What is the value or benefit of using an oversized neutral? The telecom guys insist that it limits interferance to their equipment, It doesn't make any sense to me. When I say oversized, on a 200A panelboard that has 3/0 conductors they are asking for a 4/0 neutral. This panel board serves a 100A sub-panel that their equipment is connected to. Can anyone make sense of this for me?
Oversized Neutral
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jimwalker
Senior Member
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- TAMPA FLORIDA
Re: Oversized Neutral
It makes some since but who is paying ?
It would give them a slight differance in voltage drop back to poco from neutral to them.In electronics even a half a volt can cause problems and with harmonics they could see problems but this is not nec mandated that you fix it
It makes some since but who is paying ?
It would give them a slight differance in voltage drop back to poco from neutral to them.In electronics even a half a volt can cause problems and with harmonics they could see problems but this is not nec mandated that you fix it
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Oversized Neutral
I am not adverse to the general notion of ?bigger is better.? However this ?oversized? neutral, being but one size larger, amounts to a 26% oversizing. I have heard of a 200% neutral, and I have seen a 140% factor for calculating the neutral load in certain specific circumstances (220.22). But a 26% oversizing? I think this is a random grasp for something bigger, and not a calculated or empirically determined selection of the ?correct? oversizing.
I am not adverse to the general notion of ?bigger is better.? However this ?oversized? neutral, being but one size larger, amounts to a 26% oversizing. I have heard of a 200% neutral, and I have seen a 140% factor for calculating the neutral load in certain specific circumstances (220.22). But a 26% oversizing? I think this is a random grasp for something bigger, and not a calculated or empirically determined selection of the ?correct? oversizing.
Re: Oversized Neutral
Well I am a telecom power engineer and can shed some light on the subject, however I doubt it will satisfy you.
Number one reason is to keep the voltage drop between N-G below 2 VAC P-P. The reason is the way switch mode power supplies operate.
Number two reason, to a lessor degree is neutral over heating. Harmonics cannot be controlled by the size of the neutral. We use delta/wye transformers to control harmonics and isolate them.
I have worked for three telecom's, SBC, MCI, and currently Alltel over the last 25 years and all three have similar practices which go something like this.
All prefer 480/277 service using a 480 delta UPS system to supply critical 120 VAC loads via a device called a PDU. The PDU uses a K-13 step down transformer with built-in filtering, monitor, alarms, and distribution. The PDU allows a new ground reference point to be established via N-G bond. General purpose recepts use a K-4 or standard transformer. The main difference between all of them is the size of the neutral they use, and for what type of loads. Could be 100, 141 or 200%. I use 100% neutral feeder for general purpose loads, 141% neutral feeder for critical equipment loads, and never 200%.
The rest of the equipment like HVAC, mechanical, rectifiers, etc are all 480 delta, and lighting is 277.
Keep in mind telephone companies primarily use -48 VDC operated equipment. Harmonics are not a problem there. It is the data equipment like servers, routers, hubs ect that use AC power, and the evolution of what we call collo hotels that complicate things. Collo hotels are areas set up to lease space to companies like AOL, Earthlink and the like to give them direct access to the network we provide. BIG BUCKS. The problem is every Tom, Dick, and Harry do things differently if they even have a clue what they are doing, and we have to provide an environment that accommodates all their needs. If not someone else will.
Well I am a telecom power engineer and can shed some light on the subject, however I doubt it will satisfy you.
Number one reason is to keep the voltage drop between N-G below 2 VAC P-P. The reason is the way switch mode power supplies operate.
Number two reason, to a lessor degree is neutral over heating. Harmonics cannot be controlled by the size of the neutral. We use delta/wye transformers to control harmonics and isolate them.
I have worked for three telecom's, SBC, MCI, and currently Alltel over the last 25 years and all three have similar practices which go something like this.
All prefer 480/277 service using a 480 delta UPS system to supply critical 120 VAC loads via a device called a PDU. The PDU uses a K-13 step down transformer with built-in filtering, monitor, alarms, and distribution. The PDU allows a new ground reference point to be established via N-G bond. General purpose recepts use a K-4 or standard transformer. The main difference between all of them is the size of the neutral they use, and for what type of loads. Could be 100, 141 or 200%. I use 100% neutral feeder for general purpose loads, 141% neutral feeder for critical equipment loads, and never 200%.
The rest of the equipment like HVAC, mechanical, rectifiers, etc are all 480 delta, and lighting is 277.
Keep in mind telephone companies primarily use -48 VDC operated equipment. Harmonics are not a problem there. It is the data equipment like servers, routers, hubs ect that use AC power, and the evolution of what we call collo hotels that complicate things. Collo hotels are areas set up to lease space to companies like AOL, Earthlink and the like to give them direct access to the network we provide. BIG BUCKS. The problem is every Tom, Dick, and Harry do things differently if they even have a clue what they are doing, and we have to provide an environment that accommodates all their needs. If not someone else will.
Re: Oversized Neutral
Never witnessed a failure, but I have seen several over heat from excessive current. All fixes were the same, K rated transformer with at least 100% neutral feeder and dedicated neutrals. You could just use a oversized transformer rather than a K rated, but where is the fun in that?
- Location
- Bremerton, Washington
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- Master Electrician
Re: Oversized Neutral
I have dad students in my classes who worked for a school district and have seen overheated and burned neutrals from computer loads.
I have dad students in my classes who worked for a school district and have seen overheated and burned neutrals from computer loads.
lady sparks lover
Senior Member
Re: Oversized Neutral
Believe it or not, I have a lot of dealings with harmonic loads, and oversized neutrals, because it has come to me and the company I work for that the neutral does at least carry %100 or more.
The K-rating helps with overheating, but what happens to the current on the neutral, it remains. Therefore you still have to oversize the CCC to compensate for 4CCC in one conduit. %200 might be a bit much, but hey with the increase in the number of loads that carry harmonics (computers, fluorescent lighting, and yes...VFDs), why not.
I think I mentioned this before, I've seen the contractor argue with me about having it...and lets just say he's paying for it because it caused a fire!
Believe it or not, I have a lot of dealings with harmonic loads, and oversized neutrals, because it has come to me and the company I work for that the neutral does at least carry %100 or more.
The K-rating helps with overheating, but what happens to the current on the neutral, it remains. Therefore you still have to oversize the CCC to compensate for 4CCC in one conduit. %200 might be a bit much, but hey with the increase in the number of loads that carry harmonics (computers, fluorescent lighting, and yes...VFDs), why not.
I think I mentioned this before, I've seen the contractor argue with me about having it...and lets just say he's paying for it because it caused a fire!
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