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54067323

join:2012-09-25
Tuscaloosa, AL
reply to garys_2k

Re: How to calculate the amperage of paralleled conductors???

said by garys_2k:

Do you have two circuits that you need to parallel to run a single 56 A load (where the total of four conductors includes two "hots" and two neutrals, EACH being paralleled to two pairs of conductors will carry the total current)? Or, you have four circuits (four hots, four neutrals) that you will parallel to run this load?

i need about 50 amps so what we are doing at the feed end is to take as many number 10 wires as we can prove out (8 or more) and connecting them to a multi-tap which is connected to a 60 amp outlet on a distrobox, then at the other end we are pulling the wires out of junction boxs close to where we need the power chopping lose from their "loads" then using more multi-taps combining them to the inlet of another distrobox and from there the lighting is connected.

the existing wiring is tinned so it's easy to work with but the junction boxes are rusted out rat bottoms that I have had to use a cold chisel and a 5 pound sledge on to get the covers off.

so far we powered up two areas wth no problems.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to whizkid3

said by whizkid3:

2. There is no valid reason to parallel small conductors, aside from being too cheap to purchase the right size wire in the first place.

It's a valid reason when that is all there is to work with and there is no way to replace what is there.


nunya
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:12
Reviews:
·Charter
·voip.ms
·surpasshosting

1 recommendation

If it's temporary, you bring in PROPERLY SIZED and RATED temporary cables. What you are doing is probably more dangerous at sea than it would be on dry land.
Ever been on a ship that's been on fire? Let me tell you firsthand - it's a whole lot of no fun. Port or not.
--
If someone refers to herself / himself as a "guru", they probably aren't.



ropeguru
Premium
join:2001-01-25
Mechanicsville, VA

said by nunya:

If it's temporary, you bring in PROPERLY SIZED and RATED temporary cables. What you are doing is probably more dangerous at sea than it would be on dry land.
Ever been on a ship that's been on fire? Let me tell you firsthand - it's a whole lot of no fun. Port or not.

While I agree, somehow I think that there probably isn't much left on the ship to burn.

"A couple of weeks and then we blow the ship up."

Bob4
Account deleted

join:2012-07-22
New Jersey
Reviews:
·Optimum Online

2 recommendations

reply to 54067323

said by 54067323:

take as many number 10 wires as we can prove out

the junction boxes are rusted out rat bottoms that I have had to use a cold chisel and a 5 pound sledge on to get the covers off.

So you can't inspect the wires (you're just checking for basic continuity) and have no idea as to their condition, but you know the junction boxes are all rusted. And you're going to be putting 50 amps through them.

What could go wrong?


LazMan
Premium
join:2003-03-26
canada
reply to 54067323

Maybe it's just me, but fudging it with unknown wiring in unknown condition sounds like a lot more work and a lot more dangerous then running an appropriate chunk of Teck90 or some cam-lock cables to where you need to go...

Plus, as was mentioned, you've got to ask the right question to get the right answer...



Thespis
I'm not an actor, but I play one on TV.
Premium
join:2004-08-03
Keller, TX
reply to 54067323

Unfortunately, this kind of approach to wiring is not unusual on movie locations. That's usually nothing compared to the rigging...
In the theatre, we're a little more afraid of burning the place down.
--
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Pick two...



54067323

join:2012-09-25
Tuscaloosa, AL
reply to nunya

said by nunya:

If it's temporary, you bring in PROPERLY SIZED and RATED temporary cables.

i would love to do that but it is not possible given the constraints I am working under, my idea was to lay in stingers but the "safety" guy shot down that idea, now we are using the ships wiring which he has signed off on.

If you have a better idea on how to rewire a steel ship built in the 50's within three days I would love to here it.

Ever been on a ship that's been on fire? Let me tell you firsthand - it's a whole lot of no fun. Port or not.

Yea about 12 years ago I burnt a cigarette to the waterline, had a little fuel problem in the engine room something lit up and the suppression system went off killing the engines and about a minute later boom.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to ropeguru

said by ropeguru:

While I agree, somehow I think that there probably isn't much left on the ship to burn.

It’s pretty much steel on steel, the wiring is in steel conduit as is every junction box and everything is threadd together.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to Bob4

said by Bob4:

So you can't inspect the wires (you're just checking for basic continuity) and have no idea as to their condition, but you know the junction boxes are all rusted. And you're going to be putting 50 amps through them.

In the boxes we have opened the wiring looks good and megs out just fine.

What could go wrong?

Hopefully nothing


nunya
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:12
Reviews:
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reply to ropeguru

It isn't about the ship burning. The whole damn ship can burn and the hull will still float. It's about things like old fuel/oil, insulation, wires. It's the smoke that kills below deck, not flames.
--
If someone refers to herself / himself as a "guru", they probably aren't.



flex

@tmodns.net
reply to 54067323

Does nec not apply cause you are in internatiomal waters?

Movable cables need to flex. They are made with more and finer strands. And with softer insulation. Like welding cable. Their ampacity is different from stiff thhn etc house wire.

Ampacity partly based on the area of the strands. So the general rule applies - founling the diameter quadruples the area. And ampacity.

