If so, to what equipment, under what circumstances?


A real rough rule of thumb is to add one wire size for each 100' of one way circuit distance.

So no, in most houses I do not think voltage drop is much of a problem.

My guess is something was changed in this house and as you said the piece of equipment only needed a 30 amp breaker. ![icon_smile.gif](upload://b6iczyK1ETUUqRUc4PAkX83GF2O.gif)

I only brought up Voltage drop as an example of when we oversize conductors.

Bob


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Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: dhartke
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Thanks Bob,


You're comments are always welcome. Hope you get to feeling better soon.
I have a similar problem. I'm sick of being bored and want to go to work.


Dave


Originally Posted By: jmyers
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Bob,


Does there ever come a time when the wire is too big?

I would have to assume that is why it was installed in the first place, although the run in not even close to 100 feet, probably about 40 to 50 feet tops.

Joe Myers


Originally Posted By: rmoore
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Joe…


At first I thought your question was purely theoretial, but your last post suggests that this was an actual situation. Still no problem with the wire size but what was the circuit serving? I could see this happening if perhaps an electric range had been replaced with a duel/fuel unit or it may just be some smart (read unusual) redundance in case a device is replaced with a larger unit later.

BTW...Bob B. Get well soon!


--
Richard Moore
Rest Assured Inspection Services
Seattle, WA
www.rainspect.com

Originally Posted By: Bob Badger
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jmyers wrote:
Bob,

Does there ever come a time when the wire is too big?


No, not from the eletrical standpoint, it is just a bigger pipe to move the electrons.

Just trying to get the larger conductors onto the equipment gets tough.

We will use a termination like this, it is crushed onto say a 4/0 condutor and has an offset pin that would be a smaller size like 2 AWG. (The size of the breaker could not exceed 2 AWG rating.

The offset is nessary to get all three conductors onto a breaker.

But for this I was thinking along the lines as this.

Quote:
I could see this happening if perhaps an electric range had been replaced with a duel/fuel unit


Bob

By the way, Thanks


--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: jmyers
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Thanks to everyone for their thoughts.


Bob B,

Here is the scenario. There was a two pole, 30 amp breaker servicing an air conditioning unit, running on #6 aluminum. After thinking about it for a while, I could not think of any codes which made it illegal or wrong.

The #6 would be rated for 40 amps which is one size larger than needed, given it was aluminum wire. The air conditioning unit was totally inaccessible so the name plate rating could not be determined. The wire run was relatively short, say 40 to 50 feet, so the only reason that I figured they could have done it would have been voltage drop. Guessing on that one!

Here is my concern. With the wire being oversized, would the breaker still trip at the rated amperage even though the wire size is increased? Meaning, if I take the #6 wire and put 35 amps through it continuously, will the 30 amp breaker still trip within the breaker amperage rating?

For all that matters, if I hook up 4/0 to the 30 amp breaker and put 35 amps through it continuously, will it still trip?

I was just thinking out loud and the brain is in overload mode today! ![icon_biggrin.gif](upload://iKNGSw3qcRIEmXySa8gItY6Gczg.gif)

Joe Myers


Originally Posted By: rmoore
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Joe…


It may help if you think of the breaker as a simple fuse. Doesn't matter what size the conductor is on either side*...it the amount of current or amps trying to squeeze through the small fuse wire that will blow it (or trip a breaker). This load is dictated by the draw of connected devices.

*Having said that, undersized conductors would cause some voltage drop and depending on the device**, could draw more amps. Once the conductors are up to a certain size, then any further increase in size is going to cause such miniscule variations in voltage that I think we could ignore their effects on breakers.

The way you put your question though doesn't even need the above explanation. If you put a fixed amperage (35 amps in this case) through that breaker then it will (or won't) trip the same regardless of conductor size.

I'm sure someone will put this better...

**More than willing to be corrected on this but it's my understanding that a lightbulb will just be dimmer at lower voltage with no increase in amperage, while an elctronically controlled motor, for instance, would require more amps to maintain the same RPM or torque.


--
Richard Moore
Rest Assured Inspection Services
Seattle, WA
www.rainspect.com

Originally Posted By: jmyers
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Richard,


That is exactly what I was getting at. I just wanted to make sure the breaker would trip at its given rating, independent of wire sizing.

Thanks.


Can you further explain the light bulb, motor load theory. It was always my understanding that lowering the voltage would lead to an increase in amperage. Hence given the fact that most commercial buildings are the higher voltages because if you were to wire everything 120 volt it would take equipment capable of thousands of amps to service it, versus hundreds of amps.

What that means is take a motor and wire it 120 volt and it will draw more amps than the same motor wired for 240 volt. The 240 volt motor will usually draw less amps.

Joe Myers


Originally Posted By: rmoore
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Quote:
Can you further explain the light bulb, motor load theory?


Joe?LOL?That is about the extent of my electrical theory. I also have a theory that electricity is really magic. I?m fairly good, and getting better every day thanks to forums like this, at knowing how to use it in homes safely, but I?m still somewhat mystified by the electrical beast itself. At the risk of getting myself in trouble, I?ll try to answer your question.

