ECN Forum Forums General Topics of the Trade General Discussion Area Sub panel wire size

Forgive me, our posts are overlapping so I am a little behind.

It is doubtful that the starting load of a number of residential garage motors will be acting in unison. In a commercial application you could have workers numbering anywhere from 2 to 5 to infinity but in the typical home shop you have one or two. Again, you need to do a load calc to have any idea what you will need and #2 Cu may be a better choice than #3 for some garages (especially ones 175' away!). One thing you need to realize is that some motors will blink the lights no matter what. Unless you have wired right to the ragged edge, using a #2 over a #3 may not help. The home panel may be overloaded or underwired. I have been called out to homes (wired by others) that experience a slight blinking of the lights when the a/c kicked in. All connections can be tight, relay contacts can be good, the load on the panel can be ESTREMELY small, the wire feeding the panel and the a/c can be large in diameter and short in distance but the problem persists. In those cases, a new a/c or a larger lateral from the power company MAY have solved the problem. The point is, oversizing a subpanel feeder is not a cure-all. A larger wire provided by the electrician does not somehow act as a capacitor for an extra burst of energy when needed (I realize you know this). Voltage drop in the subpanel feed is just one of many possible causes and may not factor in at all.

You have to draw a line somewhere and use some math skills rather than making a shot in the dark and hoping for the best. If you talk your customer into paying for a wire that is 1 or 2 or 3 sizes larger than another contractor quoted, and his lights blink anyway when the job is done, what is he going to think about your competence then. Without data to back-up your claims then you have nothing to fall back on. Like I said, from my experience, some loads will blink the lights no matter what actions you take (do a search here on a/c loads for verification).

...all those words and I don't think I have answered your question yet iwire...

Of course you need to add all the continuous and noncontinous loads together (here I am using the code definition of those words). The starting load of a motor is not a continuous or a noncontinuous load for use in calculations. For motors you need to use the full load current found in the codebook. No pc program can take into account: the condition of the motors involved, the load already on the main panel, how well that main panel was wired, etc..

I notice that the more I write, the more I ramble and the harder I am to understand. Thus I will sum up all my words in one sentence.

You can not assume a proper size panel without load calculations much less sizing the wires that feed it.

Triple, I think you are right, we are closer in opinion on this then I first assumed.

Here is the info we where given.


Sure it would be good to know if the panel is feeding a 100 watt lamp and a garage door opener, or a full machine shop, we don't.

Given that info, I will stand by 2 AWG, yes this could still be overkill or undersized, we do not know, but IMO based on the info given is what I would do.


Very true, on the flip side an undersized wire creates a restriction to the current available from the utility service.



Of course the panel can be loaded to 100%...non-contiuous.

If you do the calculations, the continuous loads on a 100 amp panel can be 80 amps, non-continuous loads can increase the load to 100 amps.

If this was not the case there would be no reason to figure non-continuous loads separately from continuous loads.

Anyway it looks like we both want to provide a good job.

Again, Just My Opinion, as voltage drop is not really a NEC issue that is all any of this discussion is.

Bob



[This message has been edited by iwire (edited 02-29-2004).]

December we did the same job. 4+ car garage fed off pool house about 200'. We used #2. Why? Because we get hit all the time with oh by the way we are going to have this. Car hoists, electric floor heat, finish the 2nd floor, tennis / basketball court lighting, furnace, A/C, electric gate, ???.

We only had a few breakers but we still used a 20 space panel. Why? Because we are ready to do more work for them. If the customer spends more than my home cost on a garage then they desirve more that NEC minimums. There is also the other side of the spectrem where a person is running an auto shop out of his back yard.

The added costs for us was little. Does that make us a bunch of hacks for not calculating the draw of only what is on the print? Are we bad for making the EMT and boxes too big for the wire used also?

Tom