If something goes wrong and u didnt follow code you are liable. As is anyome else who helped you. And if you know code and still dont follow it you are liable for treble damages



54067323

join:2012-09-25
Tuscaloosa, AL

said by flex :

Does nec not apply cause you are in internatiomal waters?

Not really once off shore it will only be maybe a half mile out.

None the less after glossing over the code last night it appears we would be in compliance under 90.4 as the safety guy is an "insurance inspector" makinghim the authority having jurisdiction and he signed off on it.

Movable cables need to flex. They are made with more and finer strands. And with softer insulation. Like welding cable. Their ampacity is different from stiff thhn etc house wire.

that is why 99% of the cables I work with are 600v SJO and the single conductors are what I call welding cable because like welding cable they are real flexable and have some rather tough 600v insulation on them.

Stuff like this.



founling the diameter quadruples the area.

??


dandeman
Premium,MVM
join:2001-12-05
Chapel Hill, NC
Reviews:
·AT&T Southeast

2 edits

The problem will paralleling conductors is that in practical application, there's no assurance that the current will be divided evenly..

Unless you have engineering experience in very low impedance systems it will be hard to see why..

Same gauge, same length cables will have small differences in the resistance, as my work has involved, down in the micro-ohm range. If one length of cable turns out to be for example 400 micro-ohms, and another cable 100 micro-ohms, then ohms law says the 100 micro-ohm cable will carry the by ratio the brunt of the current.. Connector crimping and other small mechanical connection diffierences can contribute to these small variations. Even if you start out, evenly matched, corrosion and other degradation over time can cause differences to develop..

The only way to do this safely for fire protection would be to protect each cable with it's own breaker rated for the individual conductor, then if the current divide developed enough difference and started overloading one of the cables that c/b would trip.. then of course the load would shift to the other cables, likely causing progressive tripping of all the breakers and dumping the load..

I posted this just to explain the failure mechanism that could lead to conductor overload and fire only, and not as a justification or rationale to violate code.

Of course if it's an off shore, special application beyond what prescriptive code practices cover, then do, or get your own professional engineer to review it, sign off with his stamp on it rather than ask for opinions here......



flex

@tmodns.net
reply to 54067323

Thanks. I meant
doubling the diameter of a circle quadruples its area.

Please keep your ears open to the folks who from theory and experience are telling you not to do this. If u do it anyway be prepRed for worst case of redhot melting dripping bare wire. and to lowells - tie in and see god.

Plan b might be to suggest led lights instead of hmi



cdru
Go Colts
Premium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
reply to 54067323

said by 54067323:

None the less after glossing over the code last night it appears we would be in compliance under 90.4 as the safety guy is an "insurance inspector" makinghim the authority having jurisdiction and he signed off on it.

So you're using the NEC to try to justify that it's ok, but earlier you said that the work doesn't fall under the NEC.

Also, AHJ doesn't just work that way where if an "insurance inspector" says it's ok then everything is fine and dandy. For safety issues, usually AHJ is an individual that is granted authority by statue. E.g. electrical inspector, fire marshal, etc.

Bob4
Account deleted

join:2012-07-22
New Jersey
reply to dandeman

I not sure he knows if the cables are really the same length or not.



leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET

1 edit

Probably not, but there are a number of ways to determine ahead of time whether the individual wires selected for bundling have the same electrical characteristics.

That wouldwouldn't solve any code compliance issues but it does address safety concerns.

Other things that can be done to improve safety are using significant margins (e.g. calculated current carrying capacity more then 300% of design load) and 200% load test prior to the actual event (if possible doing thermal imaging of conduits and junction boxes).

Edit: oops, would => wouldn't
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flex

@bellsouth.net
reply to 54067323

also fyi

standard ampacity is for single wires in open air at standard room temperature. if the wires are in the hot sun and/or hot ambient and/or confined in conduit and/or bundled, each of those conditions needs a derating affect. the code book will specify.



54067323

join:2012-09-25
Tuscaloosa, AL
reply to cdru

said by cdru:

said by 54067323:

None the less after glossing over the code last night it appears we would be in compliance under 90.4 as the safety guy is an "insurance inspector" makinghim the authority having jurisdiction and he signed off on it.

So you're using the NEC to try to justify that it's ok, but earlier you said that the work doesn't fall under the NEC.

Na just pointing out something I never realized is possible, you see the majority of my work connects to services that are already "approved" or we bring our own power.

Though I do believe if it was connected to shore power then the NEC would apply and what we are doing would not be permitted as the local electrical inspection department would not go along.

Now I don’t get involved in that type of work as it is normally handled by the house electrician who works with the inspection department.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to dandeman

said by dandeman:

If one length of cable turns out to be for example 400 micro-ohms, and another cable 100 micro-ohms, then ohms law says the 100 micro-ohm cable will carry the by ratio the brunt of the current..

That makes a sense the current will always prefer the path of least resistance, but what might be the actual difference in the current carried by the other conductors?

As an example what I am doing, impedance aside, let’s take 60 amps divided by 4 wires equaling 15 amps per number 10 conductor and lets say I think those conductors are 100 feet long but unknown to me two of those conductors are 110 feet in length.

