Resistance is Futile

[Dennis Alwon]

I know when I ask these questions my brain does not do well with the answers yet I try again

First off I do not understand Table 9 in Chapter 9 at all. I am guessing that larger conductors tend to have a different resistance when installed in pvc conduit as opposed to steel conduit. Why?

What I started out trying to find out was the ampacity of a bare 12awg wire at 100'. I see Table 8 and it has the ohm/kft but that is for dc resistance-- not sure why that is different either.

I understand that temperature change affects the results. I found online that 12 awg is 1.588 ohms/kft so 100 feet would be .1588 ohms.

Now if I use ohms law I=V/R and I have a 120v circuit I get 755 amps.

Is this correct?

Lots of question- any help would be nice.

Sorry that ohm reading was at 20C

 

[George Stolz]

Table 9 deals with impedance, which would lead to a more accurate answer when dealing with AC.

Table 8 is usually close enough for my purposes, so I never look at Table 9.

 

[Dennis Alwon]

At 60C it is 1.932/kft = .1932

so 120/.1932 = 621 amps

 

[Dennis Alwon]

Table 9 deals with impedance, which would lead to a more accurate answer when dealing with AC.

Table 8 is usually close enough for my purposes, so I never look at Table 9.

Table 8 has a few questions for me--- The bare copper we buy is coated or uncoated-- I assume uncoated... Again Table 8 also has DC resistance-- how can you use that ??? Number are way different for 12 awg although the 1.93 is the same as what I looked up.

The problem I had was dc resistance vs table 9 that had ac resistance

 

[Smart $]

Table 8 specifies no conditions other than 75?C for the values. We have to assume this is in free air, but I have no idea what the ambient air temperature is... or perhaps it's simply measure when ambient is 75?C???

Table 9 specifies the same temperature but adds the conditions 3?, 60Hz, and 3 single conductors in conduit. These will vary the resistance from the DC value.

 

[Dennis Alwon]

Table 8 specifies no conditions other than 75?C for the values. We have to assume this is in free air, but I have no idea what the ambient air temperature is... or perhaps it's simply measure when ambient is 75?C???

Table 9 specifies the same temperature but adds the conditions 3?, 60Hz, and 3 single conductors in conduit. These will vary the resistance from the DC value.

Okay-- I get it but why DC value-- is there an ac measurement for ohms?

The tables I found online show 20C and 60c since Table 98 matches the 60C I suspect that is what it is.

So in free air can #12 really handle 621 amps-- The online shows 41 amps at 60C-- confused about this

 

[iwire]

Okay-- I get it but why DC value-- is there an ac measurement for ohms?

I am not expert by any means so I may learn something too but I don't believe there really is an AC resistance, if it is AC it is impedance and it seems every detail of circuit can change the impedance. From the raceway type to the layout of the conductors.

But I think most of us electricians do like George, treat it like DC and call it good enough. Engineers would go deeper and make it right.

 

[Dennis Alwon]

I am not expert by any means so I may learn something too but I don't believe there really is an AC resistance, if it is AC it is impedance and it seems every detail of circuit can change the impedance. From the raceway type to the layout of the conductors.

But I think most of us electricians do like George, treat it like DC and call it good enough. Engineers would go deeper and make it right.

Thanks-- that was my take but what about the use of ohms law and the discrepancy in amperage. Is ohms law not appropriate here? Why?

I don't think I ever used either of these tables

 

[iwire]

Thanks-- that was my take but what about the use of ohms law and the discrepancy in amperage. Is ohms law not appropriate here?

Strictly speaking I don't think Ohms laws is ever truley apropriate, or perhaps accuate with AC circuits. Ohms law is for DC.


Here is how Wikipedia describes impedance, I find it kind of interesting.

http://en.wikipedia.org/wiki/Electrical_impedance

Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.

In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude. When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.

It is necessary to introduce the concept of impedance in AC circuits because there are two additional impeding mechanisms to be taken into account besides the normal resistance of DC circuits: the induction of voltages in conductors self-induced by the magnetic fields of currents (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part.

I have to be honest, I do not understand how knowing the resistance or impedance of a conductor can lead us to the current carrying ability of a conductor.:?:huh:

Or am I misunderstanding what you are trying to do?

 

[Dennis Alwon]

Strictly speaking I don't think Ohms laws is ever truley apropriate, or perhaps accuate with AC circuits. Ohms law is for DC.


Here is how Wikipedia describes impedance, I find it kind of interesting.

http://en.wikipedia.org/wiki/Electrical_impedance




I have to be honest, I do not understand how knowing the resistance or impedance of a conductor can lead us to the current carrying ability of a conductor.:?:huh:

Or am I misunderstanding what you are trying to do?

That is what I asked-- did you check the site I posted above-- 2nd table http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity

 

[Dennis Alwon]

BTW-- the wiki site was a good read Thanks Bob

 

[GoldDigger]

Two differences for AC:
1. Reactive components, with inductance dominating for 50/60 Hz.
2. A higher resistive component because of skin effect. A higher percentage difference as the conductor gets larger.

Tapatalk!

 

[Smart $]

Okay-- I get it but why DC value-- is there an ac measurement for ohms?

The tables I found online show 20C and 60c since Table 98 matches the 60C I suspect that is what it is.

So in free air can #12 really handle 621 amps-- The online shows 41 amps at 60C-- confused about this

Table 8 specifically states the values are for 75?C.

