ampacitiy

[Anonymous]

We are installing a 3 phase 100 amp subpanel. The run is 130’. What size wire would we want to run and what size EMT. We will be pulling 5 wires, the 3 hots, one identified ground conductor and one ground. Thanks for your help in advance.

[Wgoodrich]

2 awg copper or 1/0 aluminum service entrance conductor are rated 100 amps. If you install 4 wires sized 2 awg THHN copper and the equipment grounding conductor sized 8 awg bare copper then you may install a 1 1/4" EMT conduit.

If you installed all five conductors as 2 awg copper which is not necessary but legal you would need 1 1/4" EMT conduit.

Actually you are allowed to use the EMT as the equipment grounding conductor to that nonservice rated panel.

Then again if your structure is of steel construction using steel I-beams and if both of your panels are bolted directly to your framing steel of that building then the steel of the building may be used as the equipment grounding path.

Also if you want to do the demand load of that nonservice rated panel you may reduce your neutral in ampacity to equal the neutral demand load of that subpanel but not smaller than the equipment grounding conductor required to serve that nonservice rated panel.

Hope this helps

Wg

[Anonymous]

Thanks for your quick reply. We had planned to use 1 ¼” EMT. I am curious. If you have time to answer a follow up question? This will be in a drop ceiling, so I looked at #3 wire in table 310-16 for thhn and got 110 amps which I derated for ambient temp of 96-104 which gave me a multiple of .91 X 110 or 100.1.
What did I miss? Distance?

[Wgoodrich]

You missed temperature limited ampacity requiring your conductors smaller than 1 awg to be required to use the 60 degree column regardless of the temp rating of your insulated conductor.

You are correct in using the higher temp rating to start your ampacity deration but you only concerned your self with ambient temp derating. Did you think of the more than 3 current carrying conductors in a raceway. In your case you would have four current carrying conductors in your raceway which would not make a lot of difference in your calculation but to be correct you would have to make a further reduction of 20% of ampacity due to table 310.15.B.2.A, but wanted to point out two derations must be considered and combined in ampacity deration calculations. This 4th current carrying conductor would be due to the 3 phase wye connection using the neutral. The neutral must be counted in your deration calculation per 310.15.B.2.A as a current carrying conductor. See Article 310.15.B.4.B and C

Then once you have done your calculation then you must consider 110.14.C requiring you to compare your ampacity deration calculation to the requirements of 110.14.C calling for you to use the 60 degree column for conductors smaller than 1 awg due to temperature limited rating. Then if your deration calculation answer when starting your deration using the actual temp rating of your conductor's column in table 310.16 must then be compared to the 60 degree column called for by 110.14.C. Whichever gives you the lowest ampacity rating is the ampacity rate you must use. See the following copied section of 110.14.C that may help.

COPIED SECTION OF 2002 NEC
110.14.(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(1) through (6).
(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60°C (140°F)
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used
(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors
(4) For motors marked with design letters B, C, D, or E, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.

The following link may help you being a chapter found in our Homewiringandmore.com web site concerning a explaination of ampacity deration calculations.

http://www.homewiringandmore.com/homewiringusa/2002/definitions/ampacity/amp.html

Hope this helps

Wg

[Anonymous]

This is just a thank you note. I don’t know why you would take so much time to help someone you don’t know but I appreciate it. Thanks for keeping us out of trouble and for the education. If you need a donation to keep this going please say so. This has been invaluable to us. I thank you again

[Wgoodrich]

George, Thank you for taking the time to show you noticed. Thanks also for the offer of donation. We have made a concerted effort to provide this forum and the homewiringandmore.com web site free to all so that they can learn and help themselves doitthemselves.

From time to time I have seen gracious offers of donating. While we appreciate the thought and at this time we have no method set up for those who want to donate. Don and I have been paying out of our pocket and seem to be doing fine. Maybe someday we will offer programs etc. on a side bar for sale or may even get to where we need financial support if these web sites and forums keeps growing but right now our intent is to help. Really haven't taken the time to think of money out of pocket yet. Been here a year or so and we believe we may have saved a life or two and maybe even promoted knowledge in the electrical industry to a certain extent. The thank you s do mean a lot to us. Also hearing from one who was here and was successful in their quest to get a job done safely and may even having done it themselves successfully kind of lights up our day.

Again thank you

Warren and Don

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