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Randell Tarin 01-21-2008 10:20 PM

How do panels get their amperage rating?
I was wondering how service panels get their amperage rating. If there is no main breaker on the panel, wouldn't the amperage be based on the size of the service feed and the load? Is the rating just arbitrary?

The reason I'm asking, I need a 100 AMP service with 12 slots. Couldn't I just as easily use, for example, a 70 amp panel if the service feed was adequate for 100 amps?

Stubbie 01-21-2008 11:20 PM


First the minimum service to a single family dwelling in the USA must be 100 amps. Second you must have overcurrent protection (main disconnect) on the line side of any panel used as a service rated panel. That disconnect can be in the panel or remote from it between the meter and the panel. If my main disconnect is rated 100 amps I must have a panel that is rated 100 amps or more. I cannot supply a 70 amp panel via a 100 amp main disconnect. For one a 70 amp panel will only have 4 spaces sometimes two. Second panels are ctl meaning they are circuit limiting which means manufacturers are limited to the number of spaces they can make available depending on the amperage rating of the panel. This is not to mean you add up the breakers but considering a diversified load.
For instance it wouldn't make sense to have a panel with forty spaces for branch circuits and only rate the panel for 70 amps. There are other reasons that maybe some of the others will respond with but this would take a book to cover the technical aspects of the panel rating.

Randell Tarin 01-21-2008 11:46 PM

What if this was a subpanel? Could one service a 125 AMP panel with cable only rated for 100 amps IF the load did not exceed 100 amps? Could you use a 100 amp breaker for the main in this same panel?

I've run my cable (for 100 amps), but haven't purchased the box for the sub yet. I have access to a free 125 AMP box so It would save me some money if this is kosher.

220/221 01-22-2008 12:41 AM


Could one service a 125 AMP panel with cable only rated for 100 amps
Sure. The amp rating on the panel is MAXIMUM amps.

elkangorito 01-22-2008 09:54 AM

The current rating of consumer panels (& other panels) is usually restricted to;

a] the busbar rating within the panel and/or,
b] the "temperature rise" characteristics of the panel.

Having said this, if you wish to install a 100 Amp panel "downstream" of a similarly rated panel (100 Amps or 125 Amps), it would be very wise to have some "discrimination" between the Main Breaker sizes. There are 2 types of discrimination to observe;

1] overload current discrimination &,
2] fault current discrimination.

Most circuit breaker manufacturers provide "discrimination" & "cascading" tables for their breakers. Therefore, it is not a good idea to mix brands of circuit breakers within an installation since the "time/current" curves will be different.

What I'm trying to say is that if your Main Switchboard has a main breaker size of 125 Amps & your downstream switchboard has a main breaker size of 100 Amps, there will not be enough discrimination between these breakers to inhibit the upstream breaker (the 125 Amp unit) from opening if a high current fault occurs at the downstream switchboard/circuits. With such a high current fault, you will have both breakers opening & thus lose all power.

As a general rule, "overload" (low current) discrimination is achieved when the upstream breaker size is between 1.4 to 1.6 times that of the "overload current" of the "downstream" breaker. Eg upstream breaker is about 150 Amps & the downstream is 100 Amps. This configuration will guarantee discrimination with an overload fault condition.
It's not the same for "fault current" discrimination. In this case, the "time/current" curves must be compared between 2 breakers to ensure that both will not open on a high current fault.

There is nothing unsafe about having breakers that do not discriminate between fault is just inconvenient when all the power goes out because a sub-panel had a high current fault.