What the conductor can handle in amperes is inappropriate for the application. With the temperature constrained, you would not be able to have any current on the conductor whatsoever, as that would raise the temperature and increase the resistance.

In practice, the ambient temp is lower, allowing current on the conductor to raise the temperature to the indicated value (75?C). Where the voltage exceeds an amount that would raise the conductor to that temperature, additional resistance or impedance (loads) must be added to the circuit. We use this principal in reverse.

 

[Dennis Alwon]

Table 8 specifically states the values are for 75?C.

What the conductor can handle in amperes is inappropriate for the application. With the temperature constrained, you would not be able to have any current on the conductor whatsoever, as that would raise the temperature and increase the resistance.

In practice, the ambient temp is lower, allowing current on the conductor to raise the temperature to the indicated value (75?C). Where the voltage exceeds an amount that would raise the conductor to that temperature, additional resistance or impedance (loads) must be added to the circuit. We use this principal in reverse.

So how does the table online give a value of 41 amps?

 

[winnie]

You have a bunch of different issues here.

1) Ampacity: this is the current that a wire can handle without overheating. Heat is mostly generated by current flowing in the wire, and the temperature of the wire is set by the equilibrium between heat generation, the heat generated by other wires nearby, and the heat being carried away to the surroundings.

The lower the resistance of a length of wire, the less heat generated by a given current flow. The better the heat dissipation of the wire (say free air, or blowing air) then the lower the temperature caused by a given amount of heat production.

The _length_ of the conductor does not change the ampacity. If you double the length of the wire, then you will double the resistance and double the heat production for a given current...but at the same time you double the area to dissipate the heat, so the ampacity remains the same.

2) Conductor resistance: this is what causes voltage drop. When you take a voltage and divide it by a resistance, then this gives you the current necessary to get that resistance to 'drop' the entire voltage. When you take your supply voltage and divide it by the wire resistance you get a current in amps that is basically your short circuit current; not the current that the wire can deliver to a load, but the current which will flow if the wire is the _only_ thing resisting the flow of current.

3) Reactance has to do with energy cyclically stored and released from AC circuit elements. The concept of reactance allows one to use Ohm's law equations for AC circuits involving inductors and capacitors, however reactance is a different physical effect than resistance. Resistance is all about energy lost in the system because of current flow, reactance is about energy stored in one part of the AC cycle and released during another part.

4) When current flows in a wire, a magnetic field is generated around the wire. This magnetic field stores energy. A wire has inductive reactance, where energy is stored in this magnetic field during part of the AC cycle, and then the collapsing magnetic field dumps energy back into the circuit. If you put this wire in a ferrous conduit, the inductive reactance will change because the magnetic properties of the conduit mean greater inductive reactance.

5) When AC current flows in a wire, the magnetic field tends to 'push' the electrons toward the surface of the wire. Effectively less of the cross section of the wire is available to carry current. This isn't really significant at 60Hz, except for _really large_ conductors. But it does mean that the _resistance_ of a wire will be higher for AC than DC.

6) The reactive impedance and skin effects are usually negligible except when you are figuring short circuit current.

-Jon

 

[iwire]

So how does the table online give a value of 41 amps?

I don't think the 41 amps listed has anything to do with the other values.

 

[Dennis Alwon]

I don't think the 41 amps listed has anything to do with the other values.

That's how I see after Jon's response.

Thanks Jon believe it or not that made sense to me

This is what I was actually looking for

2) Conductor resistance: this is what causes voltage drop. When you take a voltage and divide it by a resistance, then this gives you the current necessary to get that resistance to 'drop' the entire voltage. When you take your supply voltage and divide it by the wire resistance you get a current in amps that is basically your short circuit current; not the current that the wire can deliver to a load, but the current which will flow if the wire is the _only_ thing resisting the flow of current.

So hypothetically it may be possible to run 621 amps on the 12 awg if it is the wire is the only resistance? Is it physically possible to have that or someplace near it in the real world? Just curious-- has nothing to do with anything I am working on or plan to do...

 

[fmtjfw]

That's how I see after Jon's response.

Thanks Jon believe it or not that made sense to me

This is what I was actually looking for




So hypothetically it may be possible to run 621 amps on the 12 awg if it is the wire is the only resistance? Is it physically possible to have that or someplace near it in the real world? Just curious-- has nothing to do with anything I am working on or plan to do...

The short circuit ampacity of #10 copper wire is about 600A for 100 cycles with 90?C insulation. #12 wire would probably last less time.

 

[winnie]

So hypothetically it may be possible to run 621 amps on the 12 awg if it is the wire is the only resistance? Is it physically possible to have that or someplace near it in the real world? Just curious-- has nothing to do with anything I am working on or plan to do...

A 12AWG copper conductor couldn't carry 621 amps for any sort of extended period of time. See http://www.powerstream.com/wire-fusing-currents.htm for an estimate of the current which will cause a bare conductor in free air to melt.

On the other hand, the real world application of this sort of calculation is to determine what sort of current will flow in the event of a short circuit. In the event of a short circuit with 100 feet of 12AWG (50 feet out and back again), the current would be about 600A (give or take, depending on the actual supply voltage and the voltage drop in the transformers and feeders), which would last until the breaker trips.

-Jon

 

[GoldDigger]

Until the breaker trips or the wire melts. The latter would be a more credible option for service wiring direct from the POCO secondary than for customer wiring protected by at least a main breaker.

Tapatalk!