service calculations for a campground with single phase loads on three phase service

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Rsteenson

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Hello, I am doing some calculating for a new campground service install and I keep tripping myself up on single phase load on a three phase system.
According to 551.73 I need to provide 9600 VA per site with a 50 amp 208v hook up (SINGLE PHASE). 30x9600=288,000 VA. according to 551.73 I can reduce to 42% giving me 120,960 for the service entrance conductors. So....whats the most elementary way for me to calculate the size of my three phase main breaker and transformer????? The more I read on this the more confused I get! Its been too long since I was in night school and I keep confusing myself with all of the multipliers :)
 
Rsteenson said:
Hello, I am doing some calculating for a new campground service install and I keep tripping myself up on single phase load on a three phase system.
According to 551.73 I need to provide 9600 VA per site with a 50 amp 208v hook up (SINGLE PHASE). 30x9600=288,000 VA. according to 551.73 I can reduce to 42% giving me 120,960 for the service entrance conductors. So....whats the most elementary way for me to calculate the size of my three phase main breaker and transformer????? The more I read on this the more confused I get! Its been too long since I was in night school and I keep confusing myself with all of the multipliers :)
Click to expand...
Do you really mean 208 single phase, as in 208 to grounded conductor and no 120V option?
Or are you referring to a two hot, one neutral service consisting of two of the phase conductors along with the neutral? (Derived from a source 208Y/120 service.)
If you are talking about a two-out-of-three phase system, you have to keep firmly in mind that when you have 208V (line to line) loads, the full load current will be present on both phase lines at that campsite, BUT when you have a balanced system over multiple two phase line hookups, the current in each line conductor upstream will end up being lower then the sum of the individual currents on that same line.
The sum of the currents from line to neutral (120V) loads, on the other hand, will add numerically when you get back to a multi-site feeder or the campground service.
So you will have to treat the load calculations (in terms of line current) differently at different points in the system or else design conservatively as if all major loads were line to line.
 
GoldDigger said:
Do you really mean 208 single phase, as in 208 to grounded conductor and no 120V option?
Or are you referring to a two hot, one neutral service consisting of two of the phase conductors along with the neutral? (Derived from a source 208Y/120 service.)
If you are talking about a two-out-of-three phase system, you have to keep firmly in mind that when you have 208V (line to line) loads, the full load current will be present on both phase lines at that campsite, BUT when you have a balanced system over multiple two phase line hookups, the current in each line conductor upstream will end up being lower then the sum of the individual currents on that same line.
The sum of the currents from line to neutral (120V) loads, on the other hand, will add numerically when you get back to a multi-site feeder or the campground service.
So you will have to treat the load calculations (in terms of line current) differently at different points in the system or else design conservatively as if all major loads were line to line.
Click to expand...

2 hot one neutral. (L1 to N =120 , L1 to L2 =208)
 
iwire said:
Assuming a 208Y/120 service I believe just 120,960/1.73/208=336 amps.
Click to expand...

Without looking into the specific NEC sections you referenced I have to agree with iwire. You have to assume a balanced system, although the actual conected load will vary phase to phase due to different camp sites using different equipment, etc. I'd provide a 350A 3-phase, 4-wire service.
 
where does the 1.73 factor come from? Square root of 3 for three phase service? You also recommend a 350A 3 phase 4 wire service for a 336A load. NEC Article 100 states a double pole breaker must be down rated to 80% for a continuous Load on a double pole breaker
 
I am building a 30 campsite park with all 50amp pedestals. In your above calculation where does the 1.73 factor come from? Square root of 3 for three phase service? You also recommend a 350A 3 phase 4 wire service for a 336A load. NEC Article 100 states a double pole breaker must be down rated to 80% for a continuous Load on a double pole breaker
 
  1. In all three-phase calculations, the factor of "the square root of 3" is going to make its way into the calculation in some way.
  2. There is nothing that says the 336 amps is continuous. So a 350A breaker is sufficient.
  3. This is a 6 year old thread, and I am closing it.
  4. If you wish to continue this discussion, then please open a new thread.
  5. If you want to discuss technical issues, we can allow that.
  6. We can also assist an engineer in the design of an electrical system.
  7. But we cannot allow a person who is not an electrician to post questions related to the installation of electrical systems.
 
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