600 C-9 Bulbs

We are putting light bulbs on a large outside tree in a city park. We have six 100 foot strings of 100 C-9 lights. We have two 20 amp GFI
circuits available. I plan to put 300 lights on each circuit. My math
shows 2300 watts available and C-9 bulbs use 7 watts. This comes to 2100 watts. I will use 12 guage extension cords. The light strings and bulbs are commercial grade. This all looks good on paper, but I am
worried because there is not much room for real world error. Does anybody have experience with this many lights? Wasn't there a movie about a guy causing a large scale blackout when he threw the switch on his house decorations? I don't want to be that guy. Any help you can give me would be appreciated. Thanks, Gus
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Well Gus here is what you do. Call out a qualified electrician, pay him to do the work and you get to be the hero for not fudging something up.
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I do have a electrician coming out to inspect the whole set up. I am more concerned with the theory about the number of lights. Thanks, Gus Brian wrote:

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You appear to have made one or more errors, get advice from the electrician before you do the work. Not just the math, there are many code requirements in regard to the materials and methods.
j
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gus wrote:

Depending on voltage drop, you may be okay, or you are going to be tripping the breakers. What country are you in? What did they do last year? How much trouble to get another circuit or reduce the load? You do know that someone makes a LED replacement for the c-9 bulbs which use way less power and last lots longer.
--Dale
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Thanks for the help. I found this link that answers all my questions. http://www.centsibleholidaylighting.com/calculating-watts-amps.htm If you have a brown out around 7:00 pm (MST) on Friday after Thanksgiving you know what happened. They did not light up this tree last year and someone else ordered the lights. When asked they gave me a blank look and said "What's a watt?" Merry Christmas Gus Dale Farmer wrote:

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gus wrote:

It's not the brownout we should worry about, it's the fire. You realize, of course, that you cannot plug one of your strings into the end of another?
Ed
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| We are putting light bulbs on a large outside tree in a city park. We | have six 100 foot strings of 100 C-9 lights. We have two 20 amp GFI | circuits available. I plan to put 300 lights on each circuit. My math | | shows 2300 watts available and C-9 bulbs use 7 watts. This comes to | 2100 watts. I will use 12 guage extension cords. The light strings | and bulbs are commercial grade. This all looks good on paper, but I am
Normally the design current for a 20 amp circuit should be 80%, which would be 16 amps here. Your wattage calculation appears to have assumed 115 volts. What matters is what the total current is for the 100 bulbs with the voltage supplied (should be close to 120 volts in the USA).
If the bulbs are truly 7 watts at 120 volts, this is 17.5 amps total for three strings on one circuit. You could get away with this working. If you can find a couple shorter strings and back the number of bulbs down to say 500, you can make it work more within the code requirements. Reducing one string in each of the 2 sets to 74 working bulbs, either by a shorter string or by leaving in some dead bulbs clustered in the back of the tree, would get you under 16 amps.
| worried because there is not much room for real world error. Does | anybody have experience with this many lights? Wasn't there a movie | about a guy causing a large scale blackout when he threw the switch on | his house decorations? I don't want to be that guy. Any help you can | give me would be appreciated.
That's just movie fiction to impress people. There's also the urban legend of the kid hitting a power pole with his bat just as the lights went out in a past big NYC blackout.
You are not going to do much more with a big error in this than burn out some plug or cord. At worst, it could burn down the tree of the hot spot created is close to it.
Surely you have an electrical inspector office around there. While this might not fall under inspection issues by not being a permanent install, maybe they can give you some advice on a short term setup, especially if you are an employee of the same jurisdiction (city?).
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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I read all this scary stuff about terrible things that might happen if you put 18 amps on a 20 amp circuit but I think it is largely bullshit. 310.16 tells us 12 ga wire is really good for 25a (at the 60c rating). That 80% safety factor you want comes in 240.4(D) that limits the overcurrent device to 20 amps. The branch circuit will be fine. If he splits his load across 2 cords in each duplex outlet the down stream load is safe. Why all this hysteria? It is certainly not backed up by the code.
On 20 Nov 2006 22:33:54 GMT, snipped-for-privacy@ipal.net wrote:

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snipped-for-privacy@aol.com wrote:

It is standard practice to limit the continuous current on a circuit breaker to 80 per cent of its rating unless the circuit breaker is listed for continuous loading. Although Table 310.16 lists 25 amperes as the ampacity that a No. 12 AWG copper conductor with 60 degree C. insulation can carry continuously, the asterisk sends us to 240.4(D) that requires that the maximum OCPD be 20 amperes. You know this, I am sure.
The bottom line is long established practices of using the 80 per cent rule has kept buildings safe for many years. Too often these temporary installations become rather permanent and last for years. Just how far do we want to go in shaving the Code rules when we know good and well when we follow them we get a safe installation and escape the liabilities that may come with a burn down, death, or both. And liabilities are a big item. If there is a fire, then there is an investigation, and experts are called in, and if someone has died or been injured the money involved can become very large. I know of one case where the settlement was about $3 million, and the payer was held accountable because they had violated several Code rules. In this case it was the NESC and not the NEC.
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snipped-for-privacy@electrician2.com wrote:

I forgot to give the reference to continuous load:
210.20 Overcurrent Protection. Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 210.20(A) through (D).
(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the RATING of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
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On 20 Nov 2006 16:33:21 -0800, snipped-for-privacy@electrician2.com wrote:

My contention is the 80% is built into 15 and 20 a circuits by 240.4(D). The writers of the NEC know they have no control over the user and it is expected they may be loaded to the max. BTW where is the hard and fast "rule" that you can't plug in more than 80% of the circuit ampacity on a 20a receptacle circuit? 210.23(A) says (1) Cord-and-Plug-Connected Equipment. The rating of any one cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating. (2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires (lighting fixtures), shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.
"One" of his strings is less than 80% (or 50%) of 2400w You might even argue they are 600 "luminaires" which makes (2) moot and none are reallty "fastened in place". They are just draped on a tree.
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On Tue, 21 Nov 2006 01:32:38 -0500 snipped-for-privacy@aol.com wrote: | On 20 Nov 2006 16:33:21 -0800, snipped-for-privacy@electrician2.com wrote: | |>The bottom line is long established practices of using the 80 per cent |>rule has kept buildings safe for many years. | | My contention is the 80% is built into 15 and 20 a circuits by | 240.4(D). The writers of the NEC know they have no control over the | user and it is expected they may be loaded to the max. | BTW where is the hard and fast "rule" that you can't plug in more than | 80% of the circuit ampacity on a 20a receptacle circuit? | 210.23(A) says
My bet is that the 15 and 20 amp circuits, the ones most common in homes, and the ones most subject to cheap contractor work, are more clearly specified because otherwise there will be more common errors or bad intentions in choosing what to actually use.
| (1) Cord-and-Plug-Connected Equipment. The rating of any one | cord-and-plug-connected utilization equipment shall not exceed 80 | percent of the branch-circuit ampere rating. | (2) Utilization Equipment Fastened in Place. The total rating of | utilization equipment fastened in place, other than luminaires | (lighting fixtures), shall not exceed 50 percent of the branch-circuit | ampere rating where lighting units, cord-and-plug-connected | utilization equipment not fastened in place, or both, are also | supplied. | | "One" of his strings is less than 80% (or 50%) of 2400w | You might even argue they are 600 "luminaires" which makes (2) moot | and none are reallty "fastened in place". They are just draped on a | tree.
"Utilization Equipment Fastened in Place" isn't easily moved somewhere else to get to a different circuit, and I guess they would frown on a practice of using more extension cords (I'm trying to plan my home wiring so I never need to have extension cords even for temporary utilization).
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@aol.com wrote:

My understanding is that the 80% limits are because a fuse or thermal CB may trip if the load is over 80% and over 3 hrs. I don't think it generally has anything to do with safety of the rest of the circuit.
Matching this from the fount of all knowledge - Wikipedia: fuse http://en.wikipedia.org/wiki/Fuse_%28electrical%29 "Overcurrent devices installed inside of enclosures are "derated" at least per the US NEC. This is a hold-over from the first mounting of electrical devices on the surface of slate slabs. The slate was the insulating material between devices mounted in air. So, rather than change the fuse rating, it became common to allow only 80% of the current value of the overcurrent device when the circuit is in operation for 3 hours or more (continuous loading)."
-------------- A few code cycles back there were several proposals to change the prohibition of plugging in loads over 80% - may have been 240.4-D or maybe 210.21-B-2 which limits plug-in loads to 12A on a 15A receptacle on a 20A circuit. One argument was that this is a limitation on how the system is used after it is wired and inspected. The code generally stays away from such limits (and this one is particularly impractical). As you said, the NEC "has no control over the user". (The CMP response was 'the code is the way we want it' thus explaining the logic behind the rule.) Another argument was that some UL listed devices, like hair dryers, with current draw of 12-15A can have 15A plugs - UL thinks plug-in devices at 100% are OK. (The CMP response was 'we are right, they are wrong'.) (Both arguments for change make sense to me.)
-- bud--
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| One argument was that this is a limitation on how the system is used | after it is wired and inspected. The code generally stays away from such | limits (and this one is particularly impractical). As you said, the NEC | "has no control over the user". (The CMP response was 'the code is the | way we want it' thus explaining the logic behind the rule.) | Another argument was that some UL listed devices, like hair dryers, with | current draw of 12-15A can have 15A plugs - UL thinks plug-in devices at | 100% are OK. (The CMP response was 'we are right, they are wrong'.) | (Both arguments for change make sense to me.)
That's how I look at it with respect to the first half of 210.6(A)(2) :-)
The code board certainly wants to avoid issues like those outlets you can easily picture with a stack of 2 or 3 triple taps and a mass entanglement of plug-and-cords converging into a fire starter's convention. Of course the code's solution is more outlets, especially in the kitchen, which is why my plan is for one of these: http://phil.ipal.org/usenet/aee/2006-11-22/ks-1.html http://phil.ipal.org/usenet/aee/2006-11-22/ks-2.html The latter is more likely because I haven't found a plate that has the arrangement of the former.
So why not call a 15A circuit a 12A circuit, and a 20A circuit a 16A circuit?
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snipped-for-privacy@ipal.net wrote:

The 80% limit is for a single plug-in on a circuit of 2 or more receptacles. The "15A" circuit is still rated 15A.
The third argument made for not trying to limit a single plug-in load to 80% was that it is inconsistent with the rest of the code, where 80% only applies to continuous loads.
-- bud--
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snipped-for-privacy@ipal.net wrote:
So why not call a 15A circuit a 12A circuit, and a 20A circuit a 16A circuit?
Because we call them by their rating.
2005 NEC 210.3 Rating. Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. The rating for other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.
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Just a follow up. We went with 500 lights because the Lions Club needed some spot lights for their manger display. We are drawing about 17 amps per line and have had no problems with long term running. We have 120 volts input and none of the strings are tied together. Thanks for the opinions and interesting discussions. I agree with the first answer to have an electrician look at it, I was trying to educate myself. It is like getting information about an illness before you see the specialist. Gus snipped-for-privacy@electrician2.com wrote:

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On 27 Nov 2006 21:07:48 -0800 snipped-for-privacy@electrician2.com wrote:
| snipped-for-privacy@ipal.net wrote: | | So why not call a 15A circuit a 12A circuit, and a 20A circuit a 16A | circuit? | | | Because we call them by their rating.
You're taking my question out of context. I'll rephrase it so it is harder to do that:
Why not refer to the rating of a circuit as the value of the largest single load that can be placed on the circuit?
| 2005 NEC | 210.3 Rating. Branch circuits recognized by this article shall | be rated in accordance with the maximum permitted ampere | rating or setting of the overcurrent device. The rating for other | than individual branch circuits shall be 15, 20, 30, 40, and 50 | amperes. Where conductors of higher ampacity are used for | any reason, the ampere rating or setting of the specified overcurrent | device shall determine the circuit rating.
Stating the rule I was suggesting to be changed as support for why not to make the change doesn't make any sense.
The answer to my question should focus on why the 80% rule exists and what its meaning is in terms of the theories and issues that brought about such a rule. Is the limit of 80% for one single load there because some fuses or breakers could act at 80% current in the worst case scenarios (they can, based on real trip curves and variations in other tolerances of voltages and loads). Or is the limit of 80% to avoid overloading a general use circuit (if so, then it wouldn't have meaning for a dedicated circuit).
If I have a truly 15 amp load and will be providing a single dedicated circuit to supply it, what is the reason for having a rule that requires the overcurrent protection rating for it to be higher (20 amps in this case)?
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snipped-for-privacy@ipal.net wrote:

The facts are the NEC load calculations over kill by 200 to 250 per cent. No. 12 wire is tested per the UL standard using 40 amperes. There is a substantial safety margin built into the code rules that allow for a lot of mistakes, because over the life of an electrical system that lasts from 30 to 50 years many mistakes are anticipated. Who knows who is going to try to modify these systems? So the 80 per cent rule is a good engineering practice. Likewise, the NEC does not have built in reserves for expanding an electrical system. I personally have wire a restaurant el cheapo, only to see the expansion take place before opening. The service had to be redone. I think a 50 per cent load should be maximum for circuits to allow for expansion.
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