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I am putting lights around my house this season, and am finding myself concerned about safety and what the company who makes the bulbs says is safe. I think they are being way over-cautious, because they say we can only connect two strands. But I'm reading different things on the web, and I'm not sure which is correct.

I want to connect six strands of incandescant C9 bulbs, which have 25 bulbs each (150 bulbs total). I've carefully read all the safety instructions, but the only recommendation for how many strands to connect is on the box itself, and it says only two. This is very inconvenient because there will be three extension cords going across our yard and walkway.

I found this equation via Google: Number of total bulbs = Volts from your outlet X amps on your circuit X 80%/Wattage from a single lamp

So for a 120V outlet, and a 15A circuit, and each bulb having 7W, we should be ok with 205 bulbs.

And so, is it ok to connect all six of these strands, with the above equation in mind? Is the company that makes the lights just being super-cautious so that they don't get sued? What would be the major concern (blowing a fuse, or something else)?

And to clarify previous questions, my lights would in this case be connected in series (if it were possible to safely do so). Each strand is wired in series.

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    \$\begingroup\$ What is the proper voltage and current ratings for the bulbs individually and are they wired in series or parallel in a strand? \$\endgroup\$
    – Andy aka
    Commented Dec 7, 2015 at 22:16
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    \$\begingroup\$ Considering only available wattage vs wattage needed you would be correct, but that's not all you need to consider. They are probably limiting it to two strands because the wires can't handle the current more bulbs would require. The major concern would be the wiring insulation melting and the excess heat setting something on fire. \$\endgroup\$
    – I. Wolfe
    Commented Dec 7, 2015 at 22:20

3 Answers 3

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If the strands are connected "in series" - strand A plugs into an outlet, strand B plugs into the end of strand A, strand C plugs into the end of strand B, etc. then all current for all of the strands will have to flow through the wires of strand A, which are probably too small to carry the current of more than two or three strands. If you have eight strands connected "in series", the wires in the first couple of strands will likely overheat and melt the insulation.

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  • \$\begingroup\$ Please see my comment on philbrooksjazz. \$\endgroup\$
    – Mark
    Commented Dec 7, 2015 at 22:30
  • \$\begingroup\$ @Mark you have accepted an answer that potentially contradicts your question (no offence to Mr. Bennett of course). In your question you mention that to get 6 strands to work you'd need three extensions across the yard. This implies 2 strings per extension as you have said but you haven't stated that the strings are wired in series (or in parallel). I have asked you to clarify under your question. Maybe you don't understand the question? \$\endgroup\$
    – Andy aka
    Commented Dec 7, 2015 at 22:34
  • \$\begingroup\$ @Andyaka Hi, I can only guess that the wires are in series, as I have a limited understanding of wiring and EE. But in my case, anytime that I'm going to plug in to the lights, I will be taking an extension cord and pluggin it into an outlet in the house, and then plugging in the first strand to that ext. cord, then plugging in the next strand to the end of the first strand. It would be great if I could continue with all the strands in this manner but I don't think I can do that, as the first two answers on this question say that that would be a fire hazard. \$\endgroup\$
    – Mark
    Commented Dec 7, 2015 at 22:42
  • \$\begingroup\$ @Mark OK fair play, you don't know. \$\endgroup\$
    – Andy aka
    Commented Dec 7, 2015 at 22:44
  • \$\begingroup\$ @Andyaka Do you know if these kind of lights are ever wired in parallel? And if so would that change the way I could connect them? \$\endgroup\$
    – Mark
    Commented Dec 7, 2015 at 22:45
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Very definitely, Peter's answer is correct, if you string all these lights together, you'll be putting yourself at a risk for an electrical fire. The big problem is the lamps you're using, those 7 watt incandescent bulbs are a thing of the past. You need to go get some LED type christmas lights, these draw less than 1/2 watt each. A typical string of 100 lights is ~ 40 watts, so I'm guessing you could string up to 5-6 of these together, but definitely follow the recommendation on the package.

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  • \$\begingroup\$ I see, ok, this is very good to know. While on the topic, how did they used to light houses in the old days with these incandescents? Did they just use different outlets every few strands? Seems cumbersome, but of course, safety first if that was what it took. \$\endgroup\$
    – Mark
    Commented Dec 7, 2015 at 22:29
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    \$\begingroup\$ You can still get away with up to 8 strands per 15 amp circuit, because each strand is ~ 175 watts, that is ~ 1.5 amps, which is roughly 8 strands (with 20% headroom), but the problem is that the diameter of the wire of any one of the strands has to handle all the current of ALL of the strands, so you have no choice but to use additional extension cords and a power strip. \$\endgroup\$ Commented Dec 7, 2015 at 22:52
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    \$\begingroup\$ Also, Mark - I'm guessing that your lights need to string in such a way that they cover a long distance - like your roof line, which is why using a power strip won't work. If you're stringing a christmas tree, you can probably get away with a 'parallel' solution, i.e. all strings plugging individually into a power strip, and not end to end (series), in any case the low power LED lights are much safer, and you'll save a lot of money on your power bill. \$\endgroup\$ Commented Dec 7, 2015 at 23:02
  • \$\begingroup\$ Ok, I see. We do have some long extension cords and a few different outlets outside. It's ugly in the day when you can see the extension cords, but beautiful after dusk! \$\endgroup\$
    – Mark
    Commented Dec 7, 2015 at 23:04
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If you can wire one string to the AC power then you can wire two strings in parallel to the same AC mains providing you don't exceed the 15A. OK so wire two in series and then take another two wired in series and wire these in parallel to the original two series strings.

Keep going until you run out of terminal blocks or you start to create an energy shortage in your district. That's a joke by the way but I'd hate to see anyone's xmas lunch spoilt by a power outage.

Just do it safely!!

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