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Spot Networks Calculator

Two source spot network Would you like to have incoming utility power that was five times more reliable than what you have now?

How can this be done? Did you know that in some circumstances you could parallel two independent utility sources, greatly reducing the likelihood that a problem on one source will cause a problem at your facility. This is because one source can fail and your load continues to receive power without a bump. Other bumpless transfer solutions such as solid-state transfer switching are far more costly.

However, paralleling multiple sources to a common bus will not be permitted by a utility in many instances. The most common reason that the utility does not permit the paralleling of two sources is that the two sources are not sufficiently matched. Unmatched sources cause concerns for the utility including the concern that power could flow in a reverse direction through one of the sources.

If the problem can be solved by having matched sources, how close must the two sources be to be considered "matched" and what other parameters could "make or break" the application?

You will need some information to know if you are even a candidate for a spot network installation. Use this calculator to determine whether your application would be suitable for a spot network. Use the "Quick Check" calculator below as a first-pass analysis. Use the Complete Check Calculator to provide a more in depth evaluation on whether your diverse sources are candidates for a spot network.

Quick Check Calculator
1. What is your facility's maximum load?1

2. What is your facility's minimum load?2
Unknown (will assume 20%)

3. What is your facility's incoming voltage?

4. What is the new Utility Source 1 transformer rating?2
Not sure (select this and we will just estimate based on your facility's maximum load)
kVA %Z3
Isc (Available short circuit amps)

5. What is the new Utility Source 2 transformer rating?4
Same as above (select this and we will use the same value you selected for the first utility transformer)
kVA %Z3
Isc (Available short circuit amps)

1 Obtained from your utility bill as "Demand", "Peak Demand", "Billed Demand" or similar. If you don't have an electric bill enter the "amp" rating found on the main breaker for the facility.
2 Lower loads means more likelihood of reverse current on one of the networks. Worst case is zero load. Select the lowest normal load you expect.
3 Starting at the main breaker or fusible switch, trace the circuit towards the utility until you reach the first transformer. If this transformer is owned by the utility, a call to the utility will usually result in your obtaining the information. If unsuccessful, select the "Not Sure" and the program will estimate the transformer rating as five times your incoming needs (assumes other customers are also using this transformer). Estimating that the upstream transformer is oversized is a conservative estimate since the likelihood of two mismatched sources successfully supplying a spot network decreases as the upstream transformer increases (actually as the upstream impedance decreases, but that is saying the same thing).
4 "%Z" is percent impedance. Provided on nameplate of transformer.
5 This is the rating of the proposed second source for your facility. As with question 4, you can obtain this information from your utility. If unsuccessful, select "Same as above" and the program will assume that the values for Source 2 are the same as for Source 1. For best operation, it is beneficial to have both source transformers having the same kVA and impedance ratings.


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