voltage drop calculation question

I'm installing some new fans in greenhouses that draw 6 amps at 120 volts. If they are wired on opposite legs of the 240 at the panel, then the amp draw should only be 6 amps total, not 12? Is this correct? In addition, there is a third fan in a farther greenhouse that draws 5 amps at 240. Trying to calculate if voltage drop will be too large with the additional load. Greenhouse is fed with a AWG 10 copper, but its the third sub panel and already 200 feet from the meter. Using resistance values for the various conductors between subpanels and estimated amps through each, I'm calculating a voltage drop at the farthest fan of about 5 volts. This seems well within tolerable limits, but I am l little confused because some things use 120 and some 240.

If you have 2 equal 120v loads on opposite sides the current on neutral at load center will measure zero. The current on power legs will measure 6 amps. So no voltage drop on neutral. There's still voltage drop on power legs.
Use amprobe and see for yourself.
George

Watts however will be amps times volts. WHich is 6X120 for one fan, or 6X240 or 1440 watts for 2 of them. No free lunch. Jim

Sounds like free energy.
You could have a booth at the fair.
Keep it up and the solar panels will be glowin.
Free enerdgee

If they shared a neutral there would be zero current on the neutral but 12 amps from two breakers.

Really not free energy. The two currents in neutral are 180 degrees out of phase so they cancel each other. That's also why the neutral I the service entrance is a smaller wire And why you try to balance load center. Have equal 120v loads on each leg.

Many don't understand the phase difference.
George

I learned about phase difference when I tried to split a 240 load on opposite load centers with the breakers parallel to each other nothing happened then I staggered the breakers so they where either in phase or out not sure but it worked--- A/C current

I'm just an apprentice electrician so I'm sure there are lots of people here with more knowledge.....but I don't understand why you are mentioning legs and phase differences if we are assuming the original poster is talking about a standard residential service. Standard service is single phase, at least it is around these parts.

240 is single phase from one side of the hot incoming service to the other. It is also single phase to neutral, but 120 volts. When looked at from the 120 volt point of view, there are 2 branches that are out of phase to each other. When loading the distribution breaker panel, every other 120 breaker from top to bottom is on the opposite branch. When a 240 load is connected, the double breaker attaches to both branches automatically. Best practices splits the intended heavier 120 circuit breakers so they are on opposite branches. Keeping the applied load balanced to reduce the load on the neutral, and share the load through the mains. Jim

Yippee. Something I did a couple of years ago. Putting new end on a 12ga. 100 foot long power cord. Now you would think you have a real bad ars power cord! Take your hairdryer "heat shrink gun" and plug it into the socket and turn it on. Listen to the sound and pitch of the gun. Now plug in your newly fixed cable and plug the gun into that. You will be surprised how much the pitch drops useing that 100 feet of cord. Real good physic and electrical lesson. Very surprising. Just remember this wasn't a crummy little 18ga. Extension cord. Give it a try.

Have your instructor show you what 180 degrees phase shift looks on a dual trace oscilloscope.

Most people don't understand what 240 vac really looks like, how the power transformer makes both 120 vac and 240 vac.

Let us know if your instructor has a dual trace scope.
George

Ray, you asked an electrical question so don't be surprised to see all sorts of answers, it goes with the territory on here. Here's the real deal as I best recall, but I've been retired from electrical power distribution design for years and rusty on the latest codes and NEC so NO WARRANTY ????..Things may change you know...?..


If you have two 120 volt fans and each draws 6 amps, two of them will draw 6 + 6 or 12 amps.

If one is on one leg L1 of a typical residential 120/240 Volt Single Phase Three Wire Service Transformer fed panel and the other was on the opposite (180 out of phase) leg L2, THE NEUTRAL CURRENT (transformer to panel Neutral buss) WOULD BE ZERO because L1 is 180 out of phase with L2 and they cancel each other out.....

If BOTH were on the same leg L1 or L2 and each drew 6 amps, the NEUTRAL CURRENT (transformer to panel Neutral buss) WOULD BE 12 AMPS

NOTE regarding the above two examples: Each fans individual branch circuit Neutral would carry its own 6 amps of return current, but the Neutral current from the transformer to the panel Neutral buss is what would be zero or 12 !!!!!!!!!!!!!!!!!!! The individual 120 volt branch circuit (Line and Neutral) to supply a 120 volt fan that draws 6 amps would have 6 amps of current flow in each conductor WELL DUHHHHHHHHHHHHHHHH.

A 240 Volt appliance WOULD DRAW ZERO AMPS OF NEUTRAL CURRENT, there's NO NEUTRAL even connected to it, only two Hots L1 & l2 plus an equipment ground (typically).

The way a typical residential 120/240 Volt Single Phase Three Wire service is obtained is by use of a transformer with a 240 volt end to end L1 to L2 Secondary winding. That transformer is tapped in its center (Neutral) such that its only 1/2 or 120 from either end to the center tap (Neutral), thus you have two legs of 120 from the outer end to the center tap (Neutral) but 240 volts single phase from end to end L1 to L2

There are a gazillion line voltage drop calculators on the net, choose one. Line voltage drop is a function of Current,,,,,,,,,,,Wire gauge,,,,,,,,,,,,Wire length. Add up all the loads,,,,,obtain the wire gauge,,,,,,,,,obtain the wire length THERES YOUR ANSWER

Hope this helps, post back any questions

If in doubt where fire or life safety is concerned consult competent trained professional electricians and engineers, local authority and the NEC, don't risk your life or a fire on opinions posted here, mine included.

John T

+1....................

Will it operate, yes . Ideal, barely . The fans don t require starting torque like a reciprocating compressor so they will spin up to speed.
The 240V fan and having the 120V fans on the opposite half of the secondary to have zero neutral current was a wise decision .
I would be more concerned now about everything being properly grounded .

(quoted from post at 11:20:38 01/02/20)
Will it operate, yes . Ideal, barely . The fans don t require starting torque like a reciprocating compressor so they will spin up to speed.
The 240V fan and having the 120V fans on the opposite half of the secondary to have zero neutral current was a wise decision .
I would be more concerned now about everything being properly grounded .

Click to expand...

orderline on topic, but I wanted opinion of buickanddeere, as I recall you are/were a power line person. These are pictures of 3 wire 120/240 to residence from transformer on pole. The two hots are truly 'hot' as in glowing red, but neutral not. Homeowner said never any problem/damage/etc. to home or service entrance. Power company came & said, 'whatever it was it must have resolved itself as we seen nothing wrong now'. No recurrence. My problem is, where could all that current have been going and end up with no damage at home service? Any ideas?

That is real good question . Either lines are shorted to each other , ,both lines are shorted to the neutral or each line is shorted to ground . Or much overloaded . Perhaps the home owner added a 200amp breaker panel to the original stack or mast .
3rd possibility is somebody growing "medicinal herbs" has clamped on about 200amps of grow lights ahead of the meter .
Have to wonder what would draw that kind of current without opening the transformers primary fuse. Unless several houses are supplied form a large transformer .
The service appears to be very old and small being either a 30 or 60 amp .

(quoted from post at 15:14:10 01/02/20)
That is real good question . Either lines are shorted to each other , ,both lines are shorted to the neutral or each line is shorted to ground . Or much overloaded . Perhaps the home owner added a 200amp breaker panel to the original stack or mast .
3rd possibility is somebody growing "medicinal herbs" has clamped on about 200amps of grow lights ahead of the meter .
Have to wonder what would draw that kind of current without opening the transformers primary fuse. Unless several houses are supplied form a large transformer .
The service appears to be very old and small being either a 30 or 60 amp .

Click to expand...

ost of that neighborhood was built in early 1940s I believe and it is typical to have 4 to 6 homes on a single transformer, thus making it reasonable that under fault conditions, a single home service lines could draw very large current. Owner said power company just left without any explanation. He is an electrical engineer, but not power (Giga Hertz digital), and has far too much to risk by taping ahead of meter. I'm with you in that there had to have been a fault somewhere that was not found, or as you say, more & more load was added over 80 years and service lines were never upgraded.
Thanks for your insight/response.