Reason for 125%

[Ponchik]

On a continuous load I know we have to use the 125% to size the conductors and then the OCPD. But if the OCPD is designed to hold at 100% and the conductor is rated for that current, remind me again what was the purpose for the 125%?

Thank you

 

[jim dungar]

Basically, the factor accounts for the increase in temperature when items are installed in enclosures versus when they are tested in open air.

 

[wwhitney]

Basically, the factor accounts for the increase in temperature when items are installed in enclosures versus when they are tested in open air.

There's a thermal effect from enclosing a single OCPD, and then there's an additional thermal effect from putting other heat sources (OCPD) in that enclosure. Any thoughts on the relative magnitude of those two factors for, e.g. a residential load center with 20 breakers?

For example, I would guess that if you put a regular breaker in a single OCPD enclosure in say a 30C ambient temperature, it would be likely to hold at its rating continuously. I.e. I'm guessing that the thermal effect of the enclosure would be outweighed by lowering the ambient temperature 10C. Is that plausible?

Cheers, Wayne

 

[jim dungar]

There's a thermal effect from enclosing a single OCPD, and then there's an additional thermal effect from putting other heat sources (OCPD) in that enclosure. Any thoughts on the relative magnitude of those two factors for, e.g. a residential load center with 20 breakers?

I believe people on this forum spend more time rationalizing this 125% factor than the NEC or standards people ever did.

IMO, close to 100 years ago someone said let's add a fudge factor of 25% and call it a day. Since then all product development and testing has been based on this decision.

 

[Ponchik]

Someone asked me "if my EV charger draws 27Amps and the NM cable 10 is rated for 30, why can't I use a 30A breaker"

Besides explaining that the 125% and the wire size is code required, I could not give a reason why 125%, why not other number and why even we do it.

 

[jaggedben]

Someone asked me "if my EV charger draws 27Amps and the NM cable 10 is rated for 30, why can't I use a 30A breaker"

Besides explaining that the 125% and the wire size is code required, I could not give a reason why 125%, why not other number and why even we do it.

Breakers have tolerances, and there's a more than negligible chance that a 30amp breaker will trip if run continuously at 27A for three hours.

I think if the 'fudge factor' (as Jim calls it) ensures that people don't cut things too close and then upsize or bypass breakers when they trip, it's probably justified.

 

[tom baker]

The 125% results in a larger wire connected to the CB. This larger wire acts as a heat sink to prevent a TM breaker from tripping on heat. For justification of this, look at Mike Holts accepted change that oversizing the neutral is not required for a continuous load

 

[jim dungar]

Breakers have tolerances, and there's a more than negligible chance that a 30amp breaker will trip if run continuously at 27A for three hours.

I think if the 'fudge factor' (as Jim calls it) ensures that people don't cut things too close and then upsize or bypass breakers when they trip, it's probably justified.

Even allowing for tolerances, a breaker or fuse can carry 100% load indefinitely in an open air ambient temperature of 40°C. The three hours is a time limit from the NEC not the protective device. The problem is devices are mounted in enclosures often with other heat producing items.

 

[Ponchik]

Thank you all fro the replies.

 

[Tulsa Electrician]

The 125% results in a larger wire connected to the CB. This larger wire acts as a heat sink to prevent a TM breaker from tripping on heat. For justification of this, look at Mike Holts accepted change that oversizing the neutral is not required for a continuous load

Would you be so kind to post a link so I can read.

 

[Jraef]

Even allowing for tolerances, a breaker or fuse can carry 100% load indefinitely in an open air ambient temperature of 40°C. The three hours is a time limit from the NEC not the protective device. The problem is devices are mounted in enclosures often with other heat producing items.

Which is why, when you want a “100% rated” circuit breaker, it usually has to be either a Main, where it stands alone by itself connected to bus bars, a separately enclosed breaker, or all by itself in a switchgear / MCC cubicle.

The only exception I have seen to this is Sq.D I-Line panels and I think that’s because their direct bus connection is designed to help dissipate heat away from the breakers.

When a breaker is listed to be APPLIED at 100% of its rating continuously (aka “100% rated”), testing establishes that the correct operation of the circuit breaker can carry the full rated current without tripping and that it does not exceed temperature limits that could render it unsafe. That test is required to be done in an enclosure with a specific minimum air volume, which is why it is really difficult to get a breaker to pass when mounted in a panel with other breakers. It’s also required that the conductors connecting to a 100% rated breaker are 90C rated, but sized based on the 75C column.

So back to the issue of breakers being “80% rated”, it’s indirect. If, for a basic continuous load or one that is mixed with continuous and non-continuous, you size the conductors for 125% of that load as you are required, then you size the breaker to protect those conductors, you will END UP with the breaker only carrying 80% of its rating continuously. Because the panel manufacturers know that this is how the breakers will be APPLIED, the heat rise testing they do for them MOUNTED IN THE PANEL is based on that 80% application.