Fifth jaw for meters

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Coppersmith

Coppersmith

Senior Member
Location
Tampa, FL, USA
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Electrical Contractor
In a meter can, what is the fifth jaw for? Can I use a five jaw meter can in a four jaw application? is the fifth jaw removable?
 
I think it is needed when using on a 208/120 system with only two phases and neutral being metered. Not certain exactly how the metering works but with 120/240 and 180 degree phase angle you can meter accurately by monitoring only the two ungrounded conductors, but the 120 degree phase angle of the wye system has inaccuracies if only using a 4 wire meter so it needs a neutral reference to be accurate. Same should be true on any other wye system though 208/120 is the most encountered where you might see only two phases plus a current carrying neutral.
 
kwired said:
I think it is needed when using on a 208/120 system with only two phases and neutral being metered. Not certain exactly how the metering works but with 120/240 and 180 degree phase angle you can meter accurately by monitoring only the two ungrounded conductors, but the 120 degree phase angle of the wye system has inaccuracies if only using a 4 wire meter so it needs a neutral reference to be accurate. Same should be true on any other wye system though 208/120 is the most encountered where you might see only two phases plus a current carrying neutral.
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Here the POCO has always required 5 jaws for everything with the 5th jaw in the 9 o'clock position.

-Hal
 
Coppersmith said:
In a meter can, what is the fifth jaw for? Can I use a five jaw meter can in a four jaw application? is the fifth jaw removable?
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If it's a new area to you . Usually the counter guys at the local supply house will tell you what you should install.
They may not know why, But they'll say this is what everybody buys...
 
Buck Parrish said:
If it's a new area to you . Usually the counter guys at the local supply house will tell you what you should install.
They may not know why, But they'll say this is what everybody buys...
Click to expand...
Good point, they do.
Also, from what I've seen each Home Depot stocks the one that's required in the area where that store is located
 
I'm installing a meter main at a residence. I consulted the book from the POCO that lists the allowable units. I was having a hard time finding a unit I can use. I found one that is a five jaw and is listed as allowed for residential, but I've never seen a five jaw meter can in use at a residence. I wasn't sure if there was some other criteria I need to know about like it can only be used for 120/208 services. It doesn't list any other criteria in the book. I thought maybe if the fifth jaw was removable and I removed it, I wouldn't get turned down by the POCO when they come to inspect.
 
Coppersmith said:
I'm installing a meter main at a residence. I consulted the book from the POCO that lists the allowable units. I was having a hard time finding a unit I can use. I found one that is a five jaw and is listed as allowed for residential, but I've never seen a five jaw meter can in use at a residence. I wasn't sure if there was some other criteria I need to know about like it can only be used for 120/208 services. It doesn't list any other criteria in the book. I thought maybe if the fifth jaw was removable and I removed it, I wouldn't get turned down by the POCO when they come to inspect.
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Be careful here. I admit I dont fully understand the difference, but a factory "5 jaw" meter seems to be for 3 phase 3 wire, where a 4 jaw with a 5th jaw kit added is for single phase 120/208. Take a look at this link:


The note on the bottom says "for single phsae 120/208 use a 4 jaw with a 5th jaw kit"

edit: I used the 114TB recently for a single phase 120/208 service. POCO loved it.
 
Coppersmith said:
I'm installing a meter main at a residence. I consulted the book from the POCO that lists the allowable units. I was having a hard time finding a unit I can use. I found one that is a five jaw and is listed as allowed for residential, but I've never seen a five jaw meter can in use at a residence. I wasn't sure if there was some other criteria I need to know about like it can only be used for 120/208 services. It doesn't list any other criteria in the book. I thought maybe if the fifth jaw was removable and I removed it, I wouldn't get turned down by the POCO when they come to inspect.
Click to expand...
It really shouldn't matter. If the meter doesn't have the fifth tab it will still plug in. That don't mean some POCO guys won't insist you need a 4 jaw meter though.
 
Coppersmith said:
I'm installing a meter main at a residence. I consulted the book from the POCO that lists the allowable units. I was having a hard time finding a unit I can use. I found one that is a five jaw and is listed as allowed for residential, but I've never seen a five jaw meter can in use at a residence. I wasn't sure if there was some other criteria I need to know about like it can only be used for 120/208 services. It doesn't list any other criteria in the book. I thought maybe if the fifth jaw was removable and I removed it, I wouldn't get turned down by the POCO when they come to inspect.
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Can you post the approved units?
I know you don't like shopping online, but you might need to. Have you checked to see if Home Depot can order it?
 
