New to the NEC, with a Transformer question

[Penwar]

I have only been involved with NEC code issues for the last couple of years and I am learning new things every day. That said, I have come across a question that I am struggling to find the answer to, so here it is.

I am considering a design option for a PV system for a client. The system will be roughly 50kW DC and will be connected to an existing 208V 3-phase utility. To keep install costs down and utilize existing technology, we are proposing using 480V inverters, then knocking it down to the 208V for connection to the grid. The question I have is in regards to the transformer and protection thereof. I am curious if I would need disconnects on each side of the transformer. Per table 450.3(b), I am seeing a need for protection on the primary and the secondary side (we would be seeing more than 9A, so 125% of FL is applicable). I am really curious about putting in a fused D/S on each side of the transformer.

Thoughts?

 

[fmtjfw]

I have only been involved with NEC code issues for the last couple of years and I am learning new things every day. That said, I have come across a question that I am struggling to find the answer to, so here it is.

I am considering a design option for a PV system for a client. The system will be roughly 50kW DC and will be connected to an existing 208V 3-phase utility. To keep install costs down and utilize existing technology, we are proposing using 480V inverters, then knocking it down to the 208V for connection to the grid. The question I have is in regards to the transformer and protection thereof. I am curious if I would need disconnects on each side of the transformer. Per table 450.3(b), I am seeing a need for protection on the primary and the secondary side (we would be seeing more than 9A, so 125% of FL is applicable). I am really curious about putting in a fused D/S on each side of the transformer.

Thoughts?

A couple of thoughts

I assume you have costed out both a 480 and a 208 installation and have found a 50KVA or 3 16KVA transformers with a low enough price to make the 480 the winner. Such a transformer will run approx $2500 for a NEMA 1 dry-type. $2500 will buy a lot of wire even at today's prices.

Since it is a PV installation the transformer will have losses 24 hours per day and the PV will generating for less 12 hours. Have you calculated the loaded and unloaded losses cost?

 

[iwire]

It seems wrong to purposely reduce the efficiency of a system that is itneded to reduce energy use.

Just my personal opinion above.


Moving on, Keep in mind that 450 only deals with transformer protection you also have to deal with conductor protection as required in 240.


As far as your question on disconnects, you will need overcurrent protection on both sides but that might be handled with the breakers on the equipment as long as you can keep conductor length short.

See 240.21(C) for protection of conductors from transformers.


One thing I am having a hard time with is deciding which side of this transformer is the primary and which is the secondary. Under normal operating conditions power will flow two directions.

 

[Penwar]

Agreed

Agreed

When we first looked at this install, it was back in 2012 and the intent was to go with (6) 8kW inverters at 208V. Since that time, SMA has brought their TL series inverters in the upper kW range. We will be using (2) 24kW inverters), but they are for the larger voltage (277/480). Our ultimate goal is to top out around 100kW within the next 5 years, which would require a lot of the smaller inverters (max in SMA TL series is the 10kW at the 208 3-phase).

As for the loaded / unloaded question, we are running those number now.