Thread: Main disconnect rating

This service panel has a 100 amp breaker labeled(common int. trip) and a double pole breaker connecting two 40 amp breakers. What would the main disconnect rating be?

There is one service panel and one sub panel. According to the labels, both panels are rated at 125 amps maximum. The two wires from the 100 amp breaker to the subpanel are 1 AWG AL; I could not read the rest but the SEC looked like 1/0 AL. Are both of these panels undersized?

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#1 Al is rated for 100 amps at 75° C. Adding up the CB's is a waste of time and is not indicative as to the service being of adequate size.

I'm talking about the panel rating at 125 amps. Do you not consider the CB's?

Originally Posted by Sean Thornton

I'm talking about the panel rating at 125 amps. Do you not consider the CB's?

From what I can see the panel is rated at 125 amps and is fed with 100 amp conductors and protected by a 100 amp CB. If all of that is true then the installation is fine. To tell if the 100 amp service is adequate for the structure you would need to perform a load calculation.

Originally Posted by Robert Meier

From what I can see the panel is rated at 125 amps and is fed with 100 amp conductors and protected by a 100 amp CB. If all of that is true then the installation is fine. To tell if the 100 amp service is adequate for the structure you would need to perform a load calculation.

The CB's in the subpanel are in excess of 125 amps; would you consider it undersized based on the breakers?

Originally Posted by Sean Thornton

The CB's in the subpanel are in excess of 125 amps; would you consider it undersized based on the breakers?

You can have 600-800 amps worth of breakers in a 200 amp panel and not be undersized. As Robert said, the only way to size a service is with a demand load calculation. Adding the breakers up does nothing but test your math skills.

Thanks, I'm beginning to catch on.

Sean

The panel is rated for 125 amps and the main breaker is rated for 100 amps, so the service is rated for 100 amps. The panel is fine for the service. If the main breaker was more than the panel or feeds could handle, it would be a problem.

Whether or not the breaker is appropriate for the panel is a different question than whether or not the service is adequate for its user.

The total of the circuit breakers will almost always be more than the service is rated for. If you draw more than the main can handle; the main will pop. That is a circuit breaker's/fuse's job.

The only way to determine if the service is adequate is as explained above.

Originally Posted by Steven Turetsky

The panel is rated for 125 amps and the main breaker is rated for 100 amps,

Actually, there are two main service disconnects there (up to 6 main service disconnects are allowed), the first main is 100 amps and supplies panel 'A', the second main is 40 amps and supplies panel 'B' ... *both* panels are (for lack of a better agreed upon term) 'distribution panels'. Neither is a "main" panel and neither is a "sub" panel.

It is true that adding up circuit breaker amperage does not accomplish much, if anything. It looks like this is a two disconnect service. One 100/2 and one 40/2, ( looks like the 40/2 feeds # 10cu, so it must be a motor load to be legal ). Since you have more than one main disconnect, the service "size" is determined by the code allowable ampacity of the wire supplying the main panel w/ those two C/Bs. If you are correct about 1/0 alum., it should be a 120 amp service, ( assuming 75o terminations ). The 100/2 likely feeds the sub-panel with all the branch circuits in it. Cannot see the grounding & bonding, but the only potential violation I can see is the # 10cu on a 40 amp C/B.

I dont see how the #10 conductors on the 40 amp CB is code compliant! The breaker should be 30 amp Max!!!

Originally Posted by James Bohac

I dont see how the #10 conductors on the 40 amp CB is code compliant! The breaker should be 30 amp Max!!!

It can be legal to oversize the C/B when it serves a motor load like an A/C or heat pump. It is done to keep the short term starting load of a motor from nuisance tripping the C/B. Most all motors have their own built in thermal protection which serves as a safegaurd for the oversized C/B. The label on the unit will usually tell you what overload is recommended. If the normal load / running current is - - - say 20 amps, you must add another 25%, ( 5 amps in this case ), and your wire must be sized for 25 amps, ( #10cu ). A 20 amp running load could require a 40 amp C/B. If the C/B is HACR rated or the fuses are motor rated, ( dual element ), they can be sized tighter to the running load and the nameplate should say that as well.

Originally Posted by James Bohac

I dont see how the #10 conductors on the 40 amp CB is code compliant! The breaker should be 30 amp Max!!!

When dealing with conductor and breaker sizing for A/c all the information you need is on the data plate. They are sized differently than a general purpose circuit and are under different rules. This should be taught in a basic HI training class.

It is entirely possible that you could have #12 on a 40 amp breaker and be in complete compliance when dealing with a motor or compressor type load.

Originally Posted by Jerry Peck

Actually, there are two main service disconnects there (up to 6 main service disconnects are allowed), the first main is 100 amps and supplies panel 'A', the second main is 40 amps and supplies panel 'B' ... *both* panels are (for lack of a better agreed upon term) 'distribution panels'. Neither is a "main" panel and neither is a "sub" panel.

I agree with you.

The point I was trying to make is that the total of the individual circuit breakers can be greater than the main breaker and determining if the main breaker and panel are compatible is different than determining if the service is ample.