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Joined: Feb 2014
Posts: 3
New Member
Hello I am a 2nd year as an electrical apprentice that missed my last 2 weeks of school after my appendix ruptured. I am attempting to learn how to calculate voltage drop for a challenge exam. I only have a few examples and the other test questions were not a problem. Am I in the zone with this example ? Is there anywhere online I can get additional help ?

Find the copper wire size. Install a 240 volt 100 Amp panel 56 meters( 184 feet) away from a meter base. Ambient temperature nominal.Considering this is a service I am using a 5% voltage drop according to 8-102(b) and am unsure how to arrive at my wire size.

From using example (9) from Table D3 this is what I have come up with.
X= distance im meters to center of distribution
X x 5(%) x 1.00(DCF) x (240/120V)= distance
10X=distance Distance =10m
Using table D3 under the 200Amp column I have picked a 3/0 conductor ???

My next issue is how do I deal with Amperages or distances that exceed chart values ?

Thanks, your frustrated apprentice

Joined: Feb 2014
Posts: 3
New Member
I just answered my own question after reading an earlier post.
What do you in the event that the wires exceed 4/0

Joined: Jul 2007
Posts: 265
I cheated and used the app on my phone, I came up with only 1.12% drop using 3/0 conductors. This is calculated at a full 100 amps. My first move would be to determine actual load on the panel, then calculate from that as it would reduce wire size. As for the wire size exceeding 4/0 just go bigger, or run paralells. On paper it all looks good but in the field you will have a tough time finding a 100 amp panel with lugs big enough to accept a 3/0 wire. In your example #3 copper is the correct size for the specified ampacity, but it is a little over your drop spec at 5.67%, so looks like one size up would do it for you #2 comes in at 2.37%.


Life is tough, Life is tougher when you are stupid
Joined: Jun 2004
Posts: 1,273
For those unaware:

The industry does manufacture "reducing lugs"... fittings that have a female opening suitable for bumped up wire... with a male protrusion that fits into the factory issue panel lug.

They are an elegant solution as compared to trying to stuff a tap-whip into the target panel.

They are so uncommon that you'll have to ask for them and wait. They are not normally available straight off the shelf.

Additionally, some manufacturers have trick lugs that can be bought as a field installed kit. They achieve the same result.

Sometimes it's just cheaper -- and quicker -- to up-size the target bus and leave many positions blank.

One reason to up-size conductors is to shift from copper to aluminum. The price spread -- for a long feeder -- is so great that it justifies a bump in pipe and panel bussing.

I had one boss who took twenty-years to figure that out. He saved $6,000 -- net -- on one set of feeders -- and that was before the big run up in metals prices.

As the price of juice is politically structured to only go up, the last thing one should design for is parasitic losses. For most retail consumers, a 3% voltage drop is now un-economic.

The copper industry made exactly this point years and years ago. Bumping up from #12 to #10 commonly pays for itself in less than 30 months -- even quicker in California. (Astonishing!) The one assumption is that the conductors pass current most of the business day.

The NEC is the wrong standard to design to. It wastes too much energy. Hence, one should use it as merely a minimum standard. All recent D-B contracts I've seen specify that the EC must be at least a full bump up from NEC minimums -- for every homerun conductor. (ie #10 is an absolute minimum)

Joined: Mar 2004
Posts: 947
twh Offline
If the load is unknown, the demand is 80% (8-102(2)).

The max ED on a circuit or feeder is 3% (8-102(1)(c)).

If you exceed the table values, you made a mistake. The questions aren't that dirty.

Check your calculations here: . They aren't guaranteed correct but should point you in the right direction.

Joined: Feb 2014
Posts: 3
New Member
Hi thanks for the help. I wrote my challenge exam and it went smoothly. I have used a Burndy crimp that will reduce a aluminum wire size at the point of termination.

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