mokelectric
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I do not understand what an AIC rating, series rating or fully rated circuit breaker is, can it be explained in simple terms?
AIC ratings on CB's, series rated CB's and fully rated CB's
- Thread starter mokelectric
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chris kennedy
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- Miami Fla.
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- 60 yr old tool twisting electrician
No:grin:mokelectric said:
AIC means Ampere Interrupting Capacity. Simply put its the maximum short circuit current that the breaker can safely interrupt. If this value is exceeded damage will occur to the breaker and switch gear.
Welcome to the Forum:smile:
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
AIC is short for Amps Interrupting Current. Actually, you would call it the IC rating of a breaker, for instance 35kAIC, meaning 35 kA (kilo Amps) IC (Interrupting Current). This is the rating that states how much current can pass through the breaker as it attempts to interrupt a fault before its components become shrapnel. When you have an instantaneous fault, such as a bolted (phase to phase) or ground short, ALL of the available current in a system attempts to flow to it. So for example if you have a big transformer that is capable of delivering 55kA even for a brief moment, all of your breakers had better have an IC rating of at least 55kA. If your favorite breakers come with 35kAIC or 50kAIC as standard, they will not be suitable, you need to go to the next level, probably 65kAIC. The "second level" is typically referred to as the HIC (High Interrupting Capacity) version, and most manufacturers have a VHIC 9Very High Interrupting Capacity) or 85 to 100kAIC (it varies from mfr to mfr, some only have 2 levels, some only one).
Series ratings have to do with using breakers in things such as panelboards and switchboards. A CB manufacturer can test breakers in series with one another for ratings that may be higher than one of them has as a stand alone device. So for example you may have a panelboard where the individual branch breaker ratings are only 10kAIC but the main CB is say 35kAIC but also of a "Current Limiting" design. Even if the available fault is 20kA in the system, the main breaker may not let enough current get through it (hence Current Limiting) to do damage to the downstream breakers. So the entire panelboard can have a 22kAIC "series rating" on it even though some of the smaller breakers have lower than that.
Fully rated? Do you mean 100% rated? If so, different subject. All standard breakers, unless specifically stated otherwise, are really rated for a maximum of 80% of the current it says it is rated for. I know that sounds odd, but it is because it follows the NEC for the wire you can connect to it. The logic follows that if you need 80A in a circuit, you must size the conductors at 125% of that 80A, so 100A conductors. Then you size the breaker for the conductor rating, so you buy a 100A breaker. But you can only load that circuit to 80A anyway, so in reality the 100A breaker is technically rated for only 80A and the breaker manufacturer knows that, so they use components rated for 80A max. If you, for some reason or another, find one of the exceptions that allows you to push the envelope, you can buy a special version of that breaker rated for 100% continuously. It will have a specific label on it that says it is a 100% rated breaker. You will also pay a little more for it. In most cases, the only difference in construction is the lugs, but the breaker MUST have that 100% rated label on it for you to use it that way.
Series ratings have to do with using breakers in things such as panelboards and switchboards. A CB manufacturer can test breakers in series with one another for ratings that may be higher than one of them has as a stand alone device. So for example you may have a panelboard where the individual branch breaker ratings are only 10kAIC but the main CB is say 35kAIC but also of a "Current Limiting" design. Even if the available fault is 20kA in the system, the main breaker may not let enough current get through it (hence Current Limiting) to do damage to the downstream breakers. So the entire panelboard can have a 22kAIC "series rating" on it even though some of the smaller breakers have lower than that.
Fully rated? Do you mean 100% rated? If so, different subject. All standard breakers, unless specifically stated otherwise, are really rated for a maximum of 80% of the current it says it is rated for. I know that sounds odd, but it is because it follows the NEC for the wire you can connect to it. The logic follows that if you need 80A in a circuit, you must size the conductors at 125% of that 80A, so 100A conductors. Then you size the breaker for the conductor rating, so you buy a 100A breaker. But you can only load that circuit to 80A anyway, so in reality the 100A breaker is technically rated for only 80A and the breaker manufacturer knows that, so they use components rated for 80A max. If you, for some reason or another, find one of the exceptions that allows you to push the envelope, you can buy a special version of that breaker rated for 100% continuously. It will have a specific label on it that says it is a 100% rated breaker. You will also pay a little more for it. In most cases, the only difference in construction is the lugs, but the breaker MUST have that 100% rated label on it for you to use it that way.
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Jraef said:
A small point, standard breakers are rated 100% non-continuous, 80% continuous. A continuous load is one that lasts more then 3 hours.
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- Wisconsin
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Fully rated means that all components in the system are rated for the full short circuit amps available at their line terminals (NEC 110.9).
Series ratings means that two (very rarely more) components when used together (hence they are in series) are rated for the available fault current. Current limiting is not a requirement for series ratings.
There are advantages and disadvantages for both fully and series components. I prefer to specify service/feeder panels as fully rated and branch circuit (400A max) panels as series rated.
It is all but impossible to have fully rated automatic transfer switches, non-fused disconnects, and motor starters all of which also need to be correctly rated per NEC 110.10.
Series ratings means that two (very rarely more) components when used together (hence they are in series) are rated for the available fault current. Current limiting is not a requirement for series ratings.
There are advantages and disadvantages for both fully and series components. I prefer to specify service/feeder panels as fully rated and branch circuit (400A max) panels as series rated.
It is all but impossible to have fully rated automatic transfer switches, non-fused disconnects, and motor starters all of which also need to be correctly rated per NEC 110.10.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Looks that way, confirmed in Jim's posting too. I just wasn't thinking it through well enough. Serves me right for posting early, left myself wide open for correction.augie47 said:
Speaking of which:
Yes, I know, just wasn't going into that additional detail because if he didn't know all this, I thought it was a little daunting to hit him with all the little ifs ands or buts. As it turns out, it was a red herring anyway.iwire said:
OK, I'll defer to your wisdom on that. As I see it, that is what is happening at some level. The "let through" current of the upstream device has to be lower than the withstand rating of the downstream device. But I see your point, "Current Limiting" is a more specific term for a design feature, I probably should not have used it.jim dungar said:
charlie
Senior Member
- Location
- Indianapolis
The following numbers match the above statements.Jraef said:
- Assume you are talking 100% rated and not fully rated as Jim Dungar has explained.
- That is only true if you have a continuous load; otherwise, you may load a circuit to 100%. With 100% rated circuit breakers, you may load up the wire to its 100% continuous rating.
- True
- True
- False. The internal construction is a bit different as well as the lugs. The reason is that circuit breakers are thermal devices on overload and they use the wire as a heat sink. If the circuit breaker is rated for 100% continuous loading, the wire will be smaller and the circuit breaker will not have as much wire for a heat sink.
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- Wisconsin
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- PE (Retired) - Power Systems
Jraef said:
A better but maybe still not completely accurate term is dynamic impedance. Dynamic impedance includes, among other items, increased resistance created by the arc which results from contact separation as one of the OCPD's operates.
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Jraef said:
Cool but many electricians are under the impression that standard breakers are only 80% rated and that is really not the case, that is why I brought in the details. :smile:
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Had not heard that term before. Looks as though I have something new to learn, thanks.jim dungar said:
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