Correct Formula to Calculate Single Phase Current
Correct Formula to Calculate Single Phase Current
(OP)
I need some help to determine the correct way to calculate the current in a 480V, single phase circuit, which services ten roadway lighting fixtures. The circuit is fed from a 2-pole (phases a-b), 480V, circuit breaker in a 480Y/277V, 3-phase, 4-wire panelboard. There are 10 - 480V rated fixtures on the circuit, 304VA each = 3040VA total circuit load. There are two additional circuits (phases c-a and b-c)fed from this same panel, with the same number of fixtures on them. I contend that the correct way to calculate the current in this circuit, for the purposes of determining the cable size, is from the formula: I = 3040/480 = 6.34A. Am I not correct?






RE: Correct Formula to Calculate Single Phase Current
RE: Correct Formula to Calculate Single Phase Current
Thanks.
RE: Correct Formula to Calculate Single Phase Current
RE: Correct Formula to Calculate Single Phase Current
Thanks.
RE: Correct Formula to Calculate Single Phase Current
If you have the same load between each 'pair' of the three phases, then the total load will be
3*3040 VA an is balanced
the line current in the Input feeder to the panel will be
(3*3040 VA)/(1.732*480V)=11A,which is same as what skogspurra said.
There are other ways to explain this, but then you should consult some basic text books on polyphase circuits.
RE: Correct Formula to Calculate Single Phase Current
RE: Correct Formula to Calculate Single Phase Current
You should also install a current transformer and cheap (such as Simpson) ammeter on each branch circuit to measure how much current is leaking to ground. It is very easy for 40 feet (12 meters) of broken underground wiring to leak 7 amps at 120 volts into the soil during a drought. The meter will resgister some ground return current when the system is new because of cable capacitance and some of it will be harmonics and some line frequency - record this value. If your electric bill goes way up or something has a tingle voltage you can figure out which circuit is the culprit right away. You would then put a clamp-on ammeter around both main hot wires at each pole to find the segment that is leaking electricity.
Mike Cole, mc5w@earthlink.net