That stated at 120 volts how much of an additional burden might be placed upon the shorter conductors due to the additional resistance of the longer conductors?


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET

said by 54067323:

let's take 60 amps divided by 4 wires equaling 15 amps per number 10 conductor

Sorry, but that is exactly the kind of thinking that will get you into trouble. The current will only divide equally over the 4 wires if their electrical characteristics are identical (and in real life they are never truly identical).

In your example you will have 2 wires with 100 * R and 2 wires with 110 * R (where R is the resistance in Ohm per feet, about 0.001 for 10awg copper) in a parallel circuit. The voltage across that circuit is equal but due to the difference in resistance the current distributes unevenly.

This is all theoretical since you are ignoring the resistance of any splicing/bonding along the way. Combined resistance of the bonded circuit is 0.0262 Ohm resulting in a 1.571V drop at 60A current (the 120V supply is irrelevant, the result is the same at 12V or 240V).
The current through the 2 shorter wires is 15.71A each while the current through the 2 longer wires is 14.29A each. This small difference in current is to be expected since there is only a 10% difference in wire length in your example (resulting in an identical 10% difference in currents: 14.29 + 10% = 15.71).
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54067323

join:2012-09-25
Tuscaloosa, AL

2 edits

said by leibold:

The current through the 2 shorter wires is 15.71A each while the current through the 2 longer wires is 14.29A each. This small difference in current is to be expected since there is only a 10% difference in wire length in your example (resulting in an identical 10% difference in currents: 14.29 + 10% = 15.71).

Based on that do you think a tinned number 10 in good condition measuring out at nearly zero ohms end to end can or cannot safely carry the differental amperage?


mackey
Premium
join:2007-08-20
kudos:12
reply to 54067323

said by 54067323:

That makes a sense the current will always prefer the path of least resistance, but what might be the actual difference in the current carried by the other conductors?

When they ran the building I work in off a generator for a week while repairing the transformer vault, they paralleled 2 4/0 cables for each phase. Most of the pairs were fairly balanced, but one pair had about 325A on one cable but only about 100A on the other. The one that was carrying 325A was quite warm....

/M

garys_2k
Premium
join:2004-05-07
Farmington, MI
Reviews:
·Callcentric
·callwithus
reply to 54067323

It sure sounds like you're going to go this way, so if you do please check the amperage on each conductor while drawing a lower, test load (say, 100 watts). Any imbalance there should scale up (assuming the connections don't change) with the larger load and you'd at least know where to look for possible problems.



leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET
reply to 54067323

I'm sure that what you are planning to do can be done safely if it is done properly but comments like "nearly zero ohms" don't inspire confidence in me that you appreciate how different "nearly zero ohm" values can be.

The 100 micro Ohm and 400 micro Ohm example from dandeman See Profile is a great example: both values are "nearly zero Ohm" (less then 1/1000th of an Ohm) but the 2nd wire has 300% more resistance which means that the first wire will carry 300% more current! That is the kind of imbalance you should try to detect and avoid.

I would not use a simple multimeter in Ohm setting to measure the conductivity of wires (the resistance is too low and the precision insufficient). The test clips may have a higher resistance then the entire length of the wire being tested. There are special meters for conductivity (inverse of resistance) but I would probably determine the resistance by applying a constant current source (lab power supply; alternatively use constant voltage and fixed resistor) and measuring the voltage. This is a much better way to get accurate results (especially when using a high test current) since voltage is what multimeters measure best.

The test suggested by garys_2k See Profile is another way to achieve confidence that the wires will share the load somewhat equally (less precise but perhaps easier to perform with what you have available).
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Bob4
Account deleted

join:2012-07-22
New Jersey
reply to leibold

Another way to calculate it is:

100/420 * 60 = 14.29 amps

110/420 * 60 = 15.71 amps

(420 is the total length of the conductors)



54067323

join:2012-09-25
Tuscaloosa, AL

2 edits
reply to leibold

said by leibold:

I'm sure that what you are planning to do can be done safely if it is done properly but comments like "nearly zero ohms" don't inspire confidence in me that you appreciate how different "nearly zero ohm" values can be.

Oh I do but at the amperage levels versus the gauge of the conductors I am working with an ohm one way or the other is not a deal breaker.

As for the work I just got in from the job and it's done and tested and worked without a hitch with a maximum voltage drop on the longest run of 3.7 vac under full load. Thankfully this weekend didn't count on the calendar so we actually completed the work on time keeping my fanny intact and un-chewed.

FWIW the reason I was using a vom (an old fluke 77) was to ensure continuity, I've had it in the past where the tone from the fox would pass through a bad connection on a conductor that should ohm out at nearly zero yet it wouldn't pass any serious amount of current due to a burnt up splice, by ohming out the conductors I can be reasonably assured I have good copper to work with.

There are days though when I feel like this guy.




But for now all I need is a six pack on ice and some sleep.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to Bob4

said by Bob4:

Another way to calculate it is:

100/420 * 60 = 14.29 amps

110/420 * 60 = 15.71 amps

(420 is the total length of the conductors)

Either way it's within a safe amperage margin for number 10 copper.