First of all?watts = amps x voltage. Watts is equivalent to energy produced or horsepower. You can get 120 watts for instance from 1 amp at 120 volts, or ? amp at 240. The higher the voltage for a given wattage (or horsepower) the less amps required.

If you wire an electric motor for 240, it will take half the amps that would be required at 120 to produce the same horsepower. Therefore you can use smaller conductors as it is amperage that determines required conductor size. A lot of motors are electronically controlled to turn at a given speed (we get into frequency here ? 60hz) and if the voltage drops a bit, are ?smart? enough to draw more amps to maintain that speed. Drop it too much and the motor starts to overheat because of excessive amperage.

A light bulb, despite it being a universal symbol for ideas, just isn?t that smart. The filament is designed to glow so much when a certain voltage is applied. Drop that voltage, as a dimmer switch does, and the energy produced, or watts also drops. No increase in amps.

Anything deeper than this I will leave to the experts.


--
Richard Moore
Rest Assured Inspection Services
Seattle, WA
www.rainspect.com

Originally Posted By: jmyers
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Richard,


Yeh, I know what you mean about that magic stuff. I have managed to weld a few things myself over the years. ![icon_biggrin.gif](upload://iKNGSw3qcRIEmXySa8gItY6Gczg.gif)

Joe Myers


Originally Posted By: Bob Badger
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Joe & Richard It sounds to me like you understand things pretty well. icon_cool.gif


Lamps, and electric heat are examples of resistive loads and those are very easy to calculate as Richard showed watts = amps x voltage.

When resistive loads are run under voltage the heat or light produced decreases along with the current.

Motors and transformers are examples of inductive loads and calculating those types of loads gets beyond me.

In a general sense if you reduce the voltage and the "work" being done stays the same the amps must go up.

So a motor with a constant load, run under voltage will draw more amps, overheat and fail if not provided with proper over current protection.

Quote:
most commercial buildings are the higher voltages because if you were to wire everything 120 volt it would take equipment capable of thousands of amps to service it, versus hundreds of amps.


Now you get right where I live, ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif) that is absolutely correct.

We would use much more copper if all the building loads where 120 volt and the services would be huge.

For comparison, a 3000 amp, 480 volt service = 2490 KW (2,490,000 watts)

The equivalent power from a 240/120 single phase service would be 10,375 amps.

As it is the services are large, office buildings we do now generally have 480/277 3 phase services from 1200 to 9000 amps (above 3000 amps it is usually multiple services)

We distribute the power through the building at 480 and put transformers all over the building to change the 480 into 208/120.

The lighting is run at 277 volts, at that voltage you can have many fixtures on a 20 amp circuit.

The HVAC equipment at 480 volt 3 phase.

Parking lot lights may run at 480 single phase. (This is a big help for voltage drop on the long circuit lengths)

Well I have rambled on enough,

Bob

Ok a little more, here is a typical 3000 amp 480 volt 3 phase service, the 3 sets of wires hanging down are paralleled 600 kcmils to feed a 1200 amp panel.

The service conductors coming in the bottom are 8 sets of 500 kcmil.



There is something unusual about the service conductors can anyone spot it?

This is a tough one. ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif)


--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: rmoore
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Bob…


Think I'll stick to residential. ![icon_smile.gif](upload://b6iczyK1ETUUqRUc4PAkX83GF2O.gif) What's the shovel for?

Joe...
Quote:
I have managed to weld a few things myself over the years.


Amazingly, I have never once done that, or been zapped, despite having done way more wiring and rewiring over the last 30 years than I should have as a non-electrician. It?s probably because I have a very healthy respect (make that morbid fear) for the invisible beast. I don?t trust breaker labeling and have always used a multi-meter to make damn sure everything is off before touching a bare wire. I still, very consciously, tuck my left hand behind my back when inspecting the interior of panels, lest it get a mind of its own and touch something. It?s not a paralyzing fear, but I am very, very, very careful.

Now excuse me?I have to go test a light bulb socket with a wet finger!


--
Richard Moore
Rest Assured Inspection Services
Seattle, WA
www.rainspect.com

Originally Posted By: roconnor
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Bob Badger wrote:
There is something unusual about the service conductors can anyone spot it?

Hmmmm ... is it a 3-phase Delta service?


--
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong

Originally Posted By: Bob Badger
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roconnor wrote:
Hmmmm ... is it a 3-phase Delta service?


I can see why you would say that, the neutral is hidden on the left side, but it is there. ![icon_smile.gif](upload://b6iczyK1ETUUqRUc4PAkX83GF2O.gif)

I am looking for something else and it is a tough one.

Here is a close up, sorry for the quality but it is a scan of a photo.




--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

Originally Posted By: jmyers
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Bob B,


You ever gotten whacked with the 500 or 600 mcm? HE HE HE, hurts like HE**!

I got cracked in the head enough with that stuff enough to learn to duck quicker!

Joe Myers