Here they allow a standard meter socket up to 320amps. Any service that is 400amps or larger requires ct metering. So if I can get a 400 amp service out of a 320 amp meter base why would it say 400amp services and larger must be ct metered?
320amp vs 400amp
- Thread starter fireryan
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Here they allow a standard meter socket up to 320amps. Any service that is 400amps or larger requires ct metering. So if I can get a 400 amp service out of a 320 amp meter base why would it say 400amp services and larger must be ct metered?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You need to get out of the mindset that the meter is somehow a limiting device on service size.
Most of the time if one has a 400 amp service that usually means there is a 400 amp main overcurrent device or devices that sum up to 400 amps.
Per NEC if we have only one service disconnect then the supply conductors (using the 400 amp example) would need to be 400 amps or at least conductors that have ampacity above next lower standard size device so at least 351 amp conductors so long as calculated load doesn't exceed the conductor ampacity.
If we have the allowed two to six service disconnects or even special conditions that permit more then that, we could easily have the sum of all overcurrent devices exceed the ampacity of the conductors that supply them - but conductors must still have ampacity that at least meets the calculated load.
Good example sticking with a 320 meter might be a six unit apartment structure that has one meter for all six apartments, but has six 100 amp service disconnects (and a net load calculation that is 400 or less factoring any continuous loads at 125%). Here we have the ability to supply 600 amps of current without tripping anything but this is still a 400 amp service because that is all it was designed to handle. Some POCO's won't always see it that way but most will. Many of them also will only supply it with a 25, 37.5 or maybe a 50 kVA transformer just depends on actual load and demand,though if it really draws 400 for any extended time they would likely need to use a 100 kVA unit.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Who is telling you that you cannot use a 320 meter socket for a 400A service?
mwm1752
Senior Member
- Location
- Aspen, Colo
I see alot of back & forth that doesn't set the tone -- the subject is a service -- you calculate the load & size per calculation -- some or most will builds a service in excess of the calculation which is not an issue -- If your service is 375 amps then you shall install equipment rated a minimum of 375 amps -- so I have a 320 amp meterbase with 2 - 200 amp panels -- your load calculation shall not exceed 320 amps due to the rating of the meterbase/main combo -- the now feeders to a 200 amp panel under usual circumstances shall feed a continuous & non continuous load rated at a maximum of 160 amps total for each panel -- The same is true for a 400 amp disconnect feeding a remote 400 amp panel except the max load on the remote panel is 320 amps. If, you have a 400 amp main breaker panel then you are still basing the load per the calculation -- But now the metering system may change from line plug in to ct metering based on the calculation -- Clear as mud???? or see my 1st post
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Mwm you have more than one error in that post.
Go back and read what I wire and I posted please. The way you rating the panels is incorrect.
Go back and read what I wire and I posted please. The way you rating the panels is incorrect.
- Location
- Massachusetts
NO!
If your properly done NEC load calculation results in 400 amps you may use a class 320 meter. It is rated up to 400 amps.
- Location
- Massachusetts
Well that may be what 'they' allow you but the class 320 meter is designed to meter up to a 400 amp service.
Remember, the meter is actually a class 320 meter, not a 320 amp meter.
Take a minute, follow this link and read what the manufactures tell us
http://www.cooperindustries.com/con...ing_equipment/ring-type_metering/Class320.pdf
(Edit: Fixed broken link)
What is 'it' who or what is telling you this.
If it is the power company that is fine, they may have made their own rules. They are allowed to do that. They could just as easily have their own rule requiring CTs for all services.
But the class 320 meter and the socket are in fact made and designed for use with a 400 amp service.
Remember the meter is of little to no concern to the NEC, however to the power company it is their cash registrar and they get to set the rules for it.
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
What they are saying is the sum of the non continuous plus the sum of 125% of the continuous load will be 400 amps or less. This would give you up to 320 continuous amps as a possibility. Doesn't matter if we are selecting the meter/socket, the conductors or the overcurrent devices.
I think the utility guys/meter designers just decided to call it 320 amps to help differentiate it from one that bolts in and could be used for 400 amp continuous loads. Those do exist and I think are even rated up to 600 amps, but most POCO usually prefer CT type metering instead of using those, I don't really know why could be cost, could be safety reasons, or maybe a little of both?
I know the one time I installed a bolt on meter socket and then POCO came back later and stripped it and placed CT's inside it, I asked one of their rep's why they did that and the response was something to the effect they didn't want their tech's to be pulling/reinstalling a 480/277 meter with that kind of arcing incident capacity that installation had. But my thoughts were that this is a single customer being supplied - why not make it standard safety policy to pull the transformer primary fuses when such an event is to happen - all the rural power companies do that same thing all the time on the hundreds of irrigation services there are in the area with self contained 480/277 metering devices.:blink:
I think the utility guys/meter designers just decided to call it 320 amps to help differentiate it from one that bolts in and could be used for 400 amp continuous loads. Those do exist and I think are even rated up to 600 amps, but most POCO usually prefer CT type metering instead of using those, I don't really know why could be cost, could be safety reasons, or maybe a little of both?
