acceptable impedance across hot/common in home circuit


Old 11-02-06, 12:28 PM
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acceptable impedance across hot/common in home circuit

Recently I bought a new multi meter for a old home rewiring project. When checking my circuits for continuity I happened to measure the impedance across the hot and common of the unpowered circuits and found that it was about 375 ohms. I thought it would/should be much higher. The manual for the multimeter says that for continuity checking anything higher than 50 ohms is considered OPEN and anything lower than that is SHORTED.

Most of the existing wiring for receptables has been dropped from the ceiling with BX grounded to metal boxes. I've found some solid conduit in the walls no longer being used, so I think the BX was probably an upgrade. Most of it has older cloth insulation within the BX and whatever is in the ceiIing as a backbone for the ceiling fixtures and wall receptacles has cloth insulation either in BX or solid conduit. As I've added receptacles, I've used MC where I've needed to feed it through existing wall structures. I've replaced all receptacles and used GFCI receptacles in the kitchen, bath and strategically for runs in other rooms.

With this 375 ohm reading, should I be concerned that there is some leakage across hot and common wires in this part of the wiring that I can't examine?
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Old 11-02-06, 12:55 PM
Join Date: Sep 2003
Location: Central New York State
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You did not measure impedance.

Stop your testing. Your results are meaningless. You are measuring resistance through any and all devices on the circuit, and across all other circuits in your house, and back through the power company.

Unless you know how to use your meter, it is nothing but a toy.

But do be careful. Using it incorrectly COULD get you killed.
Old 11-02-06, 01:06 PM
Join Date: Dec 2005
Location: USA
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blwallen - impedance is a measure of several factors (reluctance, reactance and resistance) in an AC circuit. Your multimeter is only capable of measuring resistance.

There are circumstances where resistances higher than 50 ohms can be a very serious short.

Get a book on basic electricity and another on home wiring. Once you understand the basics, then you can use your new multimeter in a meaningful way.
Old 11-02-06, 03:15 PM
Join Date: Oct 2006
Location: Wilmington
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What the others said, you are reading thru light bulbs, transformers, etc. The readings don't mean a thing, they are DC readings in an AC circuit. At least you measured dead circuit, or you meter would be smoking in your hands.
Old 11-02-06, 08:02 PM
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Sorry for the terminology ignorance. My practical ignorance isn't that profound. I bought the meter to do continuity testing on unpowered circuits and in the process checked the resistance. And yes, I do understand that I should only use the meter to test voltage or amperage on powered circuits. My primary goal is to check the integrity of the wiring before I power the circuits.

There are only two circuits involved. They aren't currently connected to that load center; they just involve runs of existing wiring to receptacles and ceiling fixtures and to new wiring, receptacles and fixtures I've added. Nothing is plugged into the receptacles, though I did just remember that there is one ceiling fixture with a fan that is still connected, so I could have been measuring across a motor. I will remove that and measure again if there is some useful measurement I can make. As noted, there are GCFI receptacles that I'd be measuring across.

I've added circuits for the kitchen and bath with THHN in thinwall conduit and added a couple of receptacles by pulling MC and connecting it to existing receptacles fed by BX. I am confident in my own work, but don't know about the condition of existing wiring that I can't see. My choice is to do some testing that will assure me of its integrity or replace all runs from the load center with THHN in wiremold.

Given the set of conditions I outlined--just wire in the walls and empty receptacles and incandescent lighting fixtures--no transformers for fluorescents or door bells, no dimmers--is there a way to check for shorts across hot and common and hot and ground, in addition to checking for continuity over common, ground and hot wires, before I apply power to the circuits?
Old 11-03-06, 03:37 AM
Join Date: Oct 2006
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Your assumption was correct, with nothing connected, no lights, no transformers/motors, etc., you should read an open.
Old 11-11-06, 05:22 AM
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Location: New York
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I also agree. With not connected, there should be essentially infinite impedance between the hot and neutral. That should also be the case between hot and equipment ground (green).
Electricians perform this test all the time in commercial work, they just use a different piece of equipment to test insulation integrity, one manufacturer is called a megger.
Old 11-11-06, 07:25 AM
Join Date: Feb 2002
Location: port chester n y
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I have inferred that you are concerned about the integrity of the conductor insulation of existing wiring of an indefinite age.
One possible problem with defective conductor isulation is "leakage-to-Ground".

A very accurate test for conductor insulation "leaking" current is performed with a GFI receptacle. Connect the conductors of the circuit to be tested to the "Load" terminals of the GFI receptacle. The GFI will detect any difference in the current-values (amps ) of the two conductors connected to the GFI.If the current-values are not exactly equal because there is leakage thru the insulation of the "live" ( 120 volts-to-Ground )conductors of the circuit, the GFI will "Trip" and open the circuit.
Old 11-11-06, 10:03 AM
Join Date: Nov 2004
Location: CA
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That meter instruction which says under 50 is short/over 50 is open is essentially worthless. It may mean the set point of the beeper on the continuity function, and that will mislead you all over the place.

Resistance of a 120 watt bulb..120 ohm. NOT open. DC resistance of a 4500 watt water heater element...about 12 ohms. NOT shorted.
Old 11-15-06, 01:27 AM
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Thanks for the recent posts.

I believe I have disconnected all devices in the unpowered circuit I am testing and am still reading about 375 ohms across hot and ground. This circuit was the original one for ceiling fixtures and receptacles in a small apartment. I have most of the boxes open that contain junctions. I can try to isolate those parts of the circuit that seem to be responsible for this resistance by disconnecting different parts of the circuit and measuring each one, then install new wiring for those sections.

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