Stubbie 01-22-2008 10:39 AM


If your installing a sub-panel fed from a breaker in your dwellings main panel that is fine. Thing is you need to know your service rating to the house main panel so look at the main disconnect and see what amperage it is.....100, 125, 200 etc... My point is in 'what is your reason' for needing the sub-panel and how much load is it going to distribute. In other words if my dwelling service is 100 amps and my diversified load on that panel is 75 amps and my sub-panel is 125 amps with a diversified load of 60 amps I will have exceeded the rating of the dwelling service by 35 amps. Remember the main panel disconnect has the current of both the main service panel and the sub-panel moving through it. So if I have 135 amps being imposed on the 100 amp main panel disconnect it will trip after a certain length of time. This goes back to what our friend in Australia (el kangarito) said about trip curves. Every breaker will hold for so long based on a time curve when subjected to an overload. So if you could give us a description of what your wanting to do and the loads your going to have on this sub-panel this would help shorten your learning curve a bunch. We need to know what your service panel rating is and what service you have to your dwelling 100 amps 150 amps etc.. And then why the sub-panel you out of spaces in your main? or wanting to operate a hot tub or something from your sub-panel that sort of thing.
One last thing...if your dwelling service panel is 100 amps you cannot subfeed out of it with an installed 100 breaker to another panel of equal amperage or greater. A 100 amp panel will only have a bus stab rating of 70 amps in the Usa this will be the largest breaker you can sub-feed with. All panels have bus stab ratings depending on the rating of the panel. The exception being panels that have feed thru lugs.

Randell Tarin 01-22-2008 09:51 PM

I have a convoluted setup. :blink: I live in the woods and came up with a solution that would keep the power company from removing half my trees.

I placed a 200 AMP mobile home meter base and panel (with pass-through lugs) near the transformer. This feeds:

1. A 200 AMP panel 300 ft away at the cabin. This panel feeds a 100 AMP sub-panel.

2. My workshop is fed by a 50 AMP sub-panel from the meter base panel.

3. The sub-panel that is the focus of this topic is going to be a 100 AMP box servicing two guest cottages. I was planning on a 100 AMP breaker upstream and a 100 AMP breaker downstream. This was also the topic of another thread because I'm using a 3-conductor cable.

Stubbie 01-23-2008 09:05 AM

Hi Randall

My you certainly have a lot going on there. I'm digesting your set up right at the moment. Just a few questions and in the next day or two I'll get back to you. Others may have some comments so keep checking the thread.


1. A 200 AMP panel 300 ft away at the cabin. This panel feeds a 100 AMP sub-panel.
Where is the 100 amp panel and what does it support. The 200 amp panel is for your dwelling that you live in correct? Is the 100amp panel attached to your dwelling or detached at another location?


2. My workshop is fed by a 50 AMP sub-panel from the meter base panel.
How are you feeding the sub-panel....from a breaker installed in the 200 amp panel in the mobile home pedestal?


3. The sub-panel that is the focus of this topic is going to be a 100 AMP box servicing two guest cottages. I was planning on a 100 AMP breaker upstream and a 100 AMP breaker downstream. This was also the topic of another thread because I'm using a 3-conductor cable.
Which panel is going to feed this 100 amp panel and is this the panel that you had to splice the cable because you relocated it?

Also are there ever going to be any metallic paths connecting any of these buildings in the future Like telephone or gas?

Please take the time to carefully answer these questions I have so I can get a better handle on what you have going on.

Randell Tarin 01-23-2008 04:24 PM

1 Attachment(s)
O.K., I've provided a diagram.

200 AMP Service Entry: This panel has a 200 AMP MAIN breaker. Mobile Home meter base with bonded neutral and ground. A grounding electrode is attached to pole. Pass-through lugs feed a second 200 AMP service at dwelling. This first panel at the meter base feeds the following sub-panels.
Sub-panel No. 1 WORKSHOP: a 50 AMP breaker feeds the sub-panel via 10/3 with ground. The MAIN and sub are metallically bonded through the bare grounding conductor. The neutral and ground are isolated in the panel.

Sub-panel No. 2 GUEST COTTAGES: A 100 AMP breaker will feed this sub-panel via 1/0 3-conductor cable. The neutral and ground will be bonded and I will be incorporating a grounding electrode here as well. No future metallic paths.
200 AMP Service Panel at Dwelling: The neutral and ground are bonded at this panel. There is a 200 AMP MAIN breaker. Grounding is accomplished through 2-grounding electrodes spaced 25' apart bond by a #4 bare copper conductor at a depth of 2'. This is also bonded to the cold water plumbing line. This panel contains breakers for: Central Heat & Air, Electric Range, Bath, Living Area, Hot Water, Washer, Dryer and other lighting circuits.

