Thread: 'Over fused' Electric Furnace

Is there a code exception (Canadian or US) allowing a circuit breaker of higher amperage than is permitted by the manufacturer of an electric appliance (i.e. if it's 'over fused' by only a small percentage)?

Specifically, I've found a 100 amp breaker protecting a furnace that states max allowed is 90 amps. I know that this is likely not permitted, but there are some unusual exceptions out there.

It's like having an 17 amp breaker where a 15 amp one is req'd (if one existed). Not right, but would it really make a difference?

Thanks.

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Below is a list of the standard size breakers and fuses. If your need falls between two of the standards you can use the next higher rating. Was that your questions?


240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers.

The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes. Additional
standard ampere ratings for fuses shall be 1, 3, 6, 10, and
601. The use of fuses and inverse time circuit breakers with

nonstandard ampere ratings shall be permitted.

James,

Useful information, but it doesn't directly answer my question (although, indirectly I believe it does).

The furnace manufacturer states to use 90 amp breaker (max) but it's being protected by a 100 amp breaker. Since 90 amp breakers exist, I'm assuming that the 100 amp is not permitted.

Thanks.

Originally Posted by Ian Currie

Is there a code exception (Canadian or US) allowing a circuit breaker of higher amperage than is permitted by the manufacturer of an electric appliance (i.e. if it's 'over fused' by only a small percentage)?

Specifically, I've found a 100 amp breaker protecting a furnace that states max allowed is 90 amps. I know that this is likely not permitted, but there are some unusual exceptions out there.

It's like having an 17 amp breaker where a 15 amp one is req'd (if one existed). Not right, but would it really make a difference?

Thanks.

Breakers do not trip , just say a 90 amp, at 90 amp. I would have to look it up but I believe it is a certain percentage like 10 or 15% over what they are labeled. So being over by 10 amp and your max supposed to be 90 amp then you 100 amp won't trip until about 110 amp draw which could do some major damage to the equipment. There is over amperage already figure in. That is how they determine the max amperage.

That is what I was thinking too!

James,

Useful information, but it doesn't directly answer my question (although, indirectly I believe it does).

The furnace manufacturer states to use 90 amp breaker (max) but it's being protected by a 100 amp breaker. Since 90 amp breakers exist, I'm assuming that the 100 amp is not permitted.

Thanks.

Ted,

Just saw your response. What you're saying makes sense. I'll call this one out - I just wanted to see if anyone knew of any exceptions before I did.

That is why they are called inverse-time circuit breakers. The greater the overload the quicker the breaker will kick out. That's why a breaker kicks out immediately when a dead short happens. The amperage goes to infinity (theoretically) when a dead short happens.

First check for an upgrade to the heat strip, it may have been changed for efficiency reasons. Staging the heat up to 3 times with sequencers. The third stage may have a higher KW than the original single stage strip, but will only be used in extreme circumstances.

Originally Posted by David Bell

First check for an upgrade to the heat strip, it may have been changed for efficiency reasons. Staging the heat up to 3 times with sequencers. The third stage may have a higher KW than the original single stage strip, but will only be used in extreme circumstances.

Good suggestion. In this case; however, the furnace is 15kW and the home is only 700 sq ft. Also, the furnace is brand spanking new (manufactured in 2010).

I have since spoken with an electrician who told me that 90 amp breakers are non-standard for residential panels so they are permitted to go the next size up (100 amps in this case). Nonetheless, the data plate states MAXIMUM breaker to be 90 amps. I wonder, if then, the next size down would work. And if so, how does a person know what the MINIMUM size breaker can be? The data plate does not provide this info.

What is the make of the panel in the house? I have never heard of a breaker manufacture who does not make all of the standard size breakers below a 100 amp. The big box stores may not carry them but they are available.

Originally Posted by Ian Currie

I have since spoken with an electrician who told me that 90 amp breakers are non-standard for residential panels so they are permitted to go the next size up (100 amps in this case). Nonetheless, the data plate states MAXIMUM breaker to be 90 amps. I wonder, if then, the next size down would work. And if so, how does a person know what the MINIMUM size breaker can be? The data plate does not provide this info.

Bullony! We have AC units changed (for example) out all the time with some weird breaker size being the maximum size....say a 45 amp maximum...We don't let them install a 50 amp. Find the correct size or it don't pass inspection.

Now sometimes there is a minimum and a maximum stated on the specifications. Say...maximum 45 minimum 40 amp. If they can find one within that range that is common then all is well!

Originally Posted by Wayne Carlisle

Bullony! We have AC units changed (for example) out all the time with some weird breaker size being the maximum size....say a 45 amp maximum...We don't let them install a 50 amp. Find the correct size or it don't pass inspection.

