Wire Size for Service

I am running an electrical service to my new garage from the meter at the street (underground utility).

Details:

200 amp service
Underground utility
170 feet long
Trench with PVC conduit

Questions:

What gauge wire should I use (I figure I'll run THHN)?

How do you figure the voltage drop for a long run when sizing the wire?

Is there a good on-line resource for proper wire sizing?

Can I run phone and coax lines in the electrical conduit or should I use a seperate conduit?

Thanks

 

Table 310-15(b)(6) says 2/0 copper or 4/0 alum for 200A service. Do not run THHN underground, it is not rated for wet. Use a conductor that has THWN insulation.

Voltage drop formulae are around (reference not at my finger tip). 2/0 copper, 240V, 170Ft, 160A load is 2.4%.

Run separate conduit for communication stuff.

 

garageman, I must have missed this one. Here is a detailed breakdown of the voltage drop formula.

VD=(2 X K X Q X I X D)/CM for Single Phase

VD=(1.732 X K X Q X I X D)/CM for Three Phase

Where: VD= Voltage Drop in volts K is a constant - resistivity in (circular- mil)ft

K= 12.9 ohms for copper conductor, K= 21.9 ohms for aluminum conductor operating temperature of 75 degrees C.

Q= Alternating Current adjustment factor for skin affect. This factor is derived by taking alternating current resistance value and dividing it by direct current resistance value, normally it is applied only on conductors AWG 2/0 and larger. For higher than 60HZ frequency it must be to smaller size conductors. For 60HZ frequency, these values can be obtained from NEC Table #8 and #9.

I= Amperes of load, for motors it is full load current, not 125% of the full load current.

D= Distance. The distance is measured from the source to the load and it is only one way.

CM= is the cross section of the conductor measured in Circular Mils or conductor(diameter measured in 1/1000 of an inch units) Squared.

So, using your imformation:

VD = (2 X K X Q X I X D)/CM

VD = (2 X 12.9 X 0.990099 X 200 X 170) = 877,200

877,200/133100 = 6.59 volts dropped @ 170 ft(round off to 6.6 volts if you like)(133100 is CM value for 2/0 conductor)

(6.59/240) X 100% = 2.74% (under the 3% thats allowed)

(240-6.59) = 233.41 volts

NEC section 210-19 FPN #4, states: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3% at the farthest outlet of power, heating, lighting loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5%, will provide reasonable efficiency of operation. See Section 215.2 for voltage drop on feeder conductors.


Hope this helps!



Fred

 

is this service for your garage only, or for your house also? if it's for your garage only, 3/0 is the smallest size cu you can use. 310.15(B)(6) table is for dwellings; a garage-only service is not a dwelling service. also, if the wire is going to be in pvc the whole length, it won't be in wet conditions, so thhn is fine. however, most (maybe all) thhn is also thwn. check the wire covering.

some inspectors also find some ambiguity in 310.15(b)(6) text. the table is for "dwelling units," which is not always the same as a dwelling. remember, the nec dictates "minimum" standards. you can always exceed them, so 3/0 is best in this case, especially when you factor in the possibility of voltage drop, given the length of these conductors.

 

There does seem to be a varying interpretation for use of table 310.15.B.6.

In my area it is accepted that if the property is for residential use by Planning and Zonning then it is illegal to use the residential garage for anything other than a dwelling unit on a residential estate.

Some hold that the definition of dwelling units are residential houses only.

My opinion is that if it is on a residential estate then it would be allowed to be sized by table 310.15.B.6 If it is an agricultural setting with a house on a farm then the outbuildings would be considered as a commercial setting requiring table 210.16 to be used. Then there is the argument that the detahced garage is for the house not the farm. And so on and so on.

Wg

 

my opinion is, given the various ways to interpret this table, to just stick to 310-16. i have seen feeders to a garage panel, for example, have to follow 310-16. the table says "dwelling," but the text says "dwelling unit," in several places. the definitions section is very specific about what constitutes each. a dwelling is a building that contains a (or many) dwelling units. If one stuck to a strict interpretation even a single family home, a dwelling, might have to follow 310-16, since the service is for the dwelling (the home space + the garage, shed, etc.) and not just the dwelling unit. my request for clarification in the 2005 has already been submitted. maybe this one won't be denied. we'll see. i'll keep using 310-16. this will keep any inspector from trying to hardball something so ambiguous.

 

solipsist9: can you explain what you mean by "most (maybe all) THHN is also THWN. check the wire covering" ? thanks.

