mmt019
Electrical
- Jun 25, 2003
- 63
I need some opinion. Attached is the image of the one line which shows a 3750 kVA transformer (XFMR-MSC-1). As it is a supervised location so as per NEC table 450.3(A), the transformer secondary protection is not required if the rating of transformer primary fuse is maximum 250%. However the protection of transformer secondary cable (CBL-SWBD-MSC-1) is still required. (CBL-SWBD-MSC-1 has 12 conductors/phase of 600 MCM with a total ampacity of 5040A.). There is a 4000A main breaker on the 480V side (secondary side) in SWBD-MSC-1 (not shown)
We can use the main breaker of the downstream switchboard (BKR-SWBD-MSC-1) as the overload protection of transformer secondary cable; because under overload condition, the whole cable will be heated and so the breaker will provide the overload protection even though it is at the ending point of the cable. This fulfils the requirement of NEC 240.92(C)(2).
But the main breaker of the switchboard cannot provide the short circuit and ground fault protection for the cable, because for that the breaker has to be at the starting point of the cable. As per NEC 240.92(C)(1)(1), the short circuit/ground fault protection of transformer secondary cable is not required when the length of the cable is up to 100 feet. In our case the cable length is 210 feet.
NEC 240.92(C)(1)(3) provides some flexibility that protection of transformer secondary cable is not required if system overcurrent devices will protect the cable for all short circuit and ground fault conditions. As shown on the TCC (as attached), the curve of the transformer primary fuse is to the left of the curve of transformer secondary cable for the maximum available short circuit current. So I think the transformer primary fuse would protect the transformer secondary cable even though the length of the cable is more than 100 feet. What do you think about this?
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1645198354/tips/BSI_-_EIC_20220218_093228_iubs4h.pdf[/url]
We can use the main breaker of the downstream switchboard (BKR-SWBD-MSC-1) as the overload protection of transformer secondary cable; because under overload condition, the whole cable will be heated and so the breaker will provide the overload protection even though it is at the ending point of the cable. This fulfils the requirement of NEC 240.92(C)(2).
But the main breaker of the switchboard cannot provide the short circuit and ground fault protection for the cable, because for that the breaker has to be at the starting point of the cable. As per NEC 240.92(C)(1)(1), the short circuit/ground fault protection of transformer secondary cable is not required when the length of the cable is up to 100 feet. In our case the cable length is 210 feet.
NEC 240.92(C)(1)(3) provides some flexibility that protection of transformer secondary cable is not required if system overcurrent devices will protect the cable for all short circuit and ground fault conditions. As shown on the TCC (as attached), the curve of the transformer primary fuse is to the left of the curve of transformer secondary cable for the maximum available short circuit current. So I think the transformer primary fuse would protect the transformer secondary cable even though the length of the cable is more than 100 feet. What do you think about this?
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1645198354/tips/BSI_-_EIC_20220218_093228_iubs4h.pdf[/url]