208 vs 240

bobdogbobdog Solar Expert Posts: 192 ✭✭
Can someone explain what the issue is with 208 volts vs 240 volts and should I care?

Comments

  • VicVic Solar Expert Posts: 3,152 ✭✭✭✭
    Re: 208 vs 240

    208 VAC is an artifact of three-phase power. It is one hot to neutral (at least in my experience).

    240 VAC in the USA is "Split-Phase" power. It is L1 to L2 -- Between two hots. And it is 120 VAC one hot to neutral -- L1 or L2 to neutral. This is what you want for residential power, unless you are doing three phase power.

    Some things will run on on 208, like some lighting. Am running 208 to the resistive water heater in my shop, as it is convenient, as the shop power is three phase Dleta, and 208 is the "high leg" to neutral.

    Do not concern yourself with 208 VAC for residential service, especially for off-grid. JMHO, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    Re: 208 vs 240

    240 VAC is the "nominal" voltage for a 120/240 VAC split phase home power in North America (with grounded neutral).

    208 is the typical available power for small office parks (at least in California) that is really 120/208 VAC 3 phase power.

    Some equipment is rated to run at 208 or 240, some equipment needs 240 specifically or it will overheat motors, or 208 to not over-voltage, etc.

    Turns out, for many reasons, that 120/208 for three phase power is easier and cheaper to balance than 120/240 VAC 3 phase power--So 120/240 VAC three phase power is not often done.

    Basically, 120/208 is a "Wye" connection... Or think of the letter "Y"... The center of the "Y" is the grounded neutral for all 3 phases.

    With a 120/240 VAC three phase setup--That is a "Delta" connection. Think of a triangle with three 240 VAC transformers. If you want 120 VAC, you have to center-tap one of the transformers... But that "neutral" is only common to one transformers... And you can draw 120 VAC only from that transformer. So, you cannot distribute 120 VAC loads to the other phases.

    There is lots of other stuff (stability of network, harmonics, etc.) about Delta/Wye 3 phase power--But is really beyond what we can talk about here (probably).

    There is a whole bunch of math (well beyond me) that explains why power companies and large customers choose different wiring/voltage/phase schemes (this was the difference between Edison/DC vs Tesla/AC--Edison did not know the theoretical math; whereas Tesla did--And why Edison stayed with DC so long).
    Several undercurrents lay beneath this rivalry. Edison was a brute-force experimenter, but was no mathematician. AC cannot be properly understood or exploited without a substantial understanding of mathematics and mathematical physics (see AC power), which Tesla possessed. Tesla had worked for Edison but was undervalued (for example, when Edison first learned of Tesla's idea of alternating-current power transmission, he dismissed it: "[Tesla's] ideas are splendid, but they are utterly impractical."[5]). Bad feelings were exacerbated because Tesla had been cheated by Edison of promised compensation for his work.[6][7] Edison later came to regret that he had not listened to Tesla and used alternating current.[8]

    Regarding Solar GT--The inverter has to have the voltage limits set correctly for 208 or 240, the minimum array Vmp voltage is typically lower for 208 VAC. Sometimes there is a reduction in output watts from the GT inverter (same max current, either at 208 or 240 volts). And utilities will (usually/always?) require all three phases to be shut down if there is a single phase failure.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bobdogbobdog Solar Expert Posts: 192 ✭✭
    Re: 208 vs 240

    Thanks for the answers. Isuppose then for my future GT system, all I need to be concerned with is 240volts. Or is it just 120 volts? I'me sorry for such "easy" questions but I know absolutely nothing about this GT stuff.
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    Re: 208 vs 240

    Most US GT inverters are native 240 VAC (some have 208 VAC options). Unless you are commercial/office park--You will probably see 240 VAC GT systems only for home installations (there are a few, typically smaller, 120 VAC GT inverters).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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