PS: condensation will cause the underground PVC conduit to eventually completely fill with water. And, yes, I know most everyone ignores this and doesn't use wire specifically rated for wet locations.

 

when you look at the labeling on most thhn wire used today, it says "thhn or thwn." again, i don't know if all thhn is manufactured this way since i never really bothered to check. the same wire can be used for either application, the difference being that one must apply the appropriate amperage and temperature rating, depending on the application. thwn has a slightly lower rating than thhn. however, i've seen some wire that also includes thwn-2 on the label, which is the same amperage and temperature as thhn, and my assumption is that anywhere you can use thwn, you can substitute thwn-2, which would allow you to get thwn protection with a 90 degree rating.
so even we assume the wire will be in pvc that may fill up with water, you're covered, albeit witha slightly lower rating. for example, if you ran #3 for a 100 amp service, as thhn or thwn-2 the wire is good for up to 110 amps; as thwn it's good for up to 100 amps (310-16). either way, you're fine.
we're probably splitting hairs here, and the only job where i ever had this become an issue was a government fcaility that made missiles. they had many engineers with obviously nothing else to do.

 

Thanks for the explanation - now I see what you mean. You're certainly right that if the wire is labelled either "THHN or THWN" it's legit for a wet location. I initially thought that by "check the wire covering" you meant there was some obscure marking, unknown to me, that thru some interpretation, meant it was also rated for a wet location. Also, I was in HD today, and you're right: All the single strand wire I looked at was labelled "THHN or THWN". (Don't know why I never noticed that before).

 

With regard to PVC underground, In Article 100, you will find the definition of location - Wet location to say that underground installations are considered wet, hence requiring the W in the insulation name.
Insulation rating/ ampacity: Even though a particular insulation is rated for 90 degrees, you must use the 75 degree column for circuits over 100A and the 60 degree colum for cirucits 100A and less. 90 degree column is for derating only.

 

although the nec "intends" for underground to mean in earth or conduit, some don't interpret it this way. i agree with your interpretation, though
i disagree with your interpretation on ampacity (although i admit that chicago does not follow the nec and the rules /interpretations may differ). 90 degree rating is allowed when the terminations are also rated for 90 degree, although most terminations are designed for 60 or 75, but this is changing. is there a reference you can point out?

 

solipsist9, I am curious, would you provide a list of examples where teminations that are listed and lables as being rated 90 degree C?

Wg

 

The termination temperature reference is NEC 1999 110-14(c).

WG, There are many terminations that have a 90 degree rating on their own, but are downgraded at 75 degree during UL testing. Almost every major manufacturer uses Ilsco or Burndy termination lugs which have 90 degree ratings (not tested as part of equipment) but when installed in switchboards or switchgear, there is always a label on the equipment stating that it is rated for 75 degree terminations max.
I do not belive any major manufacturer has attempted to re-list their equipment with a 90 degree lug and actually test at 90 degree termination capacity.

 

HandyRon I was aware that many manufacturers feel their terminals are good enough to be rated at 90 degree C. However when it comes to the NEC what the manufacturer thinks does not count, it is what the testing labs list and label that connector to be rated at. The rating must accompany either UL with a circle around the UL and an r bottom right corner or another signia by an different approved testing lab.

We need to be careful, I have seen electrical products on the market that had a UL but no circle or r accompanying that UL insignia. Without that circle or r the UL listing is not valid. Just a trick of some in the trade to bypass testing labs.

I just thought that solipsist9 knew of something new that I did not know. Still curious though what solipsist9 was talking about concering a connector listed and labeled as 90 degree C. Thought one would have shown on the market by now. Haven't seen one 90 degree rated yet though.

Wg

 

My point is that there are a lot that are UL listed at 90 degrees on their own, but not with equipment.
Check out http://www.ilsco.com/
The first one in my catalog that we specify on occasion is type AU (UL 486B 90º C Listed and is CSA certified for 600 Volts.)
The 90 degree rating doesn't get you anything when you use it to terminate on bus bar in a GE switchboard (for example), just 75 degree, since that is what GE tests the switchboard for!

 

I believe the 90 degree connections to be just around the corner, yet to my knowledge not smaller conductor is rated 90 degree C for devices etc but only 60 degree C for devices.

To my knowledge no connections are approved more than 75 degree C in any panelboard or disconnect.

Is there anything out there at this time opposite of the above statements?

Wg