Template · Generators

Emergency and standby loads on separate transfer switches single line diagram

One generator, a life-safety transfer switch and panel, and a second switch for optional standby loads. Use this for a building whose generator supplies both life-safety loads, such as egress lighting and the fire alarm, and loads the owner simply wants kept running. Each kind gets its own transfer switch and panel.

A free template with typical ratings, conductor callouts and general notes. Open it, change what differs on your job, fill in the title block and export a PDF.

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Emergency and standby loads on separate transfer switches single line diagram: one generator, a life-safety transfer switch and panel, and a second switch for optional standby loads. — Emergency and standby loads on separate transfer switches. 24 devices on an 11 × 17 in sheet with a title block and notes column.

What the reviewer checks on this drawing

The reviewer checks that the emergency wiring is kept apart from everything else, that each transfer switch has its own breaker at the generator, the selective coordination of the emergency system, and the connection for a temporary generator.

What the drawing shows

Every device on the sheet, in the order power reaches it.

Symbol	Label	Rating or detail	Conductors feeding it
Utility source	Utility service	480Y/277V 3φ 4W
Current transformer	CT cabinet	Utility metering
Circuit breaker	800A 3P	Main
Panel	Main switchboard MSB	800A · 480Y/277V 3φ 4W
Circuit breaker	600A 3P	Normal loads
Panelboard	Panel DP-N	Normal power
Circuit breaker	100A 3P	ATS-EM normal
Transfer switch	Transfer switch ATS-EM	100A · 4-pole · life safety, bypass isolation	4 #3 Cu + #8 G
Device	Temporary generator connection (TGC)	Manual transfer and, connection cabinet	4 #3 Cu + #8 G
Circuit breaker	100A 3P	Generator breaker 1
Generator	Generator	150 kW / 187.5 kVA, 480Y/277V · diesel
Panel	Emergency panel EM	100A MLO · 480Y/277V 3φ 4W
Circuit breaker	20A 1P
Lighting	Egress and exit lighting	277V
Circuit breaker	30A 3P	T-EM primary
Transformer	Transformer T-EM	15 kVA, 480V Δ – 208Y/120V	3 #10 Cu + #10 G
Panelboard	Panel EM-L	60A main · 208Y/120V, Fire alarm, exit signs	4 #6 Cu + #8 G
Circuit breaker	200A 3P	ATS-SB normal
Transfer switch	Transfer switch ATS-SB	200A · 4-pole, optional standby	4 #3/0 Cu + #6 G
Circuit breaker	200A 3P	Generator breaker 2	4 #3/0 Cu + #6 G
From (continues off sheet)	From generator	Second breaker, on the generator set
Panel	Standby panel SB	225A MLO · 480Y/277V 3φ 4W
Circuit breaker	Branch breakers	Standby circuits
To (continues off sheet)	To standby loads	Refrigeration, IT, heating

Notes on the sheet

Emergency (life-safety) loads have their own transfer switch, panel and wiring, kept entirely independent of all other wiring (NEC 700.10; CEC Section 46).
Each transfer switch is fed from its own breaker on the generator set, in separate enclosures or vertical sections (NEC 700.10), so a fault on the standby side does not take the emergency side with it. Emergency system overcurrent devices selectively coordinated (NEC 700.32).
One feeder carries the emergency loads, so ATS-EM is a bypass-isolation switch that can be serviced without dropping them; redundant transfer equipment is the other way to meet it (NEC 700.5(D), 2023 edition).
Generator 187.5 kVA ÷ (1.732 × 480V) = 226A. Where it cannot carry everything at once, the standby loads are shed first (NEC 700.4).
Four-pole transfer switches: the generator is a separately derived system, bonded and grounded at the generator.
T-EM secondary: 15,000 VA ÷ (208V × 1.732) = 41.6A; × 125% = 52A, so a 60A main on #6 Cu. Bonded as a separately derived system, with a #8 Cu supply-side bonding jumper (NEC Table 250.102(C)(1)) and a #8 Cu grounding electrode conductor to the building electrode (NEC 250.30).
The emergency system relies on one generator, so the emergency feed has a permanent means to connect a temporary one while it is out for service: a manual transfer switch and connection cabinet, interlocked so the two cannot be connected together, marked with the phase rotation and bonding arrangement, with an alarm when the permanent generator is disconnected (NEC 700.3(F)).
Sign at the service equipment giving the type and location of the emergency source (NEC 700.7).

Diagram is schematic and not to scale. Field-verify existing equipment ratings. Ratings and conductor sizes shown are typical. Confirm against the adopted code, the equipment listing and a load calculation before installation. Code references are to the 2023 NEC and the 2024 Canadian Electrical Code: see references and editions.