Cable Size

What is the correct size of THHN wire to be used for a 50 amp service (breaker Feeding is 50 amp)? The run is about 109 feet and I only have a 1990 seris ugly's book to go by. The closest table I have found is # 8 wire at 55 amps but it says no more than 3 insulated conductors in a conduit.

This circit will have 3 hot conductors and a insulated ground conductor so 4? Will the # 8 wire still be good.

http://www.electrician2.com/

That website I have found to match mostly current NEC during the handful of times I have used it.

From that page there are a lot of useful calculator pages under the "Electrical Calculators" button.

http://www.electrician2.com/calculators/wireocpd_ver_1.html

I think only current carrying conductors are counted, so the ground would nto be counted in your case.

Tim,

i don't have my book with me, but here goes.
you don't tell us what the load is, motor, lights,???

#1 what load are you pulling - voltage drop matters. i try to keep it less than 3%.
#2 the key is current carrying conductors. sounds like you have a 3 phase system. if the load is balanced, you should be ok, the ground wire shouldn't carry load current.
#3 one thing you need to remember is the temperature rating of the wire versus the temperature rating of the terminal lugs you are connecting to. if you are using 90 degree c wire and the terminals are rated for 60 degree c, you must use the current rating of the wire for 60 degree c.

you might want to goto www.mikeholt.com - its an electrical website with a lot of good people willing to help.
regards,
james

Hi Tim,
The first piece of information you require is the Full Load Amps (FLA)that will be drawn by the equipment. Sometimes this will be stamped on the nameplate of the equipment or given in the electrical drawings. Often this is given as a kVA value which can be converted to amps using (Amps = 1000 × kVA / (√3 × volts)) for 3 phase system.
Considering that your feeder breaker is 50 Amps technically the FLA should be 40Amps max leaving 20% for temperature rise in the cable duct, etc.
You will need to consider voltage drop over such a distance as stated above.
Here is a fairly comprehensive link which covers all the factors.
http://www.openelectrical.org/wiki/index.php?title=Cable_Sizing_Calculation

Regards,
Donnacha

The closest table I have found is # 8 wire at 55 amps

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That is now the rating for 90-degree C rated wire and terminals. The best NEC bet is 75-degree C rated wire insulation of 50 Amps for 3-#8 conductors (ground is not "NEC conductor") in a raceway or cable. The voltage drop is only 0.25% at 480 volts, (0.49% at 240 volts)

A 50 amp circuit ran with THHN will take #8's and a #10 ground.

A 50 amp circuit ran with THHN will take #8's and a #10 ground.

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Unless it is aluminum wire, then you would need #6 conductors.

Thank you all for the assistance.I came up with # 8 from the uglys book but there was a table about derating for conductors over 3 and that was confusing me. So as I understand a insulated ground conductor is not figurd into that correct?

Lancie1 may I ask where you got the calculator used in your example? is it a website or something downloadable?

The full load continious will be about 34 amps.

Lancie1 what is meant by you calculation where it says 40 amp load about roght for 80% breaker? I don't understand the 80% breaker part?

Voltage Drop Calculator

Lancie1, may I ask where you got the calculator used in your example? is it a website or something downloadable?

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I wrote it and used it for years, and here is a copy of the Microsoft Excel spreadsheet, and you are welcome to use it. The screen is protected only to avoid accidently changing something that normally doesn't need changing, but has no password set, so if you want to modify or update it, simply unprotect the sheet and have at it! There are some of the resistances and reactances for smaller wires that I was never able to find, so if you run across those, feel free to fill in those "#NA" boxes and post the revised version here.

Lancie1, what is meant by your calculation where it says "40 amp load about right for 80% breaker"? I don't understand the 80% breaker part?

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Most standard thermal-magnetic circuit breakers are built with an overload trip range of 80% to 100% (they will trip somewhere between 80 and 100% of the manufacturer's label). That means that you can only safely load this type to 80% of the trip rating without having a possibility that the breaker will trip. So if you have a 50 Amp breaker, then 0.80 X 50 = 40 Amps, or the safe maximun load to avoid nuisance trips. There are 100%-trip-rated breakers but they usually are much more expensive than the standard off-the-shelf version. With your 34 Amp load, you are very safe with a 50 Amp 80%-trip breaker.

So as I understand a insulated ground conductor is not figured into that, correct?

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Yes, only current-carrying conductors have to be counted in the derating factors. A ground wire is never considered to be normally current-carrying, supposedly at 0 volts. I have seen a bunch of poor not-0-volts grounding where only 1 poor old skinny rod was used in sandy soil.

Awesome, Lancie, Thanks!

Lancie1


Thank you for the spreadsheet that will help me out a lot. I really appreciate it.


I think I have a clear understanding of the 100% and 80% breaker ratings now but I have 1 additional question to that.

Is there any specific wording or terms used to describe a 80% or 100% breaker when ordering?

Is there some specific terms I should be looking for to determine which kind of breaker it is?


I know you said most are 80% but I would just like to know a 100% unit when I see it in a catalog or something and not order that if not needed.


Also is there any specific area where you would use the 100% type vs going up to the next service size?