# Science of Daisy-chaining [closed]

As everyone knows daisy-chaining power strips is very bad and dangerous. But could someone tell me why it's dangerous from a physics standpoint? (i.e. its effect on voltage, current, etc)

• "As everyone knows daisy-chaining is very bad and dangerous" Citation needed. AFAIK if done in appropriate contexts it's perfectly fine. – JMac Feb 24 '18 at 22:22
• Presumably you're talking about stringing together extension cords and power strips, but "daisy chaining" by itself does not adequately express the idea. – dmckee --- ex-moderator kitten Feb 24 '18 at 22:45
• @dmckee I agree I should probably be more specific. – Dannnnnnn Feb 24 '18 at 22:55
• @JMac - the Electrical Safety Foundation (esfi.org), created by the National Electrical Manufacturers Association, Underwriters Laboratories, and the US Consumer Product Safety Commission, begs to differ with you. Further, NFPA 70E (code in the US) also differs with your opinion. Perhaps you need a citation from a nationally recognized authority that daisy chaining is considered acceptable. – Jon Custer Feb 26 '18 at 15:14
• @JMac - plugging one power strip (plug, some feet of cord, and a box with multiple outlets) into another is expressly forbidden in NFPA 70E. If you use an outlet extender, one of the little things with one plug and say three receptacles, it must be plugged into the outlet at the wall. This is entirely separate from 'daisy chaining' lamps in a building lighting circuit together - those are hard wired and the total load has been considered to be suitable for the wire and breaker used. Totally different concepts. – Jon Custer Feb 26 '18 at 15:28

"Daisy-chaining" together power outlet strips is dangerous because it provides so many outlets into which electrical appliances can be plugged that it becomes easy to exceed the current-carrying capacity of the outlet strip(s) nearest the beginning of the chain. This can cause those strips to overheat, melt, and catch fire.

• It's more than that. There is a danger even if you only plug one load at the end. Which is why it is also dangerous to chain extension cords together. – dmckee --- ex-moderator kitten Feb 25 '18 at 2:29
• @dmckee, is that additional danger from the gauge of the wire being insufficient relative to its length? – niels nielsen Feb 25 '18 at 18:22
• More or less. A cord of length $L$ and current rating $I$ is engineered so that the total heating due to both the intended load and the parasitic resistance is safely low. But powering the load requires more and more current as more parasitic load is added, resulting in more heating. Fusing or circuit breakers should protect you, but Underwriters Laboratories, for one, isn't convinced. – dmckee --- ex-moderator kitten Feb 25 '18 at 18:39
• @dmckee, nor am I. I have seen my share of melted outlet strips and will not use one without a circuit breaker in it, and a metal housing. – niels nielsen Feb 25 '18 at 23:11

Is it not more of a problem with unfused (or improperly fused) strips and adaptors?

Or strips that are poorly designed with the assumption that the load presented at any one socket will be a certain fraction of the overall fused load which the strip is able to cope with? And therefore placing a high load on a single socket will not blow the fuse, but will put undue strain on the internal bus - especially if you use the socket furthest from the supply cord.

No strip can tell whether the load presented to it is presented by an appliance directly, or by another strip - so any danger is essentially an inherent and designed-in vulnerability to high loads.

Daisy-chaining a series of strips which serve patently low-power devices is perfectly safe.

Incidentally, there is also the problem that extension reels will be designed on an assumption about the amount of heat dissipated when the reel is uncoiled. If the reel remains coiled, the safe capacity drops dramatically (typically to less than half).

There is also the problem of voltage drop over long, high-resistance lines, causing devices to malfunction if they are operated outside their rated voltage and typically to consume a greater current.

• As noted in comments (currently) to the question above, daisy chaining power strips is against code, regardless of the planned loads. One year later nobody will remember just why you shouldn't plug the vacuum cleaner in right here at the end... – Jon Custer Feb 26 '18 at 15:42
• @JonCuster, you are in a different jurisdiction from me. I'm in the UK where we have fused plugs (unlike Europe and possibly the US). Here daisy-chaining is not (as far as I'm aware) specifically precluded - although an electrician would consider it a bad practice for a variety of reasons. Nevertheless, the safety factor is related to the load presented. – Steve Feb 26 '18 at 16:17
• I have had several secured (i.e. screwed-on), daisy-chained strips under my desk at home for many years - there are about 11 outlets in total, but all would be used only occasionally and temporarily for low-power devices. Any socket may be used for a vacuum cleaner. One strip is fully occupied at all times, another is inside a cabinet serving computer equipment and difficult to reach, and a third is secured to the underside of the desk and is not clearly visible - so the prospect of guests plugging in two arc welders are remote (and would probably cause one of the fuses to blow anyway). – Steve Feb 26 '18 at 16:24