120/240 vs. 120/208

Discussion in 'Amateur Radio Amplifiers' started by W2RWJ, Sep 4, 2011.

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  1. W2RWJ

    W2RWJ Ham Member QRZ Page

    Our electrician is installing new electrical circuits to support the RF amplifiers at the N2MO club station. Receptacles are Hubbell 2410 twist lock plugs (3 pole, 4 wire)

    As the power is derived from a 3Ø source, the power supplied is actually 120/208, as opposed to a traditional 120/240 volt Edison type feed.

    Since most amateur amps do not have taps for adjusting input voltage, after verifying filament voltages, is there anything else I should be checking in this situation?

    73 Martin
    Last edited: Sep 4, 2011
  2. W9GB

    W9GB Ham Member QRZ Page

    Martin -

    The Hubbell 2410 is a NEMA L14-20R 125/250 V, 20 amp receptacle, as such, it is designed for 120/240 VAC service.
    The FOUR wire, NEMA L14, meets 1996 changes in NEC ... X and Y are your BLACK and RED ("HOT" wires); W is your WHITE (Neutral Wire); and G is your GREEN (Safety Ground)
    The usage of 4 wires was to permit proper grounding of kitchen and laundry appliances that had both 240 VAC and 120 VAC components inside.

    Fixed appliances on three-wire circuits
    In North America, the cases of some ovens and clothes dryers were grounded through their neutral wires as a measure to conserve copper during the Second World War.
    This practice was removed from the NEC in the 1996 edition, but existing installations may still allow the case of such appliances to be connected to the neutral conductor for grounding.

    Note that the NEC may be amended by LOCAL building/electrical codes/regulations in each state and city.
    This practice arose from the three wire system used to supply both 120 volt and 240 volt loads.

    Because ovens and dryers have components that use both 120 and 240 volts there is often some current on the neutral wire.
    This differs from the protective grounding wire, which only carries current under fault conditions.

    Using the neutral conductor for grounding the equipment enclosure was considered safe since the devices were permanently wired to the supply and so the neutral was unlikely to be broken without also breaking both supply conductors.
    Also, the unbalanced current due to lamps and small motors in the appliance was small compared to the rating of the conductors and therefore unlikely to cause a large voltage drop in the neutral conductor.
    For HF amplifier with 3 wire cords and wired (jumpered for 240 VAC usage) --
    PROPER connections to a mating L14-20P are critical -- US/Canada standard appliance WIRE colors (Black, White, Green) can be confusing (and dangerous).
    CONSULT a licensed electrician, IF you don't know what you are doing.

    In some cases, the Black and White conductors in the cord are your HOT conductors (X, Y) for 240 VAC to the plug.
    The Green wire (if attached to the metal amplifier chassis frame) is attached to the G lug on L14-20P plug.

    Whether the W lug is used -- depends on the specific amplifier wiring (e.g. if 120 VAC is used within the amplifier.)
    IF the amplifier manufacturer is still in business -- ASK for their support / preferred connections for 4-wire standards (NEC post 1996).

    PRIMARY taps on the HV plate and filament transformers (insie the HF amplifier) determine your options of what is possible.
    This varies with the amplifier manufacturer (or DIY builder) and the era of the power supply design !


    Attached Files:

    Last edited: Sep 4, 2011
  3. W9GB

    W9GB Ham Member QRZ Page

    THREE PHASE POWER SYSTEMS - All About Circuits

    Three Phase - Electrical Power

    A transformer for a High-Leg Delta System (3-phase), shown below.

    240 VAC loads, such as 3-phase motors would be connected to L1, L2 and L3.
    120 VAC loads, such as Single-phase lighting would be connected L1 or L2 to Neutral (N).
    NO LOADS would be connected from L3 (High or Wild Leg) to Neutral (208 VAC) -- UNLESS the Equipment (motors, lighting) is designed for 208 VAC.

    Hubbell does have NON-STANDARD (non NEMA) plugs and receptacles for legacy equipment/connections as well as 3-phase connectors for bringing
    L1, L2, L3 and N to a 4 wire receptacle -- AVOID these approaches (industrial/commercial) for NEW installations (consumer equipment).

    It is common practice in many regions to identify 120/208Y conductors as BLACK, RED and BLUE.
    Local regulations may amend the N.E.C. The U.S. National Electric Code has color requirements for grounded conductors, ground and grounded-delta 3-phase systems
    which result in one ungrounded leg having a higher voltage potential to ground than the other two ungrounded legs.

    ORANGE is ONLY appropriate when the system has a Grounded Delta service, regardless of voltage.
    According to Article 110.15 of the 2005 National Electrical Code (NEC),
    panelboards connected to this type of transformer (Grounded Delta) must explicitly identify the High Leg, preferably by coloring it ORANGE.


