Dreaded Electrical Question

A few questions I have been pondering.

What size/type wire?
Lets use 8 gauge copper wire.
Per the chart it is rated at 40; 50; and 55 amps depending on what type wire you use.
THHN and THWN use 2 different charts.
So what chart do you use for dual rated THHN/THWN wire?

One can assume the weakest link in the wire is the connectors.
I would also want to assume that home owner use stuff sold at home centers and hardware stores is not rated for the 90 degree chart. So I can easily scratch 55 amps off the list above.
But how do I find out what the fuse box and electrical outlets are rated at and if I should use the 60 or 75 chart?
I was looking at a sub panel last night and there was no reference as to what temperature rating it met that I could see.

I believe THHN is for dry and THWN for wet locations.
Is PVC buried underground considered a wet or dry location?

Wire has voltage drop over long runs causing you to use a bigger wire.
Lets assume you run out 100 feet from the main to a sub panel.
Then out another 100 fee to a motor. Lets assume its a well pump.
Does the fact that the well motor is 200 feet from the main influence the wire size or do you just use the 100 foot legs to figure the wire size.
 
I'm no expert by any means, but I believe all wire jackets were raised to the 90 degree rating sometime in the eighties.

Also your wire should be sized according to the sub panel first, matching the wire size to the rating of the sub. Then the wire is protected by the breaker in the main panel. I don't believe a 100' run will have much effect on voltage drop. Now you size your conductors to the pump from the sub panel for the amperage rating of the pump and as before the breaker in the sub protects the wire to the pump.

This information is subject to review by the readers whom are more proficient at this than I
 
Chris English has pretty good info. I believe the voltage drop will be greatest, and important if the wire is loaded to its maximum ampacity. If it is a submersible, the depth is also added. Why push the issue. doing it once with a step up in wire size is cheaper than doing it twice with marginal wire. Jim
 
I am also no expert but years ago I put a sub panel in my shop and it was a 100 foot from main panel. I used service entrance copper good for 100 amps to the sub panel. I have an extension cord that is 60 foot long that I use for my stick welder (50 amp breaker).The extension cord was made up from the cord that run our 5 hp. silo unloader.
 
John, as most know Ive been retired from AC Power Distribution Engineering for yearsssssss and am rusty on the latest NEC so NO WARRANTY, but that being said here was my design approach back in my day:

1) I computed the maximum continuous load.

2) I sized the conductors to have a rated ampacity of at least 125% of the computed max continuous load.

3) I performed a voltage drop calculation based on amps and circuit length and if necessary, I upgraded the conductor size

4) I sized my Overcurrent Protection Device (fuse or breaker) to adequately protect the feed wires.

5) If I were feeding a sub panel (instead of say a single load) I sized the conductors with a matching minimum ampacity of
the sub panels rating.

I always considered buried PVC as a Wet or Damp Location..........I had always used THHN for dry and THWN for wet or damp locations HOWEVER my good friend and retired electrician Dusty MI says they now have the same specifications

6) AS far as Voltage Drop, I would consider the total length (Main to sub PLUS sub to motor) and upgrade the size if necessary to reduce voltage drop. THE MAIN CALCULATION concerns what the final voltage will be at the load once all the runs (main to sub plus sub to load) and size the wires accordingly.

7) If in doubt or even close, to be safe rather then sorry and to avoid having to do it twice and allow for additions, I would up the wire size to the next higher ampacity.

