Yet ONE MORE wire size/ampacity question

AudubonB · Mar 8, 2015

I'm almost apologetic (I'm never apologetic but I thought it would be politic to claim I almost am...) to begin yet another thread on wiring, but here we go with my own situation.

What's slightly different is that the run I am considering dealing with is phenomenally short. We're looking at hardly two feet from the breaker location on the main service panel through to a perfect site for an HPWC on a garage interior wall. So I upped that by 50 percent to err on the safe side.

As follows: from the ampacity tables I have then, under the conditions of:

* 240 V single-phase
* Cu wire
* 3' one-way distance
* 80 amps continuous/maximum draw

we will get voltage drops and nominal end voltage (given perfect 240V starting conditions) of:

3ga 0.04% or 239.903V
4ga 0.05% or 239.877V
6ga 0.08% or 239.805V

Further consideration: the cross-sectional diameter of the three gauges differ as follows:

6 ga = unity
4 ga: 59% bigger than 6 ga
3 ga: 100% bigger than 6ga; 26% larger than 4 ga

So......can anyone come up with a reasonable answer as to why I shouldn't make use of the far, far more tractable 6 ga than the alternatives?

dirkhh · Mar 8, 2015

Because the NEC ampacity of 6ga is only 75A even at 90°C insulation?
(not sure, is this a trick question?)

AudubonB · Mar 8, 2015

Well, Dirk, I can't tell if it's a trick Q or not...and I made up the question!

You see, from the NEC guidelines, your response is right: it allows only up to 75 amps....regardless, it appears, of the distance of the run. Yet from the data I posted, the loss (equals, more or less correctly, the heat build-up) over a three-foot run is absolutely negligible. At 0.195 volts, such loss is far less than noise in the system. So perhaps if I pose the question as follows, it would be more appropriate:

* can there be any dangers brought upon solely by the choice of 6ga rather than 3ga in the set-up as proposed?

deonb · Mar 8, 2015

AudubonB said:
* can there be any dangers brought upon solely by the choice of 6ga rather than 3ga in the set-up as proposed?

Maybe not, but let me ask you this way:

Knowing that there have been previous Tesla fires in garages, how sound will you sleep at night with the knowledge that if something happens and you file an insurance claim, the first thing they'll look at is whether the wiring was rated for the load?

Is that not worth the extra hassle and a few dollars?

Griffinlair · Mar 8, 2015

THHN #3 is about $1.79/ft, THHN #6 is $0.87/ft. Even assuming #3 for the ground, the extra cost is less than $10 including tax. Stranded copper wire is not that hard to run.

AudubonB · Mar 8, 2015

Have you tried to create with 3ga that S-curve inside the HPWC needed to cram those puppies into their home? Now, I haven't, but I've looked into it and my recollection is others haven't kind words about it. It's that - not the few dollars - that prompt the question.

And in my own and probably NOBODY else's situation, Deonb, homeowner's insurance is rarer than white-and-gold (or is it blue-and-black) angels in our part of the world. It simply doesn't exist. 'Course, that means we haven't that useless premium to shell out, either. A dozen or so years ago we had a structure catch fire (guest in our sauna). Fire truck (Alaska State Forestry) showed up five hours later - long after I'd run my bulldozer through the blaze to move the conflagration away from some other buildings.

It's different out here......

Griffinlair · Mar 8, 2015

I didn't run the wire myself. My electrician did, so i don't know how difficult it is for the "S" in the HPWC. I'm not sure of the space inside the HPWC, would a loop be easier? Anyway, as I'm a bit more conservative and my run was significantly longer, i ended up over sizing the wire instead of under sizing. BE careful if you go down this path.

AudubonB · Mar 8, 2015

You're right and I'm wrong: a loop, not an S-curve. Thanks.

Cottonwood · Mar 8, 2015

AudubonB said:
Further consideration: the cross-sectional diameter of the three gauges differ as follows:

6 ga = unity
4 ga: 59% bigger than 6 ga
3 ga: 100% bigger than 6ga; 26% larger than 4 ga

From American wire gauge - Wikipedia:

Gauge	Diameter (in)	Ratio to #6
6	0.1620	100%
4	0.2043	126%
3	0.2294	142%

My guess is that you were looking at area rations and not diameter ratios.

