I understand how to use table 430.52 in selecting the Maximum rating or setting for a motor's short circuit protection. In 430.52(B) I understand that a motor's short circuit protection shall be capable of carrying the starting current. So if the FLA is less than the size of the inverse time CB and the LRC can be carried by the setting of the inverse time CB I should be ok??? I think so but have forgotten how to prove it... I desire a REF or formula which might help select the smallest rating for a motor's short circuit protection.
To Size the smallest OCP required for a Motor
- Thread starter VinceS
- Start date
- Status
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
1 phase or 3 phase?
Let's assume 3 phase. A motor branch circuit must have 2 things; Short circuit protection and thermal overload protection. Because a motor starter already has the thermal overload protection (or at least should have), all a breaker needs to provide is the sort circuit protection and the table states the maximum size you can use.
The short circuit protection is provided by the magnetic trips, not the inverse time (thermal) trips. The reason the chart shows the the inverse time ratings is because that is how breakers are selected; the inverse time rating relates to a pre-fixed level of magnetic trips on non-adjustable breakers and the NEC has no idea what you are going to use. If your breaker has adjustable mag trips settings, that can often take care of it. This (along with the exceptions in that section) allows you to select a breaker that will not give you nuisance tripping. Below that you can do whatever you want. But the lower you go, the more likely you are to start getting trips when you don't want them, which can wear out your breaker let alone cause lost productivity (or whatever the breaker is feeding).
MCP (mag-only) breakers are used by starter manufacturers for the same reason; they don't need the redundant thermal trips and can use breakers with high ranges of mag-trip adjustability. But notice that the NEC only allows those to be used in FACTORY ASSEMBLED, TESTED, LISTED assemblies. They canot be used in the field other than as replacements.
Let's assume 3 phase. A motor branch circuit must have 2 things; Short circuit protection and thermal overload protection. Because a motor starter already has the thermal overload protection (or at least should have), all a breaker needs to provide is the sort circuit protection and the table states the maximum size you can use.
The short circuit protection is provided by the magnetic trips, not the inverse time (thermal) trips. The reason the chart shows the the inverse time ratings is because that is how breakers are selected; the inverse time rating relates to a pre-fixed level of magnetic trips on non-adjustable breakers and the NEC has no idea what you are going to use. If your breaker has adjustable mag trips settings, that can often take care of it. This (along with the exceptions in that section) allows you to select a breaker that will not give you nuisance tripping. Below that you can do whatever you want. But the lower you go, the more likely you are to start getting trips when you don't want them, which can wear out your breaker let alone cause lost productivity (or whatever the breaker is feeding).
MCP (mag-only) breakers are used by starter manufacturers for the same reason; they don't need the redundant thermal trips and can use breakers with high ranges of mag-trip adjustability. But notice that the NEC only allows those to be used in FACTORY ASSEMBLED, TESTED, LISTED assemblies. They canot be used in the field other than as replacements.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Circuit breaker or fuse?
Busman says (somewhere in the wealth of information they supply) size at 125% or next standard size, utilizing dual element TD fuse..Dave?
Busman says (somewhere in the wealth of information they supply) size at 125% or next standard size, utilizing dual element TD fuse..Dave?
More Information...
More Information...
I have a 150HP 480VAC 3Ph motor with a SF of 1.15 FLA of 180. I can't specifically recall the LRC. The size 5 starter is feed from a CH 200A inverse time CB for short circuit protection (I don't have model info). Currently the inverse time setting is at the maximum setting. This causes nuisance trips frequently, nearly 1 trip per start on a cold system, and 1 per 3 starts once system has been in run and in a warm state. Starts are infrequent once in run. The motor drives a cutter head, via a gear box, which turns freely cold or hot. I understand by not providing the LRC or specific setting range selection this post still lacks information. I desire to upsize the CB at least to a model that has a larger capacity to adjust for LRC. The system was installed as is and I'm trying to overcome the mindset of, "Its always worked this way, make it work because the original engineered design must be correct." I have come to understand the 200A CB has been replaced about every 3 to 6 months as long a anyone can remember. This is a duh. The CB is sized correctly. But causes nuisance trips, therefore should be larger to avoid the cost of frequent replacement due to ware.
More Information...
I have a 150HP 480VAC 3Ph motor with a SF of 1.15 FLA of 180. I can't specifically recall the LRC. The size 5 starter is feed from a CH 200A inverse time CB for short circuit protection (I don't have model info). Currently the inverse time setting is at the maximum setting. This causes nuisance trips frequently, nearly 1 trip per start on a cold system, and 1 per 3 starts once system has been in run and in a warm state. Starts are infrequent once in run. The motor drives a cutter head, via a gear box, which turns freely cold or hot. I understand by not providing the LRC or specific setting range selection this post still lacks information. I desire to upsize the CB at least to a model that has a larger capacity to adjust for LRC. The system was installed as is and I'm trying to overcome the mindset of, "Its always worked this way, make it work because the original engineered design must be correct." I have come to understand the 200A CB has been replaced about every 3 to 6 months as long a anyone can remember. This is a duh. The CB is sized correctly. But causes nuisance trips, therefore should be larger to avoid the cost of frequent replacement due to ware.
noonan
Member
- Location
- Bucks County, PA
- Occupation
- Electrical Project Manager
VinceS said:
Are you sure about this?
125% of 180 is 225.
- Status