Currently have 18 smoke detectors inter-connected to a Vista 20 panel with 18/4 FPL cable. We are selling (sold) the house, but have to replace all detectors since they are more than 10 years (Massachusetts Law). Current detectors are 4 wire, and unit replacement cost is ~$35. The 120v units from Homedepot are ~$12.

My question is whether the existing FPL cable can be re-wired to the cheaper units, since the cable is rated to 300 volts.

Thanks in advance for assistance

  • Hello, and welcome to Stack Exchange. I'm in no way an expert but I'm guessing code requires better than that when running 120VAC. Commented Mar 8, 2016 at 1:57
  • 1
    Even if you could find some way to adapt them, it's probably not going to be worth the time and effort to re-wire each of the 18 different smoke detectors. The cost between the cheap ones and the ones that you know will work with your panel and wiring is only $23 each, or around $400 for all o fthem - that doesn't seem like a lot of money to put into the sale of a home that's large enough to need 18 different smoke detectors.
    – Johnny
    Commented Mar 8, 2016 at 2:43

2 Answers 2


Short answer: Maybe

Here is why: The cable may be rated for 300 volts but it is not #14 gauge minimum size for power and lighting as stated here:

(A) Minimum Size of Conductors. The minimum size of conductors shall be as shown in Table 310.106(A), except as permitted elsewhere in this Code

Table 310.106(A) shows the minimum size to be #14 gauge wire.

However, if you get a 7 amp breaker or fuse marked for use with #18 ga wire or a special class of fuse, you could use this section

Article 240.4(D)(1) 18 AWG Copper. 7 amperes, provided all the following conditions are met:

(1) Continuous loads do not exceed 5.6 amperes. (2) Overcurrent protection is provided by one of the following: a. Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire b. Branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire c. Class CC, Class J, or Class T fuses

This would be very strange in a house though so your inspector may not want to allow it.

Good luck!

  • This is all well and good. Now find a 7A circuit breaker for use with 18ga wire, to fit ANY standard residential breaker panel. ...... I'll wait. Commented Mar 8, 2016 at 1:44
  • It doesn't have to fit the breaker panel. It could be installed in its own enclosure. And as I said it would be a strange thing for a house.
    – ArchonOSX
    Commented Mar 8, 2016 at 1:57
  • Regardless of how technically possible something is, sometimes the correct answer is it can't be done. Commented Mar 8, 2016 at 1:58
  • No the correct answer is that it can be done it just may not be to your liking.
    – ArchonOSX
    Commented Mar 8, 2016 at 1:59
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    And as a buyer, I hate when the seller buys new fixtures just to sell the place. It's usually cheap tenant-grade junk, or the apocryhal white carpet... Which I then have to suffer with or pay again to replace. I wish the seller would just agree with me not to replace it, stipulate that it needs doing, knock the price down by what he would've spent on the junk, and leave it to me so I can pick what I want. Commented Mar 8, 2016 at 5:11

You absolutely cannot use that cable for line voltage smoke detectors, for several reasons. One big one is that you cannot have a typical circuit smaller than 15A, which would require #14cu minimum. OK, I have to say, it is technically possible, but entirely impractical.

IMO replacing "alarm system" heads with line voltage smoke detectors is a downgrade and rather hackish.

Just how big is this house that you have 18(!!) smokes??

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