Transfer Switch

[maxamps]

When installing a 1200 amp 3 phase Transfer Switch, between incoming service and the main service disconnect (1200 amp 3 phase 480 volt breaker, 4 wire system). Contractor is proposing 4 parallel runs of 350 mcm or 3 parallel runs of 600 mcm 90 degree conductors THHN and 90 degree terminals. My calculations are 1120 and 1140 amps respectively considering de-rating for the 4th current carrying conductor (the neutral). Would either of these be sufficient to supply the 1200 amp main switch, using the next size overcurrent device as exampled in section Article 240.4 (C) ?

 

[augie47]

When the overcurrent device is greater than 800 amps the "next size up" rule does not apply, however, you might look again at 310.15(B) (4) in regard to your neutral. It only counts as a current-carrying conductor when a MAJOR PORTION of the load is nonlinear loads. I have found very few jobs over the years where that is the case.

 

[jim dungar]

Also be careful with the terminals. You do not get to choose which temperature rating the OEM used for the terminations. The lugs are not the issue, the rating depends on the complete assembly. For example, there is no listed (i.e. UL489) overcurrent device that uses conductors sized per the 90?C column, although there are several that require insulation rated that high.

 

[Smart $]

When installing a 1200 amp 3 phase Transfer Switch, between incoming service and the main service disconnect (1200 amp 3 phase 480 volt breaker, 4 wire system). Contractor is proposing 4 parallel runs of 350 mcm or 3 parallel runs of 600 mcm 90 degree conductors THHN and 90 degree terminals. My calculations are 1120 and 1140 amps respectively considering de-rating for the 4th current carrying conductor (the neutral). Would either of these be sufficient to supply the 1200 amp main switch, using the next size overcurrent device as exampled in section Article 240.4 (C) ?

Your terminal temperature limitation is likely 75?C (as Jim implied in his post).

The contractor's proposal is correct, assuming parallel conduits, 75?C terminations, and the neutral not counted as CCC... provided the ambient does not exceed 50?C (122?F) for the 600's, or 45?C (113?F) for the 350's.

Also, are you aware there has to be 1200A ocp on the incoming lines to the transfer switch?

 

[texie]

Also, are you aware there has to be 1200A ocp on the incoming lines to the transfer switch?[/QUOTE]
That caught my eye also. I thought maybe it is a SUSE rated switch.
To the OP, you must either have a service disco. ahead of it or it must be a SUSE rated switch. The type of switch also has major grounding and bonding implications.

 

[infinity]

I have a suspicion that his use of the term de-rating means a smaller neutral not ampacity adjustment.

 

[templdl]

When installing a 1200 amp 3 phase Transfer Switch, between incoming service and the main service disconnect (1200 amp 3 phase 480 volt breaker, 4 wire system). Contractor is proposing 4 parallel runs of 350 mcm or 3 parallel runs of 600 mcm 90 degree conductors THHN and 90 degree terminals. My calculations are 1120 and 1140 amps respectively considering de-rating for the 4th current carrying conductor (the neutral). Would either of these be sufficient to supply the 1200 amp main switch, using the next size overcurrent device as exampled in section Article 240.4 (C) ?

Please verify that there are in fact 90degC terminals.