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#1
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220 neutral wire question
This is perhaps a safety question. Most of the disconnects to 220v
appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral. But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance? If so is this why 220v household clothes dryers are now four wire (hot hot neutral ground) or is there something in the dryers that needs 120v and thus needs the neutral for the 120v circuit? If the disconnect were to a subpanel that is a different issue since the subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now). |
#3
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220 neutral wire question
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#4
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220 neutral wire question
Doug Miller wrote:
In article .com, wrote: This is perhaps a safety question. Most of the disconnects to 220v appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral. Right so far... But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance? Well, yes, but that's no different from a 120V appliance that develops a hot-to-case short, except that the voltage is higher. Incorrect, the voltage is still 120V. The two hot legs of the 240V circuit are 240V relative to each other, but each is only 120V relative to the ground or neutral which are bonded together at the service entrance. Pete C. If so is this why 220v household clothes dryers are now four wire (hot hot neutral ground) or is there something in the dryers that needs 120v and thus needs the neutral for the 120v circuit? The latter. In a typical electric dryer, only the heating elements are 240V. The motor and timer are 120V. Likewise in an electric range: the elements are 240V, and the control circuits are 120V. If the disconnect were to a subpanel that is a different issue since the subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now). If that ever was permitted by Code, it must have been over 20 years ago. I'm quite sure that the 1984 Code prohibited it. -- Regards, Doug Miller (alphageek-at-milmac-dot-com) Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response. Download Nfilter at http://www.milmac.com/np-120.exe |
#5
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220 neutral wire question
In article , "Pete C." wrote:
Doug Miller wrote: In article .com, wrote: This is perhaps a safety question. Most of the disconnects to 220v appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral. Right so far... But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance? Well, yes, but that's no different from a 120V appliance that develops a hot-to-case short, except that the voltage is higher. Incorrect, the voltage is still 120V. The two hot legs of the 240V circuit are 240V relative to each other, but each is only 120V relative to the ground or neutral which are bonded together at the service entrance. Pete C. Sorry, you're right of course. Thanks for catching that. -- Regards, Doug Miller (alphageek-at-milmac-dot-com) Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response. Download Nfilter at http://www.milmac.com/np-120.exe |
#6
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220 neutral wire question
Sorry, you're right of course. Thanks for catching that. We all say things slightly inaccurately now and then. If there was a short to ground, you could conceivably get 240v if you somehow also contacted the other hot. Not too likely, but not impossible. |
#7
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220 neutral wire question
ground at:
http://www.selfhelpandmore.com/homew...ound/index.htm general at: http://www.faqs.org/faqs/electrical-wiring/part1/ see floating neutral at: http://www.codecheck.com/pdf/electri...plep39elec.pdf |
#8
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220 neutral wire question
Joseph Meehan wrote: wrote: This is perhaps a safety question. Most of the disconnects to 220v appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral. But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance? Unless there is a failure of the ground wire the short to the appliance case be a short to ground and the breaker will kill the circuit and both legs will be cut if it was properly connected to begin with. Exactly what I was thinking. How people can be saying that the appliance and the disconnect case will both be hot at 240V is beyond me. If so is this why 220v household clothes dryers are now four wire (hot hot neutral ground) or is there something in the dryers that needs 120v and thus needs the neutral for the 120v circuit? They are providing for a neutral for 120V needs, like a timer. If the disconnect were to a subpanel that is a different issue since the subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now). -- Joseph Meehan Dia duit |
#9
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220 neutral wire question
wrote in message oups.com... Joseph Meehan wrote: wrote: This is perhaps a safety question. Most of the disconnects to 220v appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral. But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance? Unless there is a failure of the ground wire the short to the appliance case be a short to ground and the breaker will kill the circuit and both legs will be cut if it was properly connected to begin with. Exactly what I was thinking. How people can be saying that the appliance and the disconnect case will both be hot at 240V is beyond me. I don't think anyone was saying there would be 240v, just possibly 120v. If the ground was broken and there was a short, then there would be 120v at the box connected to the broken ground; further assuming nothing had good enough contact to ground to trip the breaker. It is not the type of thing anyone should lose sleep over, but it is possible. |
#10
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220 neutral wire question
If the disconnect were to a subpanel that is a different issue since the subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now.
If that ever was permitted by Code, it must have been over 20 years ago. I'm quite sure that the 1984 Code prohibited it. I'm looking in one of my older subpanels and it appears to me that there are separate yet bonded together neutral and ground bars. Older but under 20 years old I'm sure. In other words the line input from the main panel is separated (three wire plus ground) but on the load side at the subpanel it looks like ground and neutral are the same. The subpanel goes have its own earth ground. I'm thinking what I describe above would not meet today's code? main panel subpanel hot----------------hot hot----------------hot neutral-----------neutral + ground ground----------(to above) | | | | earth earth |
#11
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220 neutral wire question
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