8) If in doubt as to the environment and temperatures and associated ampacity ratings (like your charts with 3 different ampacities), I simply used the lower ampacity wire rating BUT THAT WAS JUST ME, IT MAY WELL HAVE BEEN (probably was) OVERKILL


John T Too long retired Electrical Engineer so see what the other experienced professional electricians and engineers have to say !!!!!!!!!!!!!!!! IM RUSTY ON THIS STUFF
 
I'll try to answer your question one at a time.First question I'm not sure what chart you are referring to So, is this in an NEC code book?. Don't worry about rating of wirenuts ,subpanel , or receptacles unless it is speced.No temp ratings for panels.PVC buried being wet or dry is not considered in the code. Almost all pipes fill with water so don't knick the wires installing. For the last question ; you would calculate the voltage drop from the main to the sub-panel for those conductors [ in many cases this would not be calculated because it was not enough distance to be a concern,] In another calculation you would check VD from the sub to the pump .
 
OK lets get a little more specific.

A sub panel in a garage 80 feet from the main panel.
Buying panel with a selection of breakers I only see 100 and 125 amp boxes.
This is way over kill for what I need.

Lets assume all wire is rated at 90 degrees.
Since finding breakers in odd sizes like 55 or 65 is hard lets just stick with round numbers.

So if I run 8 gauge wire I can protect it with a 50 amp breaker in the main box.
That puts me in the 75 degree chart so that is fine with me.

While 50 amps should be enough I was thinking like some of you and run a little over kill.
To move up to 60 amps I need 6 gauge wire.
And here comes my problem.
6 gauge is rated at 55;65;75 depending on what temperature range you use.
So if I bump up to 60 amps I am still in the 75 degree chart.
Everything still good.

But if I am going to spend the money to run 6 gauge wire to a 100 amp rated box why not use a 70 amp breaker at the main box.
Problem is that puts me in the 90 degree chart.
And we now know all wire is rated for 90 degrees so that solves that problem.

But I still need to figure out what temperature rating the lugs in the sub panel and my main panel are rated at as these connectors are most likely my weakest link.

If I look at the breakers or sub panel I see things like "rated for 120/240 VAC and 10,000 AIR"
But I do not know what 10,000 AIR means
I see no reference to 75 or 90 degrees in the specifications.

This is all still up in the air and I can go any way.
Just trying to get a cost sheet of all supplies I need for my new garage/pole shed.
 
State inspectors here enforce along the lines of what you are thinking. The lugs in the panel are NOT ever rated 90?, therefore you cannot use the 90?rating for the final numbers. However you are allowed to use this number before making deduct tons for ambient temperature and box fill. Typically figure 40 amps for #8 copper, bit 30 amps for aluminum.
As far as the wire in PVC, it is very hard to find wire that is not dual rated THHN/THWN. When you do find it, it is usually in rated assemblies such as romex. Again the state, subject to local rules, inspectors here say that even of it is in conduit but in a wet location you need to use THWN rated wire. So I cannot run romex from the wall of my house through 2'of seal tite to an air conditioning disconnect, but have to go right into the back of the disconnect or a Junction box.
Is responsible see to the voltage drop, a rule of thumb is to step up a wire size for every 100 feet. There are voltage drop calculators on the internet to help find an exact number.
In short you can't go wrong figuring everything at the lower temp ratings and maybe consult locall authorities.
 
Thanks dr sportster
I think you answered my question while I was typing out my response above.

So if I stay with 8 gauge wire I will be limited to 50 amps.

But since boxes and breakers have no temperature rating I can use a 70 amp breaker by going with 6 gauge wire in a THHN or THWN2.

Here is the chart I was using.

a206323.jpg
 
What brand of breaker or panels are you looking at? Theredirect are PDF onto he Internet that list the temp ratings. Looking at the square d qo style lists which breakers are ratedoing for 60 or 75?c wire. The panels appeared to be rated for 75?c on the one that I looked at. Aread you planning to use single conductors or a romex style cable? There is a temp ratings difference between those also.
If you could list a brand and size we can help look.
 
I think the panels may be rated 10,000 AIR or 22,000 AIR but the power companies usually want 10,000 AIR for a residential panel. [AIR - arc interupter rating - not even really sure . This is a spec I have not payed much attention to.]. #6 is probably a good choice. 8 is a bit small.
 