This panel also feeds:
Sub-panel No. 3 100 AMP sub-panel that is metallically bonded to it's parent by metal conduit. The neutral and ground in sub-panel are isolated. This contains primarily 20 amp circuits for a bedroom, sun room, yard and low-voltage lighting.

Stubbie 01-23-2008 04:43 PM

Thanks Randall

I will look this over and get back.

Some quick observations for the moment. The 50 amp breaker to your shop is wrong for 10/3 G it should be a 30 amp breaker not 50 unless you are mistaken on wire size.
The other thing is if thats a 3 wire feed from the 200 amp dwelling panel to the attached 100 amp panel is not code compliant. That feeder should be a 4 wire feeder since the 100 amp panel is attached to the same structure. But you say neutral and ground not bonded so sounds like a 4 wire.

The rest doesn't look like any problem but I haven't put as much thought into it as I should yet...don't want to overlook anything.

You do need to be careful of demand load on that main panel at the pedestal though.

Randell Tarin 01-23-2008 05:12 PM

"The 50 amp breaker to your shop is wrong for 10/3 G it should be a 30 amp breaker not 50 unless you are mistaken on wire size."
I looked again and it is a 30 AMP breaker. I'm about to upgrade and replace that whole panel. I'm going to need a 220v outlet for welding equipment sometime in the future. I'm going to beef up the feed and the box when I do.
"You do need to be careful of demand load on that main panel at the pedestal though."
Right now it's only feeding the two sub-panels. The panel at the house is more of a direct connection because of the way it's branched. So, if I'm not loading it down at the house, there's very little pull on the main panel.

It's only my wife and me living here now. We're frugal with our electrical usage. We're retired and cheap.:laughing:

The demand will likely begin to peak on weekends, as we are about to open up a bed and breakfast. But this will add less than the 100 AMPs that the sub-panel is rated for.

frenchelectrican 01-23-2008 05:49 PM

I have only one question to ask you what state are you in right now ??

Because there is a running change on the electrical code and AFAIK at least 3 states allready adpoted 08 code cycle it can really throw the monkey wrench in your plans.

the other thing with short run subfeed to the shop if you have #10 wires it must be protected with 30 amp breaker but if you are start to lay out the plans i really suggest run a 50 or 60 amp breaker and use the #6 THHN/THWN or #4 UF wires depending how you will run this.

but from master metering device to your house you can " squeak by " with 3 wire feeder but IMO i rather run with 4 wire feeder and keep the netural and ground wire sperated.

and with the UG wire the triplex is that wire is Alum or Copper it can really change the figure with voltage drop so i want to give you a head up with it.

that part i mention because the 08 code really can give you a headache if you not plan it right.

again just a quick answer just tell us what state you are in maybe you will be lucky be on 05 or earlier code cycle.

Merci, Marc

Randell Tarin 01-23-2008 05:55 PM

I live in Texas and I've been squeaking by with aluminum 3-conductor wire with absolutely NO VOLTAGE DROP for 3 years now.

I had the guy from the power company put a voltmeter on the meter base to verify that the transformer servicing me was providing the 240 volts I should receive. He measured 244 Volts.

I then had him measure at the dwelling. I ran an electric heater at the time for a load. He measured 246 volts!! A fluke maybe, but pretty good. I was going to do something along the lines of what you suggested for the shop. The only other thing that is new is the feed to the GUEST COTTAGES.

frenchelectrican 01-23-2008 06:06 PM

Ok you should be on 05 code cycle then you should be ok by time but a wise idea to check with your inspector when you add more panels there. i dont know if they have local code do come up as well so my wise idea is ask the inspector to verify it.

to be on safe side that all.

244 volts will be fine.

when you hook up the guest cottage just make sure you termated propely and for some reason i did not catch the length between the meter pedstal to guest cottage if you don't mind tell me the mesurement but i am pretty much sure you will have to use the #2 Al triplex if there is not much load other wise #1 will work as well [ kinda little oddball but #2 is more common ]

Merci, Marc

Randell Tarin 01-23-2008 06:10 PM

100 FT. I've run 1/0 3-conductor aluminum

I live in in the county and am not subject to inspection. If I add anymore to this current layout, I'll probably upsize the MAIN.

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