    Attached Files:

    Last edited: Sep 4, 2011
  4. W2RWJ

    W2RWJ Ham Member QRZ Page

    Thanks for the detailed reply - I was already through this with our licensed electrician and an PE. The site is serviced by a 120/208 3Ø WYE system, grounded neutral.


    There is not a 120/208 volt NEMA configuration for single phase circuits. In this *specific* case, consensus was this was an acceptable use for the 2140 receptacle in a new installation.

    In this *specific* case - If the amplifier in question does not require a neutral, it can be left unconnected (on the appliance side). The neutral is available in the receptacle should a connected device require it.

    I was actually concerned about QRO operation with reduced filament and B+ on equipment that cannot be reconfigured for the slightly lower voltage.

    73 Martin
  5. W1QJ

    W1QJ Ham Member QRZ Page

    You might be interested to know that there are things called buck boost transformers. I once had to dismantle a tanning salon that was once in a 120/240 neighborhood and then later moved to a 120/208 commercial neighborhood. Since the units were originally 240v we used a heavy duty buck boost transformer that supplied an extra 32v to run the equipment. Worked out very well. They are rather expensive however you very well may find some used on EBAY at a very fair price. They are rated in KVA's. Figure out what KVA you will need and see if you can pick one or 2 of them up. SInce the amp will most likely work on 208v you will be losing some power becuase your HV will be down several hundred volts.
  6. K1CD

    K1CD Ham Member QRZ Page

    Delta 3 phase is usually upstream in the power grid. Y 3 phase is more common for neighborhood distributions (each phase to a different section) and industrial areas.

    What the concern is here is about using a split phase household feed versus 2/3 of a Y industrial feed. The lower voltage from the 2/3 of a Y feed was the subject of an RV forum discussion recently as some RV parks don't supply NEC split phase to their 50A/240V RV pedestals and that can create problems for the very few RV's that use 240v appliances - seen Sean's our Odyssey blog for how he handles this with his motorhome.

    I think there is concern about the lower voltage for the amplifiers. I'd think it'd mainly just degrade output, though. You might check the actual transformers used in the amp to see if they provide a tap that's about 10% higher, sometimes they do. Depending upon the circuit, there may be other tweaks you can do to get the filament and B+ voltages optimum for the power devices.

    re the buck/boost transformers - RV's often use an autoformer to compensate for supply voltage sags. These are usually 4 winding 10:1 transformers often used to provide 12v for lawn watering controls. By using an autoformer rather than a transformer, the amount of iron needed for the core is greatly reduced.
  7. W2RWJ

    W2RWJ Ham Member QRZ Page

    Our campus has it's own distribution: We buy bulk power at 13.2kV, step it down to 4160V for distribution, and down to 120/208 at each substation. Link to Picture: http://infoage.org/images/stories/new_power_2.jpg

    This bank is 3 seperate 50kVA transformers: One end of each transformer secondary is solidly grounded at the substation in a "wye" configuration.

    In the end, it may be cheaper to replace the transformers in the "amateur" amplifiers with transformers tapped at 208 / 240 then mess with the distribution. To their credit, the commercial gear is already tapped this way.

    In case anyone is interested: Site tour link: http://infoage.org/exhibits/ocean-monmouth-amateur-radio-club/diana-site-tour.html
  8. W9GB

    W9GB Ham Member QRZ Page

    When you mentioned this -- it reminded me of the hospital campus that I worked at ages ago .. they used both WYE and DELTA in-house.
    That is true ... since this type of distribution is more common in industrial and some larger commercial installations,
    where 208 VAC warehouse lighting and motors (e.g. air handlers, HVAC) are used -- and hard-wired.

    Instead of purchasing a custom transformer for the HF amplifier -- I would probably look at a specific buck boost transformer (208 - 240),
    near you operating position for 240 VAC (hospitals sometimes have these surplus -- as hospital grade isolation/buck boost transformers for clinical gear).

    Hard wire the primary to your 120/208 circuit and then use the NEMA L14 outlet on secondary from transformer.
    That allows you to easily change-out the HF amplifier in the future !

    Last edited: Sep 5, 2011
  9. W9GB

    W9GB Ham Member QRZ Page

    eBay seller: ccub243, auction # 290524196977
    in East Tennessee has a few
    GE Transformers 9T21B1029G02 208v-120/240v at 7.5 KVA

    $300 each, BUT shipping 147 pounds adds to the cost.

  10. K0BG

    K0BG Ham Member QRZ Page

    I agree with Greg—a buck/boost is the correct solution. Or....

    You buy a new Emtron DX-4, with a three phase power supply. With 5,000 watts of dissipation, it is not likely to fail running legal limit.
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