I know the one time I installed a bolt on meter socket and then POCO came back later and stripped it and placed CT's inside it, I asked one of their rep's why they did that and the response was something to the effect they didn't want their tech's to be pulling/reinstalling a 480/277 meter with that kind of arcing incident capacity that installation had. But my thoughts were that this is a single customer being supplied - why not make it standard safety policy to pull the transformer primary fuses when such an event is to happen - all the rural power companies do that same thing all the time on the hundreds of irrigation services there are in the area with self contained 480/277 metering devices.:blink:
- Location
- Massachusetts
I think it is much simpler than that, they confused a class with a current rating.
It is not a 320 amp meter, it is a 400 amp meter that is called a class 320 meter.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I can buy that.
What some also don't think about is a lot of other "classes" will also fit that 320 socket, the blade size and spacing are the same, but some of them would not take a 400 amp load very well at all.
You can also plug a single phase meter into a three phase socket - but will not have any connections to the center phase.
- Location
- Massachusetts
Yeah, they are not like NEMA plug and receptacle configurations, some skill must be used in selecting the right meter for the job.
Fitzdrew516
Senior Member
- Location
- Cincinnati, OH
I am in agreement with the main consensus on this thread. If it's a bit difficult to get the load calculation aspect through your mind then check out this link; it explains what has already been said with a little bit of clarity.
http://sedatacenters.com/resources/...ent-vs-80-percent-powerpact-circuit-breakers/
Theoretical question though - If I had a 100% rated 400A main then technically my calculated load could be greater than 400A as long as my connected load did not exceed 400A. Now what do you size the wire to? The calculated load or the service disconnect (i.e. 400A)?
http://sedatacenters.com/resources/...ent-vs-80-percent-powerpact-circuit-breakers/
Theoretical question though - If I had a 100% rated 400A main then technically my calculated load could be greater than 400A as long as my connected load did not exceed 400A. Now what do you size the wire to? The calculated load or the service disconnect (i.e. 400A)?
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Wire size would be to the calculated load. 400A continuous connected load would be a 500A calculated load for wire sizing.
- Location
- Massachusetts
I would say impossible to answer as the NEC has no control over the meter.
If I had a 400 amp service and utility says 'you will use a 200 amp socket' that is what I would do. The size of the meter is just like the size of the utility service conductors they are not subject to the NEC.
Fitzdrew516
Senior Member
- Location
- Cincinnati, OH
Sure, I get that, but I'm saying theoretically you have a "320A" meter from the utility and you have a 400A 100% rated breaker for your main. So as Jumper says technically you could have 500A (calculated) on that service as long as my wire is sized as such, correct?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You need to calculate load before you know what minimum size service disconnecting means is necessary.
When sizing conductors and overcurrent devices for an individual load it is simpler - that load is either continuous and 125% rule applies or it is not continuous - if using a standard overcurrent device. If using a 100% rated device there is no 125% to deal with.
Multiple loads of same type is sometimes a little more complex but somewhat similar - maybe a feeder that only supplies motors fits this category.
If you are using art 220 for calculating multiple loads then you have already factored in where needed 125% for continuous loads, and in some cases reduced the load because of allowable demand factors. If your result is 375 amps then you need a 375 amp minimum conductor and 375 amp minimum overcurrent device. You likely go next standard size higher which is 400 amps with overcurrent and the conductor will depend on ampacity adjustments that may be required on top of base ampacity.
- Location
- Massachusetts
My opinion is yes you could do that because the NEC has no say in it.
But trying to get an inspector to accept that or the utility would be more hassle than it is worth at least to me.
Besides, I do not choose the socket, the power company tells me what socket to purchase.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Dominion Power VA not only tells the size/type/style, but they give it to you for free for 600A services and smaller. Bigger stuff you have to purchase from them.
Carultch
Senior Member
- Location
- Massachusetts
For a utility service meter, I understand that you pretty much have to follow the utility standards. Some of them have come to their senses, and understand that a class 320 meter system is suitable for any and all 400A services, others may still require instrument metering.
For a customer-owned meter, installed for purposes other than the service metering, the NEC does have a say in it, and is the governing code that matters. And judging from this conversation, a class 320 meter system is suitable for any circuit with a 400A and less OCPD.
- Location
- Massachusetts
Good point and I agree.
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