Lets say I have a multi-wire 120/240v coming of a Xformer to a main then out to a branch circuit's their are two hot's one from A phase one from B phase these are sharing one grounded conductor on A phase their is a tv plugged into a recpt. then continues to a insta hot h20 heater. On a phase their is a toaster plugged into a recpt. then continues to the insta hot heater. the neutral is not landed on the bar in the panel. how can I solve for voltage applied ( or dropped across) the television? is there a formula or a way to work ohms law to solve? When the toaster - 700w @ 110v tv- 13.4a @ 130v water heater- 47 kw @ 250 v This a home work Question I have multiple like this one no matter how I work it I cant seem to fined applied voltage! Thanks for any help! :?
Help finding voltage on open neutral?
- Thread starter sparkyjr
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
It is a series parallel circuit. Draw it out, solve for the total resistance, then total current and then the voltage drop across each of the resistances.
- Location
- Illinois
- Occupation
- retired electrician
It sounded like you had more than on load on one of the hots.
Let's say...
A = black wire
B = red wire
N ≠ white wire, not connectied at panel
The TV is connected to black and white.
The toaster is connected to red and white.
The insta hot is connected to black and red.
kwired
Electron manager
- Location
- NE Nebraska
13.4 amps seems pretty high for a TV, but otherwise yes the formula is Ohm's law. Draw out your circuit, and figure which values are missing and rearrange Ohm's law to find the missing values.
The test writer probably wants to assume the TV is a resistive load for the purpose of simplifying this question, when in reality it probably is not an entirely resistive load.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
It is a series/parallel circuit, since the neutral is not connected you have the toaster in series with the TV across 240 volts, then you also have the insta-hot water heater that is parallel with the TV and Toaster series circuit, so draw the circuit with the TV and toaster in series across the supply then draw the heater across the supply, fill in the known values and use ohms law to figure out the unknown values, then calculate the total ohms of the series circuit to find the current of the circuit then use the ohms of each load to find the voltage drop across each of the loads in the series circuit the heater will only add to the current of the total on the transformer since it is in parallel with the series circuit.
Also remember Kirchoff's law of a series circuit finding the current in the series part of the circuit, this means you must solve the total resistance of the two resistors that are in series to find the current of this part of the circuit.
Also remember Kirchoff's law of a series circuit finding the current in the series part of the circuit, this means you must solve the total resistance of the two resistors that are in series to find the current of this part of the circuit.
Last edited:
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
In a test question like this it is easy to get confused as you have to find the resistance of each resistor the rated values given, such as the 130 volts for the TV and 110 volts for the toaster as well as the 250 volts for the water heater, but then with the known resistance of each resistor you must find the correct values at the voltage supplies which was given at 240 volts using ohms law.
Once you know to resistance of each resistor you can start filling in the missing items of each resistor at the supplied voltage.
To find the voltage drop across each of the series resistors you must find the total resistance of the series circuit, then you must find the current in this series circuit by dividing the supplied voltage by total resistance, this will give you the current (I) that you divide each loads resistance into, this will give you the voltage drop of each load in this series circuit, the parallel water heaters values will not affect this outcome, as it only adds to the total load on the supply.
So heres what you must solve:
I filled in the given values.
Toaster = R1
TV = R2
Water heater = R3
R1
E1= _110v__
I1= ____
R1= ____
P1= _700w__
R2
E2= _130v_
I2= _13.4a_
R2= _____
P2= _____
R3
E3= _250v_
I3= ______
R3= ______
P3 _47000w_
Now after finding the above missing values solve with the supplied 240v voltage by filling in the found resistance for each of resistors above in the spaces below, solve R1 and R2's total resistance and divide it into the supplied 240 volts to get the R1 and R2's total current, then multiply this current by each resistors resistance to get the voltage drop across each resistor in the series circuit, check your work by adding the two voltage drop to see if it equals the supplied voltage of 240 volts.
E1=
I1=
R1=
P1=
E2=
I2=
R2=
P2=
E3= 240 volts
I3=
R3=
P3=
I tell the test takers that drawing the ohms law circles one for power and one for ohms at the top of their work sheet can make it easy to reference while they are working the problems.
Once you know to resistance of each resistor you can start filling in the missing items of each resistor at the supplied voltage.
To find the voltage drop across each of the series resistors you must find the total resistance of the series circuit, then you must find the current in this series circuit by dividing the supplied voltage by total resistance, this will give you the current (I) that you divide each loads resistance into, this will give you the voltage drop of each load in this series circuit, the parallel water heaters values will not affect this outcome, as it only adds to the total load on the supply.
So heres what you must solve:
I filled in the given values.
Toaster = R1
TV = R2
Water heater = R3
R1
E1= _110v__
I1= ____
R1= ____
P1= _700w__
R2
E2= _130v_
I2= _13.4a_
R2= _____
P2= _____
R3
E3= _250v_
I3= ______
R3= ______
P3 _47000w_
Now after finding the above missing values solve with the supplied 240v voltage by filling in the found resistance for each of resistors above in the spaces below, solve R1 and R2's total resistance and divide it into the supplied 240 volts to get the R1 and R2's total current, then multiply this current by each resistors resistance to get the voltage drop across each resistor in the series circuit, check your work by adding the two voltage drop to see if it equals the supplied voltage of 240 volts.
E1=
I1=
R1=
P1=
E2=
I2=
R2=
P2=
E3= 240 volts
I3=
R3=
P3=
I tell the test takers that drawing the ohms law circles one for power and one for ohms at the top of their work sheet can make it easy to reference while they are working the problems.
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