Mdshunk- Got A Minute?

zuhow

Good evening.

Full disclaimer- I see you guys (rightly, imho) lighting up engineers w/ stupid questions- I'm not here to ask a stupid question, or at least I don't think so. I asked for mdshunk because he was quite patient with me a while back on some basic transformer questions. Others feel to chime in (pile on).

Help me fill some gaps in my education. I have a decent understanding of the workings of electrical distribution in general, but next to no practical understanding of the actual entire distribution top to bottom.

Don't worry I'm not designing something!

So today I want to ask about service entrance equipment. I really want some actual 'hands on' explanation of the pieces-n-parts that one uses to do this. Single service like my house- no problem. What I want to ask about is multiple occupancy distribution from the service entrance. Consider a mixed use occupancy- let's say a bar or store on the bottom with 2 apartments above it w/ a common hall and stair.

Your (1) service entrance comes into some 'widget' like this GE mini mod III -I haven't made enough posts to post a link- search GE's distribution web page
and on through the meter to each of 4 panels.

Finally to my questions: Is there a main interrupt before/on the buss that the meters sit on? Between it and the utility? I see the beaker before each meter- what if the panel is far away from the service entrance- do you add a disconnect between the stack and panel- if so is it a switch, a fuse, a circuit breaker- what kind of device (link to one if I'm not already asking to much)?

In short, could you explain the section of circuit, in detail -what hooks to what- between the utility last cable termination and the main on the panel located on other side of the building.

flame away

Z

mdshunk

Okay, I'm with you so far. I love these types of services.

Yeah, electricians simply call that a "meter stack". I'm not a fan of the GE brand in general, but I'm on the same page with you so far.

I might as well just boil this all down. The Mini Mod III actually doesn't contain any breakers. Don't use something like that. Use a modular metering system with integral "tenant breakers". These are actually factory wired into the busing of the meter stack after each corresponding meter. The NEC contains a general requirement that all the service disconnects for a building be "grouped". One easy way to do this is to get a meter stack with integral tenant breakers. Otherwise, you'd come out of each meter with a conduit to a seperate weatherproof disconnect, and that's just ugly.

Now then, the NEC does limit you to a maximum of 6 disconnects to kill a given structure. What if you have more than 6 meters, you might ask? Well, in that case, you'd put a fusible (or breaker) main ahead of the whole meter stack. The one big disconnect can be part of the meter stack "system", to simply plug on the side and interface with the busing, or it can be a seperate piece mounted beside. Different brands do this different ways. This is called "cold sequence metering", since you have a disconnect aheah of the meters. You still need the tenant breakers to protect each cable or set of conductors going to each panel.

I'll try. The utility's aerial drop or underground lateral simply connects to big main lugs in the meter stack. Everything inside the meter stack is prewired or pre-bused at the factory. All you do is pop in whatever size breaker you want in for each tenant main, and run your conductors or cable from there to each tenant panel. This would be a subfeed. All your service grounding is done in the meter stack (water pipe, grounding electrodes). The service bonding is already done at the factory.

If you've got more than 6 disconnects, your service drop or lateral will hit a big honking disconnect first, then feed from the load side of that disconnect to the main lugs of the meter stack. Some brands, that big disconnect will just "plug on" the side of the meter stack. Pop in a tenant breaker for each position and subfeed to each panel.

One thing to watch out for.... if the commercial space in this mixed use occupancy needs something substantially different, power wise, than the dwelling units, you'll have to select your meter stack carefully. If each dwelling unit will need 100 amps, single phase, and the retail space needs 200 amps, three phase, then you'll need to order equipment with care. Also, you will probably need to order 5th terminal accessories (if the PoCo requires) as a seperate part for each meter position.

I hope I covered most of what you wanted to know. I like to talk about these types of services, so feel free to ask for clarification.

zuhow

You're a huge help, thanks.

Let's assume single phase 200A shop and 150A apartments and a 40A panel for the building common loads. So if I read this right -the mains don't go through a disconnect prior to the tenant breaker, in this case (<6 disconnects). I assume the breaker at the stack and the panel main have the same rating- 200A/150A/40A?

What's a 5th terminal?

mdshunk

Actually, you won't need mains in your panels at all if you have them in the meter stacks. You can use main lug panels, if you want to. If you use main breaker type panels, you can use any size at all. The overcurrent protection is being provided by the breaker in the meter stack protecting the subfeed to that panel. The breaker in the panel would only be a handy disconnecting means, if you choose to go that route. Me, personally, I'd use main lug panels.

One handy tip... if you use Square D equpiment, you can realize a cost savings if you get main breaker panels because you can remove the main breakers from the panels and bolt them into the meter stack. Then, just bolt lugs in the panels where the breaker used to be. You probably save 100 bucks per position doing it that way.

A 5th terminal is an accessory for any meter base to accept a 5th prong on certain meters on network services (208, single phase). Popular in downtowns and congested areas.

orson

When you say you can use any size main breaker in the panel is that assuming it is at least as big as the breaker in the meter stack, or it completely doesn't matter?

I guess it wouldn't matter as long as it was big enough to handle the demand on the sub-panel, sorry, spoke before thinking.

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zuhow

In the case of main lug panels, wouldn't I have to kill each circuit to shut down everything in the unit? I just assumed that you would naturally have one big kill switch a normal course of operations- is that not always true?

mdshunk

No, if you already have one in the meter stack, there's no need for a second one, unless you want a main in the panel for whatever reason. It's just extra cost, is all, for little to no extra benefit. If you want the whole unit dead, you walk out to the meter stack and throw the tenant main off.

zuhow

mdshunk, thanks again.

Your comments directed me to some new references. I've got some more reading to do. I'll be back with more questions before to long, I suspect.

mdshunk

If you have a certain particular project in mind, post the details, and I can tell you how I'd do it. Naturally, there's 20 different ways to compliantly do any job.

zuhow

No particular project. I just feel it's important to understand the technology that powers our lives enough to call myself 'literate'. So while I understand the principles- I'm not up on the practices- you know, those details that actually make it happen.....