Moonwick · Mar 8, 2015

IANAEE, but I'd be afraid of running 80A over any length of 6ga. The voltage drop isn't the only thing to take into account; you may only be dissipating (0.195v * 80a) = 15.6 watts over that run of 6ga, but that's 15.6 watts over only 3 feet. 5 watts per foot for 4-5 hours straight is quite a lot of heat, especially with much of that wire within an HPWC, which seem to have a reputation for getting pretty warm under normal installations.

dirkhh · Mar 8, 2015

Moonwick said:
IANAEE, but I'd be afraid of running 80A over any length of 6ga. The voltage drop isn't the only thing to take into account; you may only be dissipating (0.195v * 80a) = 15.6 watts over that run of 6ga, but that's 15.6 watts over only 3 feet. 5 watts per foot for 4-5 hours straight is quite a lot of heat, especially with much of that wire within an HPWC, which seem to have a reputation for getting pretty warm under normal installations.

IANAEE either, but it's my understanding that it's this calculation that drives the temperature rating / ampacity rating of cable to begin with...

AudubonB · Mar 8, 2015

I was: cross-sectional area. But when you're trying to mash wires in and around, I suppose your diameter is the relevant number.

kennybobby · Mar 8, 2015

How about doubling up on the 6 or use a 6 and an 8 in parallel to make it easier to loop plus have enough copper for the amps? Maybe two 7 gage? Or three 8 gage? Plenty of ways to skin that cat...

Lloyd · Mar 8, 2015

Moonwick said:
IANAEE, but I'd be afraid of running 80A over any length of 6ga. The voltage drop isn't the only thing to take into account; you may only be dissipating (0.195v * 80a) = 15.6 watts over that run of 6ga, but that's 15.6 watts over only 3 feet. 5 watts per foot for 4-5 hours straight is quite a lot of heat, especially with much of that wire within an HPWC, which seem to have a reputation for getting pretty warm under normal installations.

Ding Ding Ding Ding!!!! Right answer!!! :tongue:

- - - Updated - - -

kennybobby said:
How about doubling up on the 6 or use a 6 and an 8 in parallel to make it easier to loop plus have enough copper for the amps? Maybe two 7 gage? Or three 8 gage? Plenty of ways to skin that cat...

Parallel circuits are not allowed at 240V. Problem is if one connection is not made properly, heat can generate and not allow the breaker to release properly and your house burns.

kennybobby · Mar 8, 2015

Lloyd said:
...
Parallel circuits are not allowed at 240V. Problem is if one connection is not made properly, heat can generate and not allow the breaker to release properly and your house burns.

Well what is a piece of 3 gage stranded wire anyway?--it's just a bunch of tiny gage (30?) wires in parallel. Or get some easy to loop welding cable, which is even smaller gage wires in parallel. Using parallel wires is not a parallel branch circuit, it's just a way to get sufficient copper in an easy-to-loop manner. Electric motors are sometimes wound using parallel conductors e.g. 5-in-hand, for just this very reason.

billarnett · Mar 8, 2015

AudubonB said:
... the run I am considering dealing with is phenomenally short. We're looking at hardly two feet from the breaker location on the main service panel through to a perfect site for an HPWC on a garage interior wall...

I have an almost identical situation. My "run" is less than a foot: the HPWC is mounted directly behind the breaker panel, the wire has only to run the distance thru the wall plus a little bit inside the HPWC and the breaker panel.

I used #3. I was indeed a pain. But much less of a pain than the #2 I had for my Roadster's HPC.

Lloyd · Mar 8, 2015

From IAEC:

High ampacity services and feeders are often installed with conductors in parallel to reduce pulling tensions and for easier handling. I’m sure you are already aware that a long list of conditions comes with permission to parallel conductors. This article reviews the requirements of Rule 12-108 Conductors in Parallel along with some significant changes for such installations now provided in the 2012 Canadian Electrical Code.
Rule 12-108 specifies that, except for neutrals, control and instrumentation circuits, parallel conductors must not be smaller than 1/0 AWG copper or aluminum. No doubt this requirement is designed to limit use of parallel conductors to such circumstances where this wiring method is truly needed. We already know, the rule contains numerous precautions to ensure conductors in parallel are loaded as equally as possible, to prevent unbalanced loading, overheating and subsequent failures.