Are you thinking of "AIC"... Ampere Interrupting Capacity?

AIC rating = "Breaking capacity or interrupting rating is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration." (WIKI)
 
John, you ask " If I look at the breakers or sub panel I see things like "rated for 120/240 VAC and 10,000 AIR"
But I do not know what 10,000 AIR means

At our facility I had occasion to deal with that issue. Breakers are rated according to the maximum level of current they can interrupt and that's referred to as the interrupting rating or Ampere Interrupting Rating (AIR).

In order to determine just what AIR we required, we had to perform a complicated set of calculations to determine the absolute maximum short circuit current that could be supplied from whatever substation that supplied service to the building. Even at the first building closest to a sub station we found the max short circuit current was way under 8,000 amps. THEREFORE the standard AIC rating of 10,000 amperes breakers were just fine. If you are located close to a sub station with big enough wire of low enough impedance there could be more then 10,000 amps of short circuit current available and you would need a higher AIC breaker, but at our huge military industrial complex NOWHERE did our short circuit current capacity exceed 10,000 amps.

Without going into all the exact details, Id rather err on the side of safety and allow room for future expansion then to design a marginal distribution system BUT HEY ITS YOUR MONEY AND DECISION NOT OURS TO MAKE

John T Too long retired EE
 
I have not really got down to a specific box.
That is why I was asking questions. To narrow down my choices.
Since this is going in a unconditioned pole shed I assume a outdoor box would be best. While the shed will be water tight it will have a dirt floor so I assume a indoor box will not work.
Something that gives me several spaces for breakers.
And something where I can separate ground from neutral.

I was going to use thwn in pvc from my main box to the shed.

Just trying to get all this straight in my head before I go talk to the parish inspector.
I really do not want to look like a fool that is unprepared to do this job myself in front of him.

So lets just pick one.
Tell me if this box meets the above.
Square D 16 Circuit 8 Space 125 Amp Main Lug Convertible Load Center
 
Ok. Thanks. The outdoor (NEMA 3R) or better panels pretty much have the same gits but everything on the housing overlaps so the rain runs off with out getting in, so I would hope with an indoor rated panel personally bit an outdoor one wouldn't hurt IMHO. Like someone else said 8 may work but 6 is a better idea. I don't have my code book In front of me but I'm not sure that I would go over 60 amps for a 6 copper. As far as breakers they don't make a 55, 65, 75, 85 etc breaker. Anything over 50 amps goes in 10 amp increments. The 70 amp box should work fine, just usually more limited on breaker spaces. John T can probably tell you better on the wire amperage (and probably a lot more) than I can off hand, but I know that "here" we cannot use the 90?c rating unless for figuring numbers.
 
" we had to perform a complictaed set of calculations" Oh yes , I remember that formula from school . Why we wire jerkers had to know that I can 't figure out but boy did I struggle with the fault current calculations. That one is better left for you engineers. A lot of math for something you would never calculate later in the field.
 
OK Greg.
Thank you very much for your time.

So a 60 amp breaker in the main box
(4) 6 gauge THWN wires run in 3/4 or larger gray PVC
And a outdoor box with the neutral isolated from ground in the shed.

I will ask the inspector about the use of a indoor box and see what he says.
 
Dr, those calculations were indeed very complex and engineering difficult, we had to know all the wire sizes, resistance, and length of all the runs and the impedances of transformers and a bunch of other stuff I cant remember, but it was a relief to discover nowhere on our huge facility did we get anywhere 10,000 amps of short circuit current capacity so the standard 10,000 AIC breakers sufficed.

John T
 
John , I do believe there was some copying of the homework from the guys that got it to those who really struggled with that one.
 
John , I don't have my codebook right here but 3/4 will annoy you pulling 4 6s . Use at least 1 inch . I will check the book when I find it . Pipe sizes in the code are tight to begin with. It may allow that but it won't pull in well.
 

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