James L said:
Can you post the approved units?
I know you don't like shopping online, but you might need to. Have you checked to see if Home Depot can order it?
Click to expand...

My GC is in a hurry. I can order several different units at Home Depot, but it will take a week. I found a useable unit at a local supplier. They have to ship it in from a warehouse overnight. (I don't have an account so I'm paying too much.) It's listed in the POCO guide as a 4/5 jaw. I hoping this means the fifth jaw is removable. The link below from Greybar lists it as a five jaw.

https://www.graybar.com/meter-main/p/97136111
 
Coppersmith said:
My GC is in a hurry. I can order several different units at Home Depot, but it will take a week. I found a useable unit at a local supplier. They have to ship it in from a warehouse overnight. (I don't have an account so I'm paying too much.) It's listed in the POCO guide as a 4/5 jaw. I hoping this means the fifth jaw is removable. The link below from Greybar lists it as a five jaw.

https://www.graybar.com/meter-main/p/97136111
Click to expand...
You need a 12S or 25S meter for this.
Generally I just tell the ECs to add the fifth clip. They will bolt right in to the hole in the 9 o’clock position. Add a jumper wire to the neutral.
I have removed a clip from an old meter base and just added it to the new meter base with the jumper wire. Why spend a fortune on a fifth clip when you throw them away all the time...
It is needed in 120/208 because with the 120 degree phase angles there could be as much as 75% metering error based on individual loading. All 208 loads will meter fine with 0% error.
More 120V loads will skew the error toward the 75% mark.
A 2S (regular residential meter) is non-blondel compliant, but is an industry standard as it will meter fine at 180 degrees difference.
The power formula for 2S is: P=Vab*(Ia-Ib). Remember, this is a vector formula.. I can’t get the arrow over the variables
The power formula for the 25S is P=Va*Ia+Vb*Ib. Again, vector math.

here’s an option also. These are sold all over

MILBANK 5T8K2 5TH TERM KIT (cips) | Gordon Electric Supply, Inc.

MILBANK 5T8K2 5TH TERMINAL KIT (cips)
www.gordonelectricsupply.com www.gordonelectricsupply.com


Siemens H659-0121 :: TALON GROUNDED 5TH JAW AT/AS LG BASE ON :: Rexel USA

GROUNDED 5TH JAW AT/AS LG BASE ONLY, Cat #: H659-0121, Mfr: Siemens
store.rexelusa.com store.rexelusa.com
 
Yup, a regular residential meter has a known error if there is imbalance between the sides of a 120/240V single phase system, but this error is usually quite small.

If you were to load up only one if the legs with 120V loads only and there were large voltage drop in the service then this could be a significant error.

A 5 jaw meter measures both legs separately and will work for 120/240 but is needed for 120/208.

Jon
 
The fith jaw was required for short amount of time on residential by the utility companies in my area for some years ago. Then they scratched it after a few years.
We would add them in as an accessory to the meter trough.
I just tossed out some out they were kicking around the shop for too long.
 
winnie said:
Yup, a regular residential meter has a known error if there is imbalance between the sides of a 120/240V single phase system, but this error is usually quite small.

If you were to load up only one if the legs with 120V loads only and there were large voltage drop in the service then this could be a significant error.

A 5 jaw meter measures both legs separately and will work for 120/240 but is needed for 120/208.

Jon
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I believe that depends on design of the meter?

I seen CT metering on 120/240 single phase that only uses one CT, and each ungrounded conductor passes through in opposite direction, making the meter effectively only see 120 volts at whatever current it is measuring. any 240 volt loads get doubled by passing the conductors in opposite directions through the CT and still would see accurate power level registered on the meter (2x current but still at 120 volts)
 
kwired said:
I seen CT metering on 120/240 single phase that only uses one CT, and each ungrounded conductor passes through in opposite direction, making the meter effectively only see 120 volts at whatever current it is measuring. any 240 volt loads get doubled by passing the conductors in opposite directions through the CT and still would see accurate power level registered on the meter (2x current but still at 120 volts)
Click to expand...

That approach would have essentially the same error as the regular single phase approach. The meter is calculating usage with the assumption that the two legs of the service are equal.

If you have a large imbalanced current but the service is 'stiff' with no voltage drop, then the result is accurate. But if the heavily loaded leg drops to say 110V and the lightly loaded leg rises to 130V, you are still being charged as if each leg were at 120V.

-Jon
 
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