What is a 400 amp class 320 Circuit breaker?

[mlnk]

So a 400 amp CB is rated to handle a 400 amp load calculation. Is it correct that the main breaker is called a 320 /400 /cb because it happens to be a 320 class designation. It does not refer to 320 AMPS. It could have been, for example, a 789 designation? (789/400 cb)

I understand that a continuous load on a 400 amp cb is limited to 320 amps, but isn't that taken care of in the load calculation by multiplying the LCL by 125% So if you have a calculated load of , say 384 amps, that includes continuous and non continuous loads, so a 400 amp CB is the proper size.

Also, what does it mean to have a CB rated for use at 100% of its rating?

 

[kwired]

So a 400 amp CB is rated to handle a 400 amp load calculation. Is it correct that the main breaker is called a 320 /400 /cb because it happens to be a 320 class designation. It does not refer to 320 AMPS. It could have been, for example, a 789 designation? (789/400 cb)

I understand that a continuous load on a 400 amp cb is limited to 320 amps, but isn't that taken care of in the load calculation by multiplying the LCL by 125% So if you have a calculated load of , say 384 amps, that includes continuous and non continuous loads, so a 400 amp CB is the proper size.

Also, what does it mean to have a CB rated for use at 100% of its rating?

Most breakers are designed that they sink some of the heat that is produced within into the attached conductors. That is a major reason we must apply a 125% factor to continuous loads when selecting conductor ampacity, to lower the termination operating temperature.

Breakers intended to be used at 100% do not introduce same heat into terminations, therefore don't need those adjustments. They still may produce heat but it is managed differently.

 

[Jraef]

All circuit breakers are designed and rated to carry 100% of their rated load continuously, so a 400A breaker is rated for 400A. There is actually no such thing as an "80% rated" breaker.

But... a breaker is there to protect the CONDUCTORS, and the conductors must be TERMINATED. So you size the conductors for the rated continuous load x 1.25 using the 75C column because most terminals are rated for 75C max. That then results in the 400A breaker having an EFFECTIVE usage at 320A (80% of it's rating) since the conductors are sized that way. Then BECAUSE the breaker is only going to be used in this manner, when the panelboard for it is designed, they ASSUME this 80% use in terms of the breaker's ability to dissipate heat when mounted adjacent to other breakers (being used at 80% max).

If you have loads that are not continuous, you can use a 400A breaker (and the conductors it protects) at 400A. But be prepared to prove that the use is not continuous, defined as 3 hours or more. If you don't know for sure, the safe bet is to use continuous rules.

What makes a breaker 100% rated is that it in order to use cables at 100%, you must use 90C cable, so the breaker will have a termination method that is rated for 90C. In addition, the breaker must have more ventilation so typically it can only be used as a "stand-alone" breaker, i.e. NOT in a panelboard directly adjacent to other breakers. This means it will be a separate "enclosed circuit breaker" but is sometimes done with MAIN breakers in MCCs or in panels if they are physically separated from the other branch breakers (i.e. not back-fed into the bus). SwitchGEAR where each breaker has its own separate cubicle can have 100% rated breakers as feeders too, but that costs a LOT more than a switchBOARD or panelboard.

 

[jim dungar]

Overall a good explanation. However,

... the breaker will have a termination method that is rated for 90C....

Is not quite true. The conductors to any breaker, listed to UL489, must be sized using the 75C tables. So, a 100% rated device requires the use of 90C conductors sized at 75C. Because the breaker runs hotter, the insulation must be rated higher.

 

[paulengr]

Another gotcha is that the actual trip unit will trip somewhere between 80% and 100% of rating on a standard breaker but at 100% on a full duty breaker. Some brands trip close to 100%, some at 80%. I've been burned by this before with noncontinuous loads that run close enough to the 100% mark to cause nuisance trips.

Sent from my SM-T350 using Tapatalk

 

[kwired]

Another gotcha is that the actual trip unit will trip somewhere between 80% and 100% of rating on a standard breaker but at 100% on a full duty breaker. Some brands trip close to 100%, some at 80%. I've been burned by this before with noncontinuous loads that run close enough to the 100% mark to cause nuisance trips.

Sent from my SM-T350 using Tapatalk

I believe listing standards will say they should carry 100% indefinitely. AFAIK that is at an ambient temp of 40C for most of commonly seen breakers anyway.

 

[retirede]

I believe listing standards will say they should carry 100% indefinitely. AFAIK that is at an ambient temp of 40C for most of commonly seen breakers anyway.

This is my understanding as well.

 

[mlnk]

All circuit breakers are designed and rated to carry 100% of their rated load continuously, so a 400A breaker is rated for 400A. There is actually no such thing as an "80% rated" breaker.

But... a breaker is there to protect the CONDUCTORS, and the conductors must be TERMINATED. So you size the conductors for the rated continuous load x 1.25 using the 75C column because most terminals are rated for 75C max. That then results in the 400A breaker having an EFFECTIVE usage at 320A (80% of it's rating) since the conductors are sized that way. Then BECAUSE the breaker is only going to be used in this manner, when the panelboard for it is designed, they ASSUME this 80% use in terms of the breaker's ability to dissipate heat when mounted adjacent to other breakers (being used at 80% max).

If you have loads that are not continuous, you can use a 400A breaker (and the conductors it protects) at 400A. But be prepared to prove that the use is not continuous, defined as 3 hours or more. If you don't know for sure, the safe bet is to use continuous rules.

What makes a breaker 100% rated is that it in order to use cables at 100%, you must use 90C cable, so the breaker will have a termination method that is rated for 90C. In addition, the breaker must have more ventilation so typically it can only be used as a "stand-alone" breaker, i.e. NOT in a panelboard directly adjacent to other breakers. This means it will be a separate "enclosed circuit breaker" but is sometimes done with MAIN breakers in MCCs or in panels if they are physically separated from the other branch breakers (i.e. not back-fed into the bus). SwitchGEAR where each breaker has its own separate cubicle can have 100% rated breakers as feeders too, but that costs a LOT more than a switchBOARD or panelboard.

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