To ensure that conductors are loaded evenly, Rule 12-108requires that parallel conductors must have the same characteristics including identical sizes, types of insulation, methods of termination, wiring materials, lengths, orientation and without any in-line splices. Appendix B shows us the required conductor configurations. These special arrangements are designed to minimize differences in inductive reactance and sharing of load currents. The wire and cable manufacturer should be consulted if it becomes necessary to employ conductor arrangements different from those given in Appendix B.

Furthermore, Rule 12-904 requires that when parallel conductors are in cables or raceways, each cable or raceway must contain an equal number of conductors from each phase and the neutral. Each cable or raceway must also be of the same material and physical characteristics to ensure that conductor impedance differences are minimized. There are some very good reasons for this requirement. Should we for example, attempt to install parallel conductors in different conduit types (say PVC and steel), we would find unequal loading in the paralleled conductors for the reasons discussed above.

But now new challenges await us with changes to this rule. As you are by now aware, some of our earlier expectations are now being tested as the 2012 Canadian Electrical Code has made two important modifications to the above requirements:

Sub-rule (2) specifies that a single splice in each parallel conductor is permitted to meet the requirements of Rule 4-006 Temperature Limitation. You will recall that this rule requires that: “where equipment is marked with a maximum termination temperature, the maximum allowable ampacity of the conductor shall be based on the corresponding temperature column from Tables 1, 2, 3 and 4.” You will also recall that this rule applies to both ends of the conductor. If for example we are using 90° C rated wiring to connect to circuit-breakers marked with a maximum 75° C temperature rating, we would normally base our conductor ampacities on the 75° C columns of Tables 1 to 4. However, this newly minted change also allows us to splice on a larger wire size to meet the maximum wiring connection temperatures specified by Rule 4-006, thereby permitting use of the 90° C temperature rating. Although permissible, it’s still not a great idea, as each splice is a potentially weak link.

Sub-rule (3) specifies that: “in parallel sets, conductors of one phase, polarity or grounded circuit conductor shall not be required to have the same characteristics as those of another phase, polarity or grounded circuit conductor.” This opens the door for parallel conductors of one phase to be of a different wire size, material (copper or aluminum), length, insulation type and termination method as long as the parallel conductors of each phase have the same characteristics. I suspect that this change was designed to make things easier for repairs rather than for the initial installations.

AudubonB · Mar 8, 2015

Thanks, all!

Last poll - for now - it appears that fewer headaches and colorful language have emanated from those who opted for the lhs knockout on the HPWC than from those who used the rear knockout. Any comments?

Cottonwood · Mar 9, 2015

AudubonB said:
Further consideration: the cross-sectional diameter of the three gauges differ as follows:

AudubonB said:
I was: cross-sectional area. But when you're trying to mash wires in and around, I suppose your diameter is the relevant number.

No worries. I know how easy it is to think one thing and write another...

As many others have said, it's the NEC that applies in most U.S. jurisdictions. Those Ampacity tables are determined for the worst of worst situations to be safe. Therefore, for a more typical situation, they have a lot of margin, but the NEC is effectively the law for home wiring.

FlasherZ · Mar 12, 2015

For what it's worth, the HPWC cable itself uses a #6 conductor at 105 degC rating. However, it's an open-air cable and isn't enclosed within any raceways or enclosed walls. The NEC's articles take into account the ability to dissipate heat in raceways and risers, perhaps next to steam pipes and other stuff.

Now, that said, you could try your hand at the formula in 310.15(C), but it's a pain in the ass and I have never seen anyone really use it...

As Cottonwood noted - it's not just effectively the law, in most jurisdictions it IS the law. Some jurisdictions actually call it a misdemeanor (more than just a minor cited penalty) to willingly and knowingly violate it. So a couple feet of #3 or #2 is probably best. I can't stop you from assuming the risk, but would just point out it's easier and greater peace of mind to follow the law.

Yet ONE MORE wire size/ampacity question

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