I don't doubt what you're saying at all. It may have been bad advice, but it was from a a local electrical contracting firm that I believe is reputable. I will definitely make a few more phone calls.

In the end, I will always 'call out' situations like this one and let the electricians deal directly with the homeowner. As I said before, I just wanted to find out if there was any leeway for larger breakers.

Originally Posted by Ian Currie

I don't doubt what you're saying at all. It may have been bad advice, but it was from a a local electrical contracting firm that I believe is reputable. I will definitely make a few more phone calls.

In the end, I will always 'call out' situations like this one and let the electricians deal directly with the homeowner. As I said before, I just wanted to find out if there was any leeway for larger breakers.

When I run into this type of problem I tell my client to ask the seller for a letter from the electrician that states his position so if in the future there is a problem the buyer can go back to the electrician for correction. If he is a licensed electrician he will not be willing to write the letter and will go ahead and fix the problem. Saying it is one thing....putting it in writing makes it official.

Originally Posted by Ian Currie

I have since spoken with an electrician who told me that 90 amp breakers are non-standard for residential panels so they are permitted to go the next size up (100 amps in this case).

The electrician (at least if he were here in the states) would be wrong.

Up there ... I don't know what the Canadian Electrical Code allows, but he would be wrong down here.

Nonetheless, the data plate states MAXIMUM breaker to be 90 amps. I wonder, if then, the next size down would work. And if so, how does a person know what the MINIMUM size breaker can be? The data plate does not provide this info.

The MAXIMUM breaker size specified on the name plate is the MAXIMUM size allowed to be installed, period. *IF* the heat strip was replaced with a larger size, the labels typically have a place for the installer to mark which of the ALLOWED size heat strips is actually installed, and the maximum rating is based on that, which is also on the label.

The MINIMUM size breaker is one which would not trip ... ... and to determine that would be trail and error (after making some calculations to come up with a close-to-minimum-size).

Jerry is correct. The maximum allowable amps as STATED BY THE MFG. OF THE EQUIP. is the absolute tops. It is their equipment and they have reasons for the rating. It is not to be exceeded. If no breaker exists to match the number then it still cannot be exceeded, the breaker rating must be lower. How low? Perhaps as low as it could be without nuisance tripping every time the unit fires up but logically the next lowest value from the data plate stated max for the application. I saw an A/C unit rated for 60A running fine on a 40A breaker.

It's common to find A/C units over fused. This situation can damage equipment or wiring or cause a fire. There are times when electrical codes are superseded by mfg. stats.

What torks me about the commonplace over fusing of A/C units is the fact that the equipment ratings are posted clearly on the case data plate yet they are routinely ignored by "professionals" (even if they have been in business for a long time)who look down their nose at home inspectors.

O.K.....I'm done venting.

Bob,

No one could miss the breaker rating on this furnace. I swear it was written in text that was an inch tall in plain sight, on the front cover.

Originally Posted by Bob Knauff

Jerry is correct. The maximum allowable amps as STATED BY THE MFG. OF THE EQUIP. is the absolute tops. It is their equipment and they have reasons for the rating. It is not to be exceeded. If no breaker exists to match the number then it still cannot be exceeded, the breaker rating must be lower. How low? Perhaps as low as it could be without nuisance tripping every time the unit fires up but logically the next lowest value from the data plate stated max for the application. I saw an A/C unit rated for 60A running fine on a 40A breaker.

It's common to find A/C units over fused. This situation can damage equipment or wiring or cause a fire. There are times when electrical codes are superseded by mfg. stats.

What torks me about the commonplace over fusing of A/C units is the fact that the equipment ratings are posted clearly on the case data plate yet they are routinely ignored by "professionals" (even if they have been in business for a long time)who look down their nose at home inspectors.

O.K.....I'm done venting.

I see that all the time and you can tell by the panel condition (rust, dust screws rusted in place) that the breakers have been in there for a very long time. Was there ever a concern? Maybe. Not many if at all I would imagine or someone over time would have replaced it with the proper breaker.

I have seen home warranty claims denied because a furnace or heat pump was over fused. I have seen it up-close and personal as a matter of fact....I had to rewire the strip heat wiring and power wiring that had melted because of an air handler being fused at 45 amp and the data plate called for a 40 amp max and the home warranty company would not cover the repair. The problem was a loose connection that showed up 8 months after the inspection but I was the one who missed the wrong size fuse so I had to fix it.

Most of the equipment I use has a max fuse size and also a min. ampacity requirement tagged on it.

William Cook, the Washington County director of public safety, says an outlet on the first floor that should have been protected by a 15-amp fuse instead had a 30-amp fuse, allowing the wire that fed the outlet to catch fire. A smaller fuse would have cut power before the wire overheated.

Official: Wrong fuse caused fire that killed 6 children in upstate New York village in June | StarTribune.com