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AC receptacles for power amplifiers

Discussion in 'Amateur Radio Amplifiers' started by WY7BG, Jan 9, 2020.

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  1. WY7BG

    WY7BG Ham Member QRZ Page

    I'm looking at installing an AC receptacle for possible future use with a power amplifier. From what I can see on the various equipment vendors' Web sites, most "full legal" amplifiers - and many with output power below that - use NEMA 6-15P power plugs, which have two "hot" prongs with 240 volts across them and then a ground. (There's no neutral, because it isn't necessary.)

    One way to power the amplifier would be to install a matching NEMA 6-15R receptacle, connected via 14 AWG wire to two 15A circuit breakers on opposite rails at the breaker panel.

    However, many of these amplifiers say, in their spec sheets, that they may draw up to 14A of power, which is uncomfortably close to the full 15A such a circuit can provide. The wires could get warm, and a surge might trip the breaker! So, I am wondering if it is better to install a NEMA 6-20R receptacle, connected via by 12AWG wire to a pair of 20A breakers. The NEMA 6-15P plug apparently WILL fit into a 6-20R receptacle, because the T-shaped slot in the outlet will accept one of the horizontal prongs of the plug.

    The thing is, plugging a 6-15P plug into a 6-20R receptacle would only be truly safe, in the event of an overload or short circuit in the amplifier or its cord, IF two things were true. First of all, if the amplifier was built to draw less than 20A, it would need to have a built-in fuse or breaker; secondly, the amplifier's power cord would need to be rated for 20 amps in case a short occurred upstream of the fuse (perhaps in the cord itself).

    Can folks here with experience with big power amps (I have never owned one that required 240V) tell me if they fulfill those two conditions?
    WA9RHU likes this.
  2. WL7PM

    WL7PM Ham Member QRZ Page

    I wire ALL household outlets with 12 gauge wire.
    I use NEMA 6-20R receptacles for legal limit amplifiers.
    As you noted, the 6-20R receptacle will work equally well with either 6-15 or 6-20 Plugs
    AE8W and K0UO like this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    When I wired my hamshack/electronics workshop, I used #12 AWG wire throughout. I used three conductor (Red-Blk-Wht) with ground cable carrying 240V to every 120V dual-gang receptacle around the room. I broke out the tab on the duplex outlets so that the top socket is wired to Red, while the lower is wired to Blk, meaning that there are 2each 120V circuits in every box, and any box can be converted to a 240V socket.

    I also ran a separate 40A 240V circuit to a special socket than now powers my daily-driver kW amp. Having 240V in every duplex wall-box has been very useful on my electronics workbench while repairing some 240V equipment.
    W1TRY and K0UO like this.
  4. W1VT

    W1VT Ham Member QRZ Page

    You may want to consider how you use the amplifier. SSB voice has a very low duty cycle that won't stress the AC circuit. On the other hand, a serious RTTY contester will put a lot of stress on the amplifier and AC circuit by comparison when running the same PEP.

    Zak W1VT
  5. W1QJ

    W1QJ Ham Member QRZ Page

    For the mere difference in cost between #12 and 14 wire it simply doesn’t pay to not wire any 240v receptacle with #12 wire and a 6-20p 20amp receptacle that takes both 15 and 20 amp 250v rated plugs. Having a 20 amp breaker and a 15a plug isn’t an issue as long as the amp has its own fuses orvCB’s. The SJ rubber cord #14 isn’t an issue. SJ cord wire ratings or higher than Romex or building wire in general. SJ cord has separate ratings in article 400 of NEC.
  6. W0KDT

    W0KDT XML Subscriber QRZ Page

    12 gauge/20 amp breakers for everything except the 60 amp garage subpanel. Double breakers for 240v circuits, not two singles. Don't worry about the socket. In the unlikely event that you choose wrong it is easy and cheap to change out.
    AE8W likes this.
  7. N3FAA

    N3FAA XML Subscriber QRZ Page

    I use #12 on a 20A circuit. The plugs can always be changed, or adapters used.
    AE8W likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There aren't any amateur radio legal-limit amplifiers that require a 20A circuit if operated legally.

    14A @ 240V = 3360VA. Assuming even 100W for filaments and 100W lost in the power supply, plus maybe 40W for a very powerful blower, that would leave 3120W for B+. Most AB2 amps operate at least 55% efficiency would yield 1716W RF output.

    In reality, most "big" amps only draw maybe 12A @ 240V, and as W1VT pointed out, unless you're running a constant carrier mode, duty cycle plays a factor in heating and the "average" current is probably 8A or less.

    A 15A 240V circuit should work fine. But if you're wiring from scratch, for another few dollars no reason to not use 12AWG copper and a 20A outlet and plug.

    My most "power hungry" amp is my 4-1000A which is grid driven so includes a regulated screen supply that wastes more power and runs AB1 which is slightly less efficient. At 1500W output power it draws about 13A from the 240V line and again it's a power hog unlike most commercial GG amps.
  9. WY7BG

    WY7BG Ham Member QRZ Page

    I'd already been planning on using 12 AWG MC cable in any case, so as to future-proof the installation. The cost difference between 14 AWG and 12 AWG is minimal, and the metal jacket will help to contain any stray RF.

    The question is whether it's really safe to plug a device with a NEMA 6-15P plug into a 6-20R receptacle on a 20 amp circuit. The 15A plug implies that the power cord may only be rated for 15 amps, so that if there were a short - or if something went wrong with the amp and it drew more than 15 amps - the cord could catch fire. So, the question is, is it really safe to do that? Or do I want a 15A outlet and 15A breakers (even if the wire is 12 AWG) just to be safe?
  10. WY7BG

    WY7BG Ham Member QRZ Page

    Yes, the two breakers for this kind of circuit come as a pair with their levers locked together. You do want to kill the whole circuit if there's a short on either leg.

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