Author Topic: Understanding USA 240 V split phase home electrical service?  (Read 6459 times)

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Offline JonnyBoats

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Understanding USA 240 V split phase home electrical service?
« on: December 18, 2014, 04:15:56 am »
At my house I have typical USA single phase electrical service with a 100 Amp panel. There are two phase conductors with a neutral coming in from the pole outside the house.

Today we had a power "outage" except instead of loosing power entirely all the lights went dim and I measured 61 volts between the phase conductor and the neutral where normally there would be 120 volts nominal.

I immediately shut off the main breaker and waited for normal service to be restored before turning it back on again.

My question is how, and why can this occur? Obviously feeding things like refrigerators with half of normal voltage is not a good thing.
 
 

Online NiHaoMike

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #1 on: December 18, 2014, 04:22:59 am »
Bad neutral.
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Offline jlmoon

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #2 on: December 18, 2014, 04:30:20 am »
I would lean towards a phase to phase fault some where in your neighborhood or near there.  Trees can also load a primary line down until the pole interconnect fuses blow.  There are many situations that can cause that.  Did you see 61v on both sides of your transformer?  If so then there was a voltage dividing fault on your primary line somewhere down the road.
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Offline SeanB

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #3 on: December 18, 2014, 04:56:23 am »
If both phases then upstream a phase was lost, and this then reflected as low voltage on the single phase by you after going through at least one 3 phase transformer. Single side low voltage is either a lost neutral in the local system ( between you and the distribution transformer) or a shorted local line to neutral fault in the area pulling the voltage low ( and you were upstream otherwise it would be gone) till a fuse finally opened to clear the overload.
 

Offline Richard Crowley

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #4 on: December 18, 2014, 05:21:55 am »
I measured 61 volts between the phase conductor and the neutral where normally there would be 120 volts nominal.
OK, so that was the voltage from ONE "phase" to neutral?  What was the voltage on the OTHER "phase"?

If the voltage on BOTH sides was the same, then the problem is likely outside your control. And your wiring is OK.

However, if the voltage is DIFFERENT on the two sides, then NiHaoMike is probably correct: Bad neutral.
And you have a significant problem in YOUR wiring which should be corrected ASAP.
 

Offline miguelvp

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #5 on: December 18, 2014, 05:26:20 am »
He did correct it ASAP

I immediately shut off the main breaker and waited for normal service to be restored before turning it back on again.
 

Offline Richard Crowley

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #6 on: December 18, 2014, 05:28:15 am »
He did correct it ASAP

I immediately shut off the main breaker and waited for normal service to be restored before turning it back on again.

Shutting off the service is absolutely NOT "correcting" the problem.  It is simply masking the symptom.
 

Offline Red Squirrel

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #7 on: December 18, 2014, 05:40:07 am »
Was the voltage on the other "phase" higher?  It sounds like a floating neutral, this is a very dangerous situation because it can fry lot of electronics.

From my understanding, your typical home electrical transformer brings 7,200 volts from 1 phase of a 3 phase system (ground being the other end of the winding) down to 240 volts, there is a centre tap which is a neutral, and ground.  At the panel they are bonded and after that they are considered different and should never be bonded again but generally will have the same voltage, 0.    The two leads coming out of the transformer are called the hots or legs and are 240v between each other, and 120v to ground or neutral.  A typical electrical panel has each breaker setup in a way that they feed off 1 of the two hots in a staggered way.  Breaker 1 is on leg 1, breaker 2 is on leg 2, breaker 3 is on leg 1 etc. (reading order along one line of breakers)  240 volt appliances like the oven and dryer use a double pole breaker as the circuit feeds from both breakers (which share each leg, so 240 volts) while a 120v item will feed off 1 breaker and then the neutral bus bar to complete the circuit.   Most 240v appliances will still have 120v stuff so there will actually be 3 wires going to them, 2 hots for 240 and a neutral for 120. 

In a floating neutral situation 240v loads will work fine, but 120v loads will be in series with the loads on the other leg of the 120.  The voltages will fluctuate based on the loads.  Ex: one side might be 100v while the other side might be 140v.   

If it was indeed a floating neutral you did well by turning off the main.  Do not turn off individual items as it might change the way loads are balanced and cause further damage.   To troubleshoot you'd then turn off every single breaker, open the cover off the panel and turn on the main with absolutely nothing else turned on.  You could then probe around as needed.... though this would probably be something you'd leave to an electrician unless you are familiar with electrical.  If you accidentally short out the live bus bar you'll be having a closed casket funeral.
« Last Edit: December 18, 2014, 05:42:45 am by Red Squirrel »
 

Offline miguelvp

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #8 on: December 18, 2014, 05:49:40 am »
He did correct it ASAP

I immediately shut off the main breaker and waited for normal service to be restored before turning it back on again.

Shutting off the service is absolutely NOT "correcting" the problem.  It is simply masking the symptom.

I might be reading too much into his sentence, but to me "waited for normal service to be restored" means he contacted the electric company and waited until it was all cleared.

I don't think he was just switching the power back on to test if it was normal without letting anyone know.
 

Offline SteveyG

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #9 on: December 18, 2014, 10:03:27 am »
He did correct it ASAP

I immediately shut off the main breaker and waited for normal service to be restored before turning it back on again.

Shutting off the service is absolutely NOT "correcting" the problem.  It is simply masking the symptom.

I think you've misunderstood the OP  :-//

Offline MadScientist

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #10 on: December 18, 2014, 11:03:46 am »
US " split phase " is not a two phase system. Its a single phase system. 
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Offline ptricks

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #11 on: December 18, 2014, 11:46:42 am »
US " split phase " is not a two phase system. Its a single phase system.

Correct. I think people confuse that quite a bit.
It is a simple transformer with a center tap.
 

Offline Zero999

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #12 on: December 18, 2014, 12:19:29 pm »
US " split phase " is not a two phase system. Its a single phase system.

Correct. I think people confuse that quite a bit.
It is a simple transformer with a center tap.
What's worse id some people don't listen. I remember having an argument with someone I used to work with about it and he'd never accept it.
 

Offline G7PSK

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #13 on: December 18, 2014, 03:22:08 pm »
Is not what the OP described a brown out often caused by too great a loading on the transformer or line.
 

Offline SteveyG

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #14 on: December 18, 2014, 03:41:44 pm »
Is not what the OP described a brown out often caused by too great a loading on the transformer or line.

Could be, but we don't have enough info.

Offline SeanB

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #15 on: December 18, 2014, 04:47:25 pm »
Actually in the USA you can have a 2 phase system, using a pair of the 3 phases off a 3 phase transformer per household, and rotating phases down the street. That way you have 120VAC between phase and neutral, but 208VAC between phases. It does not really make much difference to appliances on the 120VAC side, but does reduce the power on the appliances connected to the 2 phases slightly.
 

Offline Bassman59

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #16 on: December 18, 2014, 11:46:32 pm »
Today we had a power "outage" except instead of loosing power entirely

You lose power when the connections become loose.
 

Offline JonnyBoats

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #17 on: December 19, 2014, 04:43:03 am »
To answer some of your questions:

I had 61 volts between each of the phase conductors and the neutral. It should have been 120 volts. My service would normally have 240 volts between each of the phase conductors. This is the most common residential service in the USA. A service with 208 volts between phase conductors is possible, but not what I have.

As to where the problem occurred, it was not within my house; all the surrounding buildings had the same issue which was resolved by actions taken by the power company.

There remains the open question as to why the power company's equipment delivered reduced voltage rather than shutting off power entirely? Was this a failure of one of their relay's or other interrupting device?
 

Offline SteveyG

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #18 on: December 19, 2014, 09:01:18 am »
If you read 61V on both and 122V between them, then simply the single phase HV supply to  the transformer failed.

Offline tonyarkles

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #19 on: December 19, 2014, 05:52:52 pm »
Actually in the USA you can have a 2 phase system, using a pair of the 3 phases off a 3 phase transformer per household, and rotating phases down the street. That way you have 120VAC between phase and neutral, but 208VAC between phases. It does not really make much difference to appliances on the 120VAC side, but does reduce the power on the appliances connected to the 2 phases slightly.

That's what I used to think to, but it's wrong. As others have described, it's actually 240V, centre-tapped to give 2x 180degree out of phase hots. http://en.wikipedia.org/wiki/Split-phase_electric_power
 

Online NiHaoMike

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #20 on: December 20, 2014, 03:56:15 am »
It really is two phases of 120/208 in some urban environments.
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Offline SeanB

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #21 on: December 20, 2014, 04:21:00 am »
Even the wiki referenced has a 208V as an option if you have a 4 wire 3 phase system.

http://en.wikipedia.org/wiki/High-leg_delta

 

Offline Stonent

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Re: Understanding USA 240 V split phase home electrical service?
« Reply #22 on: December 21, 2014, 07:38:16 am »
A similar situation happened to me before at the house that I grew up in.  The house shifted and the outside meter panel had some wires that ripped out / loose. So the lights in the house were very dim and nothing else worked. 
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