Mr. Serious
Senior Member
- Location
- Oklahoma, USA
- Occupation
- Electrical Contractor
I'm building a new 400-amp single-phase service for a property that has a 3000 square foot house, a pool and spa, a large shop building which will have several RV connections for occasional use, and a new 200 square foot building that will be a small business. I did a load calculation and came up with about 315 amps of load.
Originally I was planning to use a panel with a single 400-amp service disconnect breaker, but it turned out to be much cheaper to get a 400-amp main lug panel with room for six disconnects, following the six-disconnect rule that is allowed under code versions prior to 2020. Oklahoma is on the 2014 code revision.
I understand that when building a service with multiple disconnects, the main service wires need to be sized only to handle the anticipated load, not for the full 400 amps. However I had been planning to use 400-amp rated wire anyway. I was intending to use 2 parallel runs of 350kcmil Al in the same conduit, which should be big enough to carry 400 amps when de-rated for 4 current-carrying conductors in the conduit and for temperature.
But just yesterday, I was reading a post about services not really being rated for a continuous load. Main breakers are intended for a continuous load of up to 80% of their amp rating. Perhaps the bus bars of the panel itself are rated for a full continuous load of 400 amps. In the forum thread I was reading, someone said bus bars usually are rated for full continuous load. So, if the bus bars are rated for a continuous 400 amps and I install service wires also rated for a continuous load of 400 amps, would I really be building a 500-amp service?
As I said, the load calculation comes to about 315 amps, and load calculations done under the NEC rules are NOT a continuous load rating. They represent a minimum breaker size to be used for the service main breaker, so the load calculation number has already been increased to 125% of what it would be for a continuous load. So the homeowners could hypothetically add 85 amps of load and still be good with a hypothetical 400-amp breaker, if the service had one main disconnect breaker. The equivalent continuous load would be 320 amps. So, I'm thinking if I don't want to build this service too big, I ought to size my wire for 320 amps instead of 400. Or does it really need to be sized for 400, to allow for additional loads in the future?
Originally I was planning to use a panel with a single 400-amp service disconnect breaker, but it turned out to be much cheaper to get a 400-amp main lug panel with room for six disconnects, following the six-disconnect rule that is allowed under code versions prior to 2020. Oklahoma is on the 2014 code revision.
I understand that when building a service with multiple disconnects, the main service wires need to be sized only to handle the anticipated load, not for the full 400 amps. However I had been planning to use 400-amp rated wire anyway. I was intending to use 2 parallel runs of 350kcmil Al in the same conduit, which should be big enough to carry 400 amps when de-rated for 4 current-carrying conductors in the conduit and for temperature.
But just yesterday, I was reading a post about services not really being rated for a continuous load. Main breakers are intended for a continuous load of up to 80% of their amp rating. Perhaps the bus bars of the panel itself are rated for a full continuous load of 400 amps. In the forum thread I was reading, someone said bus bars usually are rated for full continuous load. So, if the bus bars are rated for a continuous 400 amps and I install service wires also rated for a continuous load of 400 amps, would I really be building a 500-amp service?
As I said, the load calculation comes to about 315 amps, and load calculations done under the NEC rules are NOT a continuous load rating. They represent a minimum breaker size to be used for the service main breaker, so the load calculation number has already been increased to 125% of what it would be for a continuous load. So the homeowners could hypothetically add 85 amps of load and still be good with a hypothetical 400-amp breaker, if the service had one main disconnect breaker. The equivalent continuous load would be 320 amps. So, I'm thinking if I don't want to build this service too big, I ought to size my wire for 320 amps instead of 400. Or does it really need to be sized for 400, to allow for additional loads in the future?
