Multiple loads on the secondary

[MaineVoltage]

Hi guys,

I have a question that I should know the answer to but I want to be sure.

We have a transformer connected to 480V Primary and Secondary is 120/240V 6KVA. There is already a 15A load connected to the 120V secondary and a 20A load connected to the 240V secondary.

Can I connect as many loads as desired to the 120V secondary winding? From the same terminal as the existing 15A load is connected providing I don't exceed the 6KVA / 120V = 50A limit?

Thanks,
Leo

 

[augie47]

Welcome !

You must protect the transformer per Art 450. {450.3(B)}
If you don't limit the secondary protection to 125%, you must protect the primary at 125% of its current.
Once that is done, you can add secondary "taps" as desired as long as you follow 240.21(C) and other applicable Code (such as 408.36)

 

[MaineVoltage]

Augie, thanks for the quick reply!

I was looking at 450.6, which is centered around transformer to transformer connections.

At 240.21(C)(3), Number 2 states that the ampacity of the secondary conductors is not less than the secondary current rating of the transformer. This can't mean that I need to use 8AWG THHN for the run from the transformer to my circuit breaker enclosure which will be powering a 15A breaker, can it? The run will be approximately 20ft.

Also, the existing 15A breaker to the secondary is connected to the transformer enclosure with a 3" peice of conduit, and in its own enclosure. but Number 3 states that all overcurrent devices be grouped..? I should be able to tie to the secondary make my run and place the breaker there, seperate from the existing breaker?

Thanks again,
Leo

 

[cpal]

you need to look at

(2) Transformer Secondary Conductors Not over 3 m (10 ft) Long. Where the length of secondary conductor does not exceed 3 m (10 ft) and complies with all of the following:
(1) The ampacity of the secondary conductors is
a. Not less than the combined calculated loads on the circuits supplied by the secondary conductors, and
b. Not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent-protective device at the termination of the secondary conductors
(2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors are enclosed in a raceway, which shall extend from the transformer to the enclosure of an enclosed switchboard, panelboard, or control devices or to the back of an open switchboard.
(4) For field installations where the secondary conductors leave the enclosure or vault in which the supply connection is made, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary to secondary transformer voltage ratio, shall not exceed 10 times the ampacity of the secondary conductor.

 

[augie47]

Augie, thanks for the quick reply!

I was looking at 450.6, which is centered around transformer to transformer connections.

450.6 is for "tie" conductors. Unless you are dealing with two or more transformers--not applicable.

At 240.21(C)(3), Number 2 states that the ampacity of the secondary conductors is not less than the secondary current rating of the transformer. This can't mean that I need to use 8AWG THHN for the run from the transformer to my circuit breaker enclosure which will be powering a 15A breaker, can it? The run will be approximately 20ft.

I don't see it saying that

Also, the existing 15A breaker to the secondary is connected to the transformer enclosure with a 3" peice of conduit, and in its own enclosure. but Number 3 states that all overcurrent devices be grouped..? I should be able to tie to the secondary make my run and place the breaker there, seperate from the existing breaker?

as long as the individual breakers for each tap are grouped you are o.k.

Thanks again,
Leo

follow what Charlie (cpal) stated.

1.make sure you have proper primary protection (125% primary current)
2.install each tap per the reference Charlie provided
3